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.
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.