Archive for the ‘Simple Science’ Category

The Joy of Science

October 15, 2017

For the last couple of weeks I have been a helper at Averil Horton’s Science Club.

It’s  very low key

Just a few adults doing science experiments themselves. And then discussing the results.

Helping – which requires no prior work on my part – has reminded me of the simple pleasure people experience from doing stuff for themselves.

And the pleasantness of discussing what one sees with others. It is the Joy of Science.

Here are some pictures.

Setting things on fire…

Flame

Seeing what floats in what…

Jewels

Playing with colours…

Flower

Averil & Colours

And finishing with a little bit of magic…

Magician

 

Talking about the ‘New’ SI

July 3, 2017

I was asked to give a talk about the SI to some visitors tomorrow morning, and so I have prepared some PowerPoint slides

If you are interested, you can download them using this link (.pptx 13 Mb!): please credit me and NPL if you use them.

But I also experimentally narrated my way through the talk and recorded the result as a movie.

The result is… well, a bit dull. But if you’re interested you can view the results below.

I have split the talk into three parts, which I have called Part 1, Part 2 and Part 3.

Part 1: My System of Units

This 14 minute section is the fun part. It describes a hypothetical system of units which is a bit like the SI, but in which all the units are named after my family and friends.

The idea is to show the structure of any system of units and to highlight some potential shortcomings.

It also emphasises the fact that systems of units are not ‘natural’. They have been created by people to meet our needs.

Part 2: The International System of Units

This 22 minute section – the dullest and most rambling part of the talk – explains the subtle rationale for the changes in the SI upon which we have embarked.

There are two key ideas in this part of the talk:

  • Firstly there is a description of the separation of the concepts of the definition of a unit from the way in which copies of the unit are ‘realised‘.
  • And secondly, there is a description of the role of natural constants in the new definitions of the units of the SI.

Part 3: The Kilogram Problem

This 11 minute section is a description of one of the two ways of solving the kilogram problem: the Kibble balance. It has three highlights!

  • It features a description of the balance by none other than Bryan Kibble himself.
  • There is an animation of a Kibble balance which takes just seconds to play but which took hours to create!
  • And there are also some nice pictures of the Mark II Kibble Balance installed in its new home in Canada, including a short movie of the coil going up and down.

Overall

This is all a bit dull, and I apologise. It’s an experiment and please don’t feel obliged to listen to all or any of it.

When I talk to a live audience I hope it will all be a little punchier – and that the 2800 seconds it took to record this will be reduced to something nearer to its target 2100 seconds.

 

 

 

How would you take a dinosaur’s temperature?

March 15, 2017
A tooth from a tyrannosaurus rex.

A tooth from a tyrannosaurus rex.

Were dinosaurs warm-blooded or cold-blooded?

That is an interesting question. And one might imagine that we could infer an answer by looking at fossil skeletons and drawing inferences from analogies with modern animals.

But with dinosaurs all being dead these last 66 million years or so, a direct temperature measurement is obviously impossible.

Or so I thought until earlier today when I visited the isotope facilities at the Scottish Universities Environmental Research Centre in East Kilbride.

There they have a plan to make direct physical measurements on dinosaur remains, and from these measurements work out the temperature of the dinosaur during its life.

Their cunning three-step plan goes like this:

  1. Find some dinosaur remains: They have chosen to study the teeth from tyrannosaurs because it transpires that there are plenty of these available and so museums will let them carry out experiments on samples.
  2. Analyse the isotopic composition of carbonate compounds in the teeth. It turns out that the detailed isotopic composition of carbonates changes systematically with the temperature at which the carbonate was formed. Studying the isotopic composition of the carbon dioxide gas given off when the teeth are dissolved reveals that subtle change in carbonate composition, and hence the temperature at which the carbonate was formed.
  3. Study the ‘formation temperature’ of the carbonate in dinosaur teeth discovered in a range of different climates. If dinosaurs were cold-blooded, (i.e. unable to control their own body temperature) then the temperature ought to vary systematically with climate. But if dinosaurs were warm-blooded, then the formation temperature should be the same no matter where they lived (in the same way that human body temperature doesn’t vary with latitude).
A 'paleo-thermometer'

A ‘paleo-thermometer’

I have written out the three step plan above, and I hope it sort of made sense.

