I am planing to build a ROV for underwater use and at this point I am concidering to have some water leak detection system for the electronics enclosure that has to withstand underwater pressures up to 5bar / 70psi / 500kPa.
The design and the description of the probes let me think a bit about how they function. In a section of the product description it says:
Each probe can be re-used a number of times if dried out and re-compressed, but once the tip degrades you can also get replacement SOS Probe Tips.
So the probe tips are made of a sponge material that is probably enriched with some sort of salt (one could simply use NaCl for that I think), that is going to rise the conductivity of any water that is going to touch and get soaked into the tip.
If I am right I think to replicate those probes would not be much of a problem.
Moving on to the circuit board (SOS Leak Sensor):
Here is the circuit diagram from the documentation.
So far I understand that if any probe is going to short out, the circuit is going to switch the signal - that is pulled down by R3 - to high. But what are R1 and R2 doing exactly?
I need to know this because I still didn't get the relation of the conductivity/resistance of the water to where the circuit detects the "short out" and at what threshold.
I got some samples of the water in my local area, where I am planning to bring the ROV in use, and have poorly measured the resistance of them and some tap and distilled water for reference. I've used a multimeter for this and I did not had any rig to hold the two probes at a constant distance to each other but I tried my best holding them with my hand, so my measurements are not so constant. But here are some significant results:
The sample water hat a resistance of something about 500kOhm (+-100kOhm) at a probe distance of about 5mm (tip to tip). I than added a pinch of salt into a little sample of the same water and measured about 40 to 30 kOhm.
So I am sure that I can get readings with homemade probes with at least 100kOhm resistance. But back to my question, how does these relate to the circuit shown above? do I need to make any adjustments to R1 and/or R2? And if I have to, how are the calculations for that?