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
Step 1: Cat-5 used for the tether
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.
Step 2: Solder the LM35
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.)
Step 3: Copper Cladding
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.
Step 4: Operational Sensor...but not waterproof
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.
Step 5: Capping the Probe
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.
Step 6: Silicone Sealant to Waterproof
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.