So contrary to what I said at the start of this article, it is possible – at least in principle – to measure the temperature of a dinosaur that died at least 66 million years ago.

But in fact work like this is right on the edge of ‘the possible’. It ought to work. And the people doing the work think it will work.

But the complexities of the measurement in Step 2 appeared to me to be so many that it must be possible that it won’t work. Or not as well as hoped.

However I don’t say that as a criticism: I say it with admiration.

To be able to even imagine making such a measurement seems to me to be on a par with measuring the cosmic microwave background, or gravitational waves.

It involves stretching everything we can do to its limits and then studying the faint structures and patterns that we detect. Ghosts from the past, whispering to us through time.

I was inspired.

=============================

Thanks to Adrian Boyce and Darren Mark for their time today, and apologies to them both if I have mangled this story!

Do you really want to know if global warming is real?

January 28, 2017

About a year ago, I thought that Climate Change Deniers had lost the argument.

I thought that we were all moving on to answering more interesting questions, such as what to do about it.

But it seems I was wrong. It seems that in this post-truth world, climate change deniers are uninterested in reality – preferring instead alternative facts.

I am left speechless in the face of this kind of intellectual dishonesty.

Actually I am only almost speechless. I intend to continue trying to empower people by fighting this kind deception.

Rather than trying to woo people over to my view, my aim is simply to offer people the chance to come to their own informed opinion.

See for yourself

As part of my FREE University of Chicago Course on Global Warming, I have been using some astonishing FREE software. And its FREE!

1

The ‘Time Series Browser’ allows one to browse a 7000 station subset of our historical temperature records from meteorological stations around the world.

  • The data are the local station temperatures averaged over 1 month, 1 year or 1 decade. Whichever you choose you can also download this data into a spreadsheet to have fun with on your own!
  • One can select sets of data based on a variety of criteria – such as country, latitude band, altitude, or type of geographical location – desert, maritime, tropical etc. Or you can simply pick a single station – maybe the one nearest you.

Already this is enormously empowering: this is the pretty much the same data set that leading climate scientists have used.

For this article I randomly chose a set of stations with latitudes between 20°N and 50°N.

7

The bold dots on the map show the station locations, and the grey dots (merging into a continuous fill in parts) are the available locations that I could have chosen.

The data from the selected stations is shown below.  Notice the scale on the left hand side runs from -10 °C to + 30 °C.

2

In this form it is not obvious if the data is warming or cooling: And notice that only a few data sets span the full time range.

So how do we discover if there are trends in the data?

The first step

Once you have selected a set of stations one can see that some stations are warm and others cool. In order to be able to compare these data fairly, we subtract off the average value of each data set between 1900 and 1950.

This is called normalisation and allows us to look in detail at changes from the 1900-1950 average independent of whether the station was in a warm place or a cold place.

3

Notice that the scale on the left-hand side is now just ± 3.5 °C.

The second step

One can then average all the data together. This is has the effect of reducing the fluctuations in the data.

One can then fit a trend-line to see if there is a recent warming or cooling trend.

5

For this particular set of stations its pretty clear that since 1970, there is a warming trend. The software tells me it is approximately 0.31 ± 0.09 °C per decade.

What I have found is that for any reasonably diverse set of stations a warming trend always emerges. I haven’t investigated this thoroughly, but the trend actually seems to emerge quite clearly above the fluctuations.

But you can check that for yourself if you want!

Is it a cheat? No!

You can check the maths of the software by downloading the data and checking it for yourself.

Maybe the data is fixed? You download the source data yourself – it comes from the US Global Historical Climatology Network-Monthly (GHCN-M) temperature data-set.

But accessing the raw data is quite hard work. If you are a newbie, it will probably take you days to figure out how to do it.

There is more!

This ability to browse, normalise, average and fit trends to data is cool. But – at the risk of sounding like a shopping channel advertorial – there is more!

