My mission to the MARS simulator

An apparatus for simulating the atmosphere on MARS. We are going to use it test a new apparatus for measuring the atmosphere on Earth.

An apparatus for simulating the atmosphere on MARS. We are going to use it test a new apparatus for measuring the atmosphere on Earth.

Next Saturday (31st May) I will be setting off to Denmark to carry out an experiment in the MARS simulator at the University of Aarhus.

Inside the gigantic red planet sausage chamber, we will be able to change the pressure, temperature and humidity of the air to simulate the conditions in the upper atmosphere and stratosphere of Earth rather than Mars

My aim is to test a new type of combined thermometer and hygrometer.

There are at least two clever things about the new device

  • The first clever thing is that it measures the temperature of the air without ‘touching’ it – it is a non-contact thermometer.
    • It does this by measuring the speed of sound in a volume of relatively unperturbed air. From the speed of sound we can relatively directly infer the temperature.
  • The second clever thing is that it simultaneously measures the humidity in the air, again without making contact with the air.
    • It does this by shining a laser through the same volume of air. The frequency of the laser is adjusted so that exactly matches a frequency of molecular vibration in water molecules.

But inventing the device is not enough: every invention needs an acronym. So after playing with the acronym generator, I have baptised the device NCTAH (pronounced nectar) which stands for Non-Contact Thermometer And Hygrometer.

And why does it matter? 

Measuring air temperature is difficult. For example at NPL we will happily calibrate a good thermometer with an uncertainty of around 0.001 °C, if it is to be used in contact with a liquid or solid. But when used in air we have to give an uncertainty more than 50 times larger!

And one place where the measurement of temperature and humidity is particularly important is when we try to determine how close a particular sample of air is to saturation. In other words when we ask:

“How close is a particular sample of air to forming water droplets or ice crystals?”.

This is the basic process of cloud formation in the atmosphere and it is extremely difficult to study. Assessing how close air is to saturation requires accurate measurement of both the amount of water present in the air and the temperature of the air.

When conventional instruments ascend to the upper atmosphere (where it is very dry) from the lower atmosphere (where it is relatively wet) they carry up moisture with them which affects their slow-responding humidity sensors.

Additionally the temperature readings from contact thermometers frequently lag the true air temperature.

Both these effects make it difficult to know how close the air in the upper atmosphere and stratosphere is to saturation and our instrument could make a significant improvement.

Making it work.

Developing and testing this device has required experts from

  • the Gas Analysis team (Tom Gardiner and Andrew Finlayson),
  • the Humidity Team (Stephanie Bell and Jenny Wilkinson),
  • and the Temperature team (Robin Underwood and myself).

We have tested the device at temperatures from -40 °C to + 40 °C at atmospheric pressure. And we have separately tested it at room temperature at a pressure of less then a twentieth of an atmosphere – equivalent to an altitude of about 25 km.

But the MARS chamber tests in Denmark will assess the whole instrument together at the extremes of its operating range.

The facility is expensive to hire, and the logistics of moving the experimental team and all the monitoring equipment to Denmark are challenging.

So I am excited – but nervous. I will try and let you know how it goes.

This is my colleague Robin Underwood holding the Non Contact Thermometer and Hygrometer (NCTAH). Tiny microphones and speakers measure the speed of sound in the space between the parabola on the left and the acoustic 'mirror' on the right.

My colleague Robin Underwood holding the Non Contact Thermometer and Hygrometer (NCTAH). Tiny microphones and speakers measure the speed of sound in the space between the parabola on the left and the acoustic ‘mirror’ on the right.

 

Who paid for this?

You did. The work is funded in part by the UK government and in part by the METEOMET project funded by the European Metrology Research Programme of the European Union.

 

4 Responses to “My mission to the MARS simulator”

  1. teddnet Says:

    Non-contact? Surely this is minimum contact? You have to send a sound wave into the atmosphere, so something has to give the air a shove, doesn’t it? And that will do a bit of compressive heating, won’t it? Pedantic I know, and nothing like shoving a mercury thermometer into the atmosphere, but it’s not like measuring the temperature of a star by measuring its colour, because in that case your measurement cannot have an effect on the temperature at the time the light was emitted…? I mean, to the usual extent of outrageous adherence to definitions applied by NPL it’s not quite non-contact, isn’t it?

  2. protonsforbreakfast Says:

    Mmmm. If the sound wave is a frequency of 10 kHz then the sound wave will have a wavelength of around 3.5 cm. At one point in the wave the air will be briefly compressed, warming by typically 0.0001 degrees Celsius. However the heat cannot flow from this region to the cold region 1.75 cm away in time. One 20,000th of a second later the same gas expands and cools. The key point is that almost no energy is lost from the sound wave and absorbed, thus heating the gas. I think ‘non-contact’ is pretty fair.

    Enjoy the break. 🙂

  3. Sam Gibbs Says:

    Who chose the color……???

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