Waterproof a LM35 Temperature Sensor

Here is a instructable to waterproof a LM35 for use on a tethered ROV using a automobile 12V battery as a power source. This came out of a need for the MATE ROV Competition. The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. Here is the datasheet - http://www.national.com/ds.cgi/LM/LM35.pdf

I used cat-5 cable for the sensor as it will shield the sensor signals on the twisted pair (and I had a spool sitting around.) It will also easily attach to the tether.

I used six wires (three pair) for the sensor (Vs+, Vout, Gnd). I twisted colored pairs together to give me the three conductors.

Before soldering, I added short lengths of heat-shrink tubing wrap to the conductors, and two larger ones over the cat-5. Then, I soldered the LM35 to the cat-5 cable and added a 10K resistor to the Vout (center) to stablize the signal for the length of cable.

I then shrunk the wrap over the soldered joints using a torch (on it's lowest possible setting.)

I used 1/4 inch O.D. copper tubing (the kind for an automatic ice maker for your refrigerator) to make a small copper cover for the LM35.

I was able to just squeeze the LM35 into the copper tubing, then I squished the tubing just above the sensor in the vise...being very careful not to squish the sensor.

At this point, the sensor is fully operational in the air, but it is not waterproof. I decided to give it a quick test to make sure everything still works.

That's 16.7 mV (16.7 C or 62 F) on the voltmeter. Everything seems to be working.

I wanted to have the sensor on a probe so it would stick out forward of the ROV a little. Here I'm building the cap out of 1/2 inch copper pipe and cap. I drilled a 3/8 inch hole in the cap to accept the sensor with the squished copper cladding on it.

The copper cap was hammered into 1/2 inch PVC pipe to make the probe. Then, I used aquarium, silicone sealant to seal the gaps as I pushed the sensor into the probe.

The entire length of the probe was filled with sealant to complete the waterproofing. Note - However, you should probably fill it with a non-conductive grease. Then, clean the end of any grease and seal it with the silicone sealant. The sealant will never set if it is not exposed to air.