It can also access the calculations of eleven different climate models.

For the particular set of stations that you have selected, the software will select the climate model predictions (a) including the effect of human climate change and (b) without including human-induced climate change.

For my data selection I chose to compare the data with the predictions of the CCSM4 Climate model. The results are shown below

6

You can judge for yourself whether you think the trend in the observed data is consistent with the idea of human-induced climate change.

For the particular set of stations I chose, it seems the CCSM4 climate model can only explain the data by including the effect of human-induced climate change.

But Michael: this is just too much like hard work!

Yes and no. This analysis is conceptually challenging. But it is not crazily difficult. For example:

  • Schoolchildren could do this with help from a teacher.
  • Friends could do it as a group and ask each other for help.
  • University students could do this.
  • Scout groups could do it collectively.

It isn’t easy, but ultimately, if you really want to know for yourself, it will take some work. But then you will know.

So why not have a go?  The software is described in more detail here, and you can view a video explaining how to use the software here.

[January 28th 2017: Weight this morning 71.2 kg: Anxiety: Sick to my stomach: never felt worse]

4. Feedback and Climate Models

January 3, 2017

In the last two articles I have written at great length (sorry) about the way carbon dioxide affects the transmission of infrared light vertically through the atmosphere.

Changes in this transmission are – we think – causing Global Warming.

The physics of the effect on infrared transmission is beyond argument. However this is just one component in the energy flows that constitute Earth’s climate system.

What else do we need to consider before we can conclude that carbon dioxide is causing global warming?

What else do we need to consider?

atmosphericmodelschematic

There is so much! The calculations in the previous articles only considered the transmission of infrared radiation and light up and down a vertical column of air with a variable temperature and pressure.

However in reality:

  • Light transmission does not just take place in one dimension (up and down) but in three dimensions.
  • Illumination from the Sun strikes each part of the Earth at different angles.
  • The infrared radiation from the Earth also takes place at many angles, and from many different heights in the atmosphere.
  • There are clouds which dramatically change atmospheric transmission.
  • The air moves in complicated ways “up and down and round and round”.

Additionally, the energy balance is dynamic – all the above factors change from minute to minute – around the surface of the Earth. And the Earth is not a sphere, and is not uniform and does not move in a circle around the Sun. And the Sun’s output varies from year to year.

In order to calculate the long-term averages of temperature and rainfall that determine the climate, we need to take into account all – or as many as possible – of the above effects.

There will be a cascade effects caused by increased  atmospheric carbon dioxide – such as changes in the location or timing of cloud formation. Additionally changes in the Earth’s surface temperature will affect the temperature of the atmosphere.

These changes may either ameliorate or exacerbate the initial effects of increased carbon dioxide concentrations.

In the end one ends up with a complex General Circulation Model of the entire Climate of the Earth – such as that illustrated above. The MODTRAN code – or something similar – is incorporated as one element of all the extant general circulation models.

If it’s all so complicated…why are scientists so sure of themselves?

There are, I think, two or possibly three reasons.

The first concerns calculations of the future effect of increasing carbon dioxide concentrations.

Simple calculations made more than 100 years ago agree pretty well with the results of most recent complicated calculations.

This indicates that the simple calculation has captured the essence of the problem.

Secondly, there is broad agreement with experimental observations – the Earth’s surface really is warming (Data Analysis 1 and Data Analysis 2). You can download the raw data from land stations here.

The third reason – which is really just a different way of thinking about the previous two reasons – is that given its effect on infrared transmission, it would be truly astonishing if adding carbon dioxide to the atmosphere did not affect the climate at all!

Once one admits to this point it becomes a question of asking what the effect will be? And every calculation I have ever seen predicts warming. If anyone has found something different, I would love to hear about it.

What about the saturation of the carbon dioxide bands?

A friend of a friend wrote an analysis in which he argued that increased concentrations of carbon dioxide could not cause global warming because:

The bottom line is that once Carbon Dioxide reaches a concentration that makes the atmosphere completely opaque in the band where it resonates,  further increases in the concentration cannot result in any additional blocking.

