I need to measure the temperature of ground surrounding an ice storage tank buried under the ground. I plan to place temperature sensors in a hole near to the tank that reaches almost the same depth as the tank. The problem is that the holes are empty, i.e. they don't have any fluid inside them and as air is a bad conductor of heat, the temperature measurements will be of the air inside the hole and not of the soil surrounding the holes. I wish to fill the holes with a fluid, which can replicate the soil surrounding the hole, so that the temperature measurements are nearly the same as the soil (if not exact). I still have to measure the thermal properties of the soil, but let's assume that the properties of the soil surrounding the holes are : rho-1400 kg/m3, Cp- 1100 J/kgK, Lambda - 1.6 W/mK. Please suggest me a fluid that can be used to fill the hole for ground temperature measurements.
Dear Tushar Sharma , probably it would be unnecessary to fill the holes if you let the thermomether reach equilibrium, the air and the soil around should be at the same temperature and your measures would be correct.
May be you could place your sensors inside the storage tank close to its walls.
Hope it helps.
Dear Manuel Gómez , I understand your point. Yes, that's also possible. But the heat transfer coefficient for any liquid(let's say water) will be higher than that of air, and thus to achieve that equilibrium temperature we will need less time than just with air. My experiment is dynamic in nature, so if I need the exact temperature, or let's say approximately the same temperature, within a time step of 2 to 5 mins, then having just air is not a good option. Your theory might hold for a static measurement, not for dynamic. I have attached the diagram below for a better understanding of the sensor placement. At the moment, it is not possible to place the sensors next to or close to the tank. The only options are the ones you see in the diagram. And temperature sensors inside the tank will measure the temperature of the water and not of the ground surrounding the tank.
Dear Tushar Sharma , yes I was thinking that you had time to reach equilibrium. May be another solution without filling the holes could be to use IR thermometers pointed towards the hole wall. They are affordable, fast and small enough to fit even into small diameter holes.
We used some of them, wireless, which came with an USB memory where a record of the temperature variations was saved for further study.
A thread or a wire could keep them in place at 1.5 and 2.5m positions on the diagram.
Continous recording of T is quite useful, since you can follow even minute changes over time.
Hope it helps.
Best.
Dear Manuel Gómez, I appreciate your advice about using IR sensors. At the moment, we plan to use the standard 4 wire PTC sensors that are connected through long cable connections to the DAQ for data logging. Now the question is whether the IR sensor is better than the standard PTC sensors and whether they will be able to estimate the correct ground temperatures despite air resistance? Can you please state the benefits of using IR sensors over standard 4-wire PTC sensors for this particular case? Thanks in advance!
Dear Tushar Sharma , describing a sensor as better than other depends on your particular application. Resolution of PTC and IR sensors are in the same range of temperature, both are quite fast delivering the measures and probably the main difference is the way they work (IR= thermocouple, PTC=resistor) and therefore the way you can use them. IR sensors works measuring the IR radiation radiated from the body you are interested on, and the reference for calibration is a black body. Both calibration and measurement are performed - normally- under air conditions, so effects of air absorbance of IR radiation are included, which allows to measure temperature without contact through air, it even allows measure temperature of gases. However it doesn´t allow to measure temperature of the sample through a glass screen, the temperature the device retrieves will be the one of the glass surface.
PTC´s are based on resistance increment with temperarture, and probably they can be callibrated in air too, but it could cause problems as those described by you, air low conductivity, large time to reach thermal equilibrium etc.
Probably PTC´s are also available wireless, so in both cases you can skip or keep the wire for data-logging.
In your case, given you already have the PTC´s, probably the best idea may be to use them, filling the hole with the hole´s material to improve contact. May be you don´t need to fill the holes completely, just the PTC, which will allows you to remove it more easily. If material is concrete rock powder or the like, you can make a suspension with water to help to contact well around the sensor.
IR sensors are affordable and work without contact, which makes them useful too.
Dear Tushar Sharma , I know that your design includes two depth sensing positions, but may be an IR camera could help too. It could be mounted in a drone or a tower and taking images of the surface and its T distribution and changes over time.
About your PTC system, may be instead of the same material from the hole, you could use a material with a high thermal conductivity as yourself suggested and may be metal fine filings could do the job. It wouldn´t necessary to fill the hole completely, just the sensor. Since the sensors are not at the bottom, you could place a sponge at the right heigths to hold the filings in place.
Carbon nanotubes have also a high thermal conductivity, but something less exotic, graphite powder is also good.
Iron filings are a good option too and easy to remove from the holes with a magnet.
Hope it helps!
Good luck
Manuel
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