Waterproof Temperature Probe

Question

I'm looking into building a temperature monitoring system, and I want the sensor to have the ability to be submersed into boiling water. Can a thermocouple handle this? I'm looking into this one now, but I'm not sure if it's waterproof.

My other idea would be to drop a small temperature sensor into an aluminum tube and seal it off.

Any ideas or suggestions?

EDIT: Sorry, I should have provided more context with my question. To clarify some things, the sensor will be used to monitor and control water temperatures ranging from 30F to 230F, not just to test when water is boiling :). Also, the water being monitored is consumable, so the sensor needs to be food safe as well, and since this is a hobby project, I'm trying to keep the sensor as cheap as possible.

Answer 1

Sealing it inside a tube would be ideal. The thermocouple you've linked isn't waterproof. You can buy thermocouple probes in sealed housings from several sources. One popular source fo industrial users is http://www.omega.com/guides/thermocouples.html .

Rolling your own is fine as long as you select good high temperature materials for the sealant.

Answer 2

A thermocouple must be put into a thermowell if you wish to submerse it in water. However there are options, does the fluid have to be water? A dialectic oil would be fine and you could get much higher temperatures. If it must be water and you intend to make your own thermowell, then you should fill the well with oil and leave enough room for expansion before sealing. The oil will transfer the heat from the outside walls of the thermowell more efficiently than air. Also make sure the thermocouple is supported properly so not to short against the well.

I take it from the nature of your question that you may have not used thermocouples much. If that is true, then you should do some reading first. There are a lot of pitfalls you need to avoid. Such as dissimilar metal junctions in your wiring, and the proper metallurgy wire for the series TC you are using.

Hope this helps and good luck with your project.

Answer 3

I use one of these kitchen thermometers for my home-scale maple sugaring equipment:

http://www.target.com/gp/detail.html/181-2465896-2946763?asin=B0001BFJ54

The probe is foodsafe and works fine in boiling sap, although after one or two seasons it starts to pick up scale from the calcium impurities in the maple sap. (a tough problem to get around)

I'm not sure if the probe contains a thermistor or a thermocouple, but in any case it's a cheap off-the shelf solution that I'm sure you could use with your own circuitry if you experimented a bit.

Answer 4

I have two issues for you: (1) the AD590 should be strobed to a moderately low duty cycle to prevent self-heating. Check out the AD apnotes. (2) When you go underwater with wide temp swings especially if you go very deep you will probably get water intrusion. The sneakiest way water gets to the leads ( and screws the reading ) is by capillary action along the leads right through the silicon caulk or casting acrylic. Maybe there are potting compounds that could prevent that but the only way I found way to put the sensor at the end of a pipe through which the leads run to the surface. In case of condensation I even put a little sump branch below the sensor.

Good luck.

Answer 5

If you don't need too much precision and aren't dealing with very high temperatures, you can just use a thermistor instead. I recently did something similar: soldered a thermistor to a length of insulated wire, and covered the junction with silicone. Works great, and costs pennies.

Answer 6

Actually I don't think there's any problem with submersing a thermocouple in water. It's just wires, and as long as the thermoelectric effect can still take place at the point where the two metals touch, then it doesn't matter what they're surrounded by. I guess the water needs to be properly grounded though... you don't want freak voltage spikes etc getting into your thermocouple circuit.

EDIT: woops, yes, if you're looking for something that's food-safe then it's not such a good idea. Bits of food likely to get stuck in the wires, etc.

Answer 7

Additional thoughts about encapsulating your thermocouple in oil, potting compound or anything else, is thermal capacitance.

The amount of thermal capacitance is proportional to the amount and type of compound used. This will slow down the temperature response of the thermocouple due to the heat retained, or lost by the encapsulant. As long as you use substances with less thermal capacitance than the water it is surrounded by you can be sure that the temperature you are measuring is the actual water temperature, and not the delay of the thermocouple.

On the issue of joining thermocouple to copper wires. This is ideally performed in a bit more of controlled manner, and most importantly that the temperature is measured and recorded where this takes place. A simple LM35 sensor will do fine, and feed this into your ADC alongside the filtered and amplified thermocouple signals. From here use the known thermocouple polynomials and some software to calculate the temperature or use look up tables for the range you are interested in.

Answer 8

A friend in college stuck a standard temperature sensing IC into a metal tube, encapsulated it with epoxy, and then sealed the immersion end with high temperature gasket sealant. Worked fine for his purposes, but he only needed it for a semester.

There are industry standard thermocouples that are made for immersion. You can buy them from industrial warehouses like McMaster Carr. If you plan on immersion for long-term use, that's probably the way to go. The only drawback is that thermocouples don't give you a digital (or even linear analog) output like temperature IC's.

Answer 9

Analog Devices use to make a stainless steel temperature probe that contained an AD590 - temperature to current transducer. A little easier to use than a thermocouple. Not sure if they make it anymore.

The range on the AD590 is -55degC to 150degC. 1uA per degK. They are laser trimmed but we use to do a boiling point and freezing point calibration. As was mentioned you need to allow for the additional thermal mass when doing your measurements.

For the AD590 in the TO-52 package you could glue the package to a tube that is a thermal insulator and run the wires inside the tube. The tube would not add mass and would insulate the wires.

Answer 10

This may be a bit pedantic, but (in the interest of not misleading newcomers) don't imply that thermistors are imprecise. I work with thermistors that are specified +/- 0.2 degC out of the box.

Answer 11

Just regarding the comment about the temperature probe from Analog Devices. It is still available, see: www.analog.com/ac2626