He was imagining that the ‘band’ where carbon dioxide molecules resonate is fixed. He was wrong.

For the individual molecules, the frequencies at which they vibrate are fixed. And the width of their ‘natural’ absorption line is fixed by the local temperature and pressure.

But transmission through the atmosphere is complicated, and the width of the band that absorbs radiation just keeps growing in width as the concentration increases.

Additionally the height in the atmosphere at which the absorption takes place gets lower – and hence warmer – and re-radiates more radiation back down to Earth.

That’s all for this article:

Here we looked at how the MODTRAN calculations fit into more complex models of global climate.

The next (and final) article is about the conclusions we can draw from these calculations.

3. Light transmission through the atmosphere

January 3, 2017

co2_band_formation

In part 2 I looked at transmission of infrared light through a gas containing a molecule which absorbs infrared light at one particular frequency.

We saw that at higher concentrations, the absorption at specific frequencies broadened until entire bands of frequencies were ‘blocked’.

We saw that the width of the ‘blocked bands’ continued to increase with increasing concentration.

Here we look at how that insight can be applied to transmission of infrared light through Earth’s atmosphere.

This is even more complicated.

  • We are mainly interested in transmission of infrared light from the Earth’s surface out through the atmosphere and into space, but the atmosphere is not at a uniform temperature or pressure.
  • When absorbing gases are present, the air is not just a ‘conduit’ through which infra-red light passes – the air becomes a source of infrared radiation.
  • We are mainly interested in the effect of carbon dioxide – but there are several other infrared ‘active’ gases in the atmosphere.
  • Gases are not the only thing in the atmosphere: there is liquid water and particulates.

So it’s complicated: Here are a few more details.

1. Density.

If the carbon dioxide is distributed in a fixed proportion to the amount of oxygen and nitrogen through the atmosphere, then it will have more effect where the atmosphere is most dense: i.e. lower down in the atmosphere.

And density is affected by both temperature and pressure.

Since carbon dioxide molecules absorb 100% of the infrared light with wavelengths around 15 micrometres, as we saw in the previous article, increasing the concentration of carbon dioxide increases the range of wavelengths that are ‘blocked’. This is illustrated in the figure at the head of the article.

Increasing the concentration of carbon dioxide also changes the height in the atmosphere at which absorption takes place.

2. Re-radiation.

Once absorbed by a carbon dioxide molecule, the infrared light does not just disappear.

It increases the amplitude of vibration of the molecule and when the molecule collides with neighbouring molecules it shares that energy with them, warming the gas around it.

A short while later the molecule can then re-radiate light with the same frequency. However the brightness with which the gas ‘glows’ relates to its local temperature.

Some of this re-radiation is downward – warming the Earth’s surface – and giving rise to a ‘greenhouse’ effect.

And some of this re-radiation is upward – eventually escaping into space and cooling the Earth.

3. Other things.

Carbon dioxide is not only the infrared active gas in the atmosphere. There is also methane, ozone and, very significantly, water vapour.

There is also condensed water – clouds.

And then there are particulates – dust and fine particles.

All of these affect transmission of light through the atmosphere to some extent.

For an accurate calculation – all these effects have to be considered.

MODTRAN

Fortunately, the calculation of transmission through the atmosphere has been honed extensively – most notably by the kind people at the  US Air Force.

However the code is available for anyone to calculate atmospheric transmission.

David Archer and the University of Chicago kindly host a particularly friendly front end for the code.

modtran-web-interface

Aside from just clicking around, it is possible to download the results of the calculations and that is how I plotted the graphs at the head of the page.

To get that data I removed all the other greenhouse gases from the atmosphere (including water), and varied only the concentration of carbon dioxide.

Notice that the absorption lines grow into bands that continue to broaden as we add more and more  carbon dioxide. This is exactly what we saw in the simple model in the second article.

This shows that the transmission through the atmosphere is still being affected by additional carbon dioxide, and these bands have not ‘saturated’.

Asking a question

MODTRAN can answer some interesting questions.

Assuming that the Earth’s surface is at a temperature of 15 °C, we can ask MODTRAN to calculate how much infrared light leaves the top of the atmosphere (100 km altitude) as we add more carbon dioxide. The result of these calculations are shown below:

toa-radiative-power

The first thing to notice is the qualitative similarity between this graph – the result of complex and realistic calculations – with the simple spreadsheet model I showed in the second article.

The second thing to notice is that the calculations indicate that increasing the concentration of carbon dioxide in the atmosphere reduces the amount of radiation which escapes at the top of the atmosphere. And that it will continue to do so even as the concentration of carbon dioxide increases well beyond its current 400 parts per million (ppm).

Where does that absorbed radiation go? The graph below shows the results of another calculation. It imagines being on the ground and asks how much infrared light is re-radiated back to the Earth’s surface as the concentration of carbon dioxide increases.

downward-flux-graph

The graph shows that matching the decline in infrared radiation leaving the top of the atmosphere, there is a matching increase in radiation falling back down to Earth.

Importantly, both these effects still depend on the concentration of carbon dioxide in the atmosphere even as the concentration grows past 400 ppm.

Over the longer term, this increase in downward radiation will increase the temperature of the Earth’s surface above the assumed 15 °C. This process will continue until the outgoing radiation leaving the top of the atmosphere is balanced with the incoming solar radiation.

That’s all for this article:

In this article we saw that transmission of infrared light through the atmosphere is complicated.

Fortunately MODTRAN software can cope with many of these complexities.

The conclusions of our calculations with MODTRAN are similar to conclusions we came to in the previous article.

Increasing the concentration of a molecule such as carbon dioxide which absorbs at a single frequency will continue to reduce transmission through the atmosphere indefinitely: there is no limit to the amount of absorption.

The next article is about the conclusions we can draw from these calculations.

2: Light transmission through a gas

January 3, 2017

In the first article I showed experimental data on the spectrum of light travelling through the atmosphere.

We saw that some frequencies of light are ‘blocked’ from travelling through the atmosphere.

Sometimes this ‘blocking’ occurs at specific frequencies, and sometimes at ranges of frequencies – known as ‘blocked bands’.

In this article, we will consider how both single frequency absorption and blocked bands arise.

Air and Light

Air is composed mainly of nitrogen, oxygen, and argon molecules. The frequencies at which these molecules naturally vibrate are very high, typically greater than 400 terahertz. High frequencies like this correspond to light in the visible or ultraviolet part of the spectrum.

Larger molecules – ones composed of more than two atoms – can vibrate more easily.

They are – in a very rough sense – ‘floppier’ and have lower natural frequencies of vibration, typically a few tens’s of terahertz.

Frequencies in that range correspond to light in the infrared part of the spectrum.

The animation below shows qualitatively the relative frequencies of a vibrational mode of an N2 molecule and a bending mode of a CO2 molecule.

co2-animation

When light travels through a gas containing molecules that can vibrate at the same frequency as the light wave, the molecules begin to vibrate and absorb some of the energy of the light wave.

The molecules then collide with other atoms and molecules and share their energy – warming the gas around them. The light has been absorbed by the gas.

But this absorption only happens close to the specific frequencies at which the molecules vibrate naturally.

The effect of a single frequency of vibration

The figure below shows the effect of the presence of a low concentration  of a molecule that can absorb light at a specific frequency.

absorption1

The figure describes how ‘white’ light – in which all frequencies are present with equal intensity – travels through a non-absorbing gas with a low concentration of molecules which absorb at one specific frequency.

Light with a frequency – represented by a colour: yellow, orange or red – which just matches the vibrational frequency of the molecule is absorbed strongly and doesn’t make it far through the gas.

But light with frequencies on either side of this vibrational frequency is absorbed less strongly. So the percentage of light transmitted has a dip in it at the frequency of molecular vibration.

If we increase the concentration of the absorbing molecule, something really interesting happens.

absorption2

The light at the central vibrational frequency is absorbed even more rapidly. But since it is already 100% absorbed – it doesn’t affect the overall transmission at this frequency. However it does affect where the light is absorbed.

But the additional concentration of absorbing molecules now absorbs strongly on either side of the main absorption frequency.

Eventually, the absorption here becomes so strong that the absorption is 100% even for frequencies that differ significantly from the main vibrational frequency.

This leads eventually to bands of frequencies that are 100% absorbed.

Band Width

Importantly, as the concentration of the absorbing molecule increases – the width of the blocked band increases.

This increase in absorption band width isn’t a property of an individual molecule – each of which just absorbs at frequencies centred around a particular frequency.

The formation of the band – and its width – is a property of a column of gas containing many absorbing molecules

This can be modelled quite easily and the output of a spreadsheet model is animated below as a function of the concentration.

In each frame of the animation, the concentration increases by a factor 2.7 – so that the concentration range covered in the seven frames is 387 (~2.7 to the power 6).

single-line-absorption

The figure shown in percent on each frame of the animation is the fraction of light in the range from 212 to 228 terahertz which has been absorbed.

Please note that the line-widths and frequencies in the model are arbitrary and approximate. However the qualitative behaviour is universal and independent of the particular mathematics I have used.

  • As the concentration of an absorbing gas increases, the transmission at the central absorbing frequency eventually reaches zero.
  • As the concentration increases further, the absorption increase at frequencies on either side of the central frequency.
  • This eventually forms a range of blocked frequencies – and the width of this blocked range continues to increase with increasing concentration.

The fraction of light transmitted is plotted below.absorption-graph-from-single-line

Once again I would like to emphasise that the graph qualitatively characterises the absorption from a single absorption frequency as a function of concentration.

Significantly, the amount of light transmitted continues to fall even after the transmission at the central frequency reaches zero.

And notice that this broadening of the absorption bands is a property of the transmission of light through a column of gas. It is not caused by line-broadening by individual molecules.

That’s all for this article:

The story so far is that when one looks up through the atmosphere, we see ‘blocked bands’ at a range of frequencies.

In the infrared region of the spectrum, these bands arise from particular modes of vibration of specific molecules which occur at specific frequencies.

In this article we saw that even when the transmission through a gas was saturated, increasing the concentration of the absorbing molecule still reduced transmission through the gas.

This is because the width of the ‘blocked band’ is not a property of the individual absorbing molecules: it arises from transmission of light through a column of gas.

The next article is about how this effect works in Earth’s atmosphere.

1. Light transmission through the atmosphere

January 3, 2017

illustration

Light through a gas

Visible light travels through most gases almost unperturbed.

And broadly speaking, the Earth’s atmosphere is transparent to visible light.

However  if one looks in detail at the way sunlight travels through the Earth’s atmosphere, one can see some remarkable features.

The figure above is a high-resolution spectrum of sunlight. The spectrum would be about 40 times as wide as the figure above but has been ‘folded back’ on itself many times

Light

You may be familiar with the fact that light is a wave in the electric field.

  • When the wave vibrates with a frequency of approximately 430 terahertz it has a wavelength of approximately 0.7 thousandths of millimetre, and it elicits the sensation of red in our eyes.
  • When the wave vibrates with a frequency of approximately 750 terahertz it has a wavelength of approximately 0.4 thousandths of millimetre, and it elicits the sensation of blue in our eyes.

You are probably familiar with the basic features of the spectrum as it sweeps from light which elicits the sensation of ‘red‘ in our brain, to light which elicits the sensation of ‘blue‘.

But this Figure also  shows many dark lines in the spectrum. If we looked at the Sun with filters at these specific frequencies – we would see no light at all! The atmosphere would be opaque!

What has happened is that light with a very specific frequency (and hence wavelength) has been absorbed by vibrations of electrons within specific types of atoms.

Some of these atoms were in the outer layers of the Sun, and some are in our atmosphere.

Infrared Sunlight

Electrical waves exist with lower frequencies that elicit no sensation of colour or brightness in our eyes: this light is called ‘infrared’.

If we look at sunlight coming through the atmosphere at infrared frequencies, the spectrum is even more complex than in the visible region of the spectrum.

The graph below shows data acquired by my colleague Tom Gardiner. It shows the brightness of sunlight coming through the atmosphere at frequencies 10 times lower than visible light.

The brightness is plotted versus the wavelength of the light rather than the frequency because for historical reasons, that is a more common way to present the data.The wavelengths vary between 4 thousandths of a millimetre and 5 thousandths of a millimetre (4 to 5 micrometres).

slide1

There are two remarkable things about this spectrum:

  • the complexity of the spectrum – there are hundreds of peaks and troughs –
  • and the occurrence of a range of wavelengths between about 4.18 and 4.45 micrometres in which the sunlight is completely blocked!

The next two figures show the green region and the orange region in detail.

slide2slide3

If one looks at even lower frequencies (longer wavelengths), one sees the same two features – millions of sharp lines and entire ‘blocked bands’ – repeated again and again.

For example, the image  below is a modified extract from this amazing image (which I don’t have permission to reproduce) and shows details of transmission of sunlight through the atmosphere at frequencies of around 20 terahertz and wavelengths around 15 micrometres.

This particular range of blocked infrared light is caused by carbon dioxide molecules in the atmosphere. At this range of frequencies the carbon dioxide molecule can bend easily.

Amazingly, this simple ‘bendability’ of the molecule plays a significant role in determining the surface temperature of the Earth.

slide4

That’s all for this article:

The story so far is that when one looks up through the atmosphere, there are certain frequencies at which light is blocked.

This blocking sometimes occurs at specific frequencies, and sometimes as ranges of blocked transmission – known as ‘blocked bands’.

For historical and technical reasons, people usually specify the wavelength of the blocked light rather than its frequency.

The next article is about the link between specific blocked frequencies and blocked bands.

Why I love thermocouples

December 6, 2016

 

img_4656

Thermocouples are probably the simplest, cheapest and most reliable temperature sensors available.

But like many pieces of great technology, their simplicity hides a mystery!

The Mystery

A thermocouple is made of two different kinds of wire, joined at one end and connected to a voltmeter at the other end.

figure-4

When heated a thermocouple generates a voltage approximately proportional to the temperature difference between the junction of the two wires and the two loose ends of the wire.

This is really useful – and by using standard types of wire – humanity can measure temperatures in a simple way.

And the mystery? The mystery is that none of the voltage you measure is generated at the tip of the thermocouple!

How a thermocouple works

In 1821 Thomas Johannes Seebeck discovered that accompanying every temperature difference in a metal, a small voltage was generated: a ‘thermo-voltage’.

figure-1

We now know that it is caused by the differing extent to which the electrons in the metal are disturbed at different temperatures.

Seebeck noted that the voltage generated for a given temperature difference depended on the type of metal.

figure-2

So a copper wire stretched between two temperatures generated one voltage, (V1), but a nickel wire stretched between the same two temperatures generated a different voltage (V2).

In a long wire, a voltage is created across the length of the wire, and one can work out the total voltage measured by adding together all the small voltages (ΔV) due to all the small temperature changes (ΔT).

figure-3

Interestingly – and this is at the heart of the mystery – because the ΔVs are only generated by ΔTs – it doesn’t matter how long the wire is, or which route it takes!

The thermo-voltage is proportional to the overall temperature difference between the two ends of the wire.

Joining two types of wire together

A ‘thermo-couple’ is made by joining two dissimilar wires together. Because the two wires are different, the voltages V1 and V2 generated by each ‘leg’ of the pair don’t cancel, and there is a net voltage (V1 – V2) characteristic of the two types of wire, and the temperature difference from one end to the junction.

figure-5

 

So if you know the temperature of your voltmeter, then you can work out the temperature of the tip of the thermocouple by measuring the thermo-voltage.

The ‘thermo-voltage’ is usually tiny, typically only 40 microvolts per 1 °C of temperature difference, but that’s enough to make a measurement with an uncertainty of about 1 °C in many circumstances.

The Mystery

From the explanation above it should be clear that the ΔVs are generated along the entire length of the wire – but no voltage is generated at the junction!

If one puts a thermocouple in a furnace – then the ‘thermo-voltage’ corresponds to the temperature at the tip of the thermocouple. But all the delta ΔVs  are generated as the thermcoouple goes through the wall of furnace!

figure-6

If one pulls the thermocouple through the wall, then a different piece of wire generates the voltage.

So in order to get reproducible results it is important that the composition of the wire is uniform along its length. This is one of the major problems in the making thermocouples and being confident they are reading correctly..

A thermocouple thermometer

A thermocouple thermometer is actually two thermometers in one!

  • First the device has a thermometer inside – usually an electrical resistance thermometer called a thermistor – that records the temperature of the electrical terminals.
  • Secondly the device has a sensitive voltmeter that records the ‘thermo-voltage’. Based on the type of wires from which the thermocouple is made, the device works out how much hotter or colder the tip of the thermocouple is than the electrical terminals.

Combining the results of the two temperature measurements together gives the temperature of the tip of the thermocouple

Interesting places to stick a thermocouple

Because thermocouples are small and tough and light, you can stick them in places that you can’t easily stick other thermometers. You might like to try these experiments:

  • Let some ice warm up to 0 °C – and then press it down on some salt with a thermocouple trapped underneath. The temperature will fall to roughly -16 °C – really cold!
  • Try putting just the tip of the thermocouple in a candle flame. You should get an answer close to 1000 °C!
  • Try working out just how hot a cup of tea is when its just right – for me it’s close to 60 °C.

It’s hard not to love a scientific instrument that can do all that!

img_4657img_4658img_4662img_4665img_4666img_4668img_4670

°C and C are not the same!

October 5, 2016
Sometimes one has to write to the papers!

Sometimes one has to write to the papers!

<RANT>

Sometimes I am unable to stop myself writing to the papers.

Some issues – such as people not using measurement units correctly  – are just too important to let pass.

And people referring to temperature units incorrectly induces apoplexy!

For the record, the degree Celsius is an SI unit for temperature: the degrees C********e and F********t are not.

Their use in everyday language is understandable – many people use the F-word occasionally – and in the correct context, it gives no offence.

But for newspapers and media outlets to do so is outrageous!

And using the abbreviation C instead of °C is just wrong.

As I wrote to The Guardian recently:

Dear Guardian,

The measurement system that underpins all of our physical measurements of the world around us is called the International System of Units, widely referred to as ‘the SI’.

It is a staggering achievement, used daily by hundreds of thousands of scientists and engineers.

It provides a standard way of comparing measurements around the globe and of referring to those measurements. So why has The Guardian invented its own system of units?

To refer to a temperature of 25 degrees Celsius, the standard abbreviation is 25 °C. However The Guardian routinely refers to this as 25C, using the symbol ‘C’ which refers to the SI ‘coulomb’, an amount of electric charge. Why?

You might argue that your meaning is clear in context. And generally it is. But why be wrong when you can be right so easily?

Sincerely

Michael de Podesta

National Physical Laboratory.

P.S. In MS Windows™ systems, the degree symbol is [ALT] + 2 + 4 + 8 on the number keypad and in MacOS the degree symbol is [ALT] + [SHIFT] + 8. In iOS, on numeric keypad use a long press on the zero key to reveal the degree symbol.

P.P.S. There should also be a space between the number and its unit, but I didn’t want to mention that in case you thought I was being pedantic.

More seriously, reporting measurements in the correct units aids clarity of understanding and establishes the basic competence of the author.

Reporting, as The Guardian did this week, that:

“the 2016 temperature is likely to be 1.25C above pre-industrial times, following a warming trend where the world has heated up at a rate of 0.18C per decade.”

merely establishes that the writer knows nothing about measurements.

This is not a matter of style, it’s a matter of just being wrong.

</RANT>

[October 5th 2016: Weight this morning 73.5 kg: Anxiety: Low. I don’t know why, but I just felt OK today :-)]


%d bloggers like this: