TonyS
Member
I am making a simple alarm for the holding tank from a project in the "Boatowners Wiring Manual". This alarm functions when water makes contact with two probes which completes a circuit that drives the input of an op amp positive and therefore gives an output to the amplifier that in turn causes an LED to light. The circuit works when I stick the probes in a glass of water. It also works if I join the probes with a resistance under 100K. However if I put the probes of my digital meter on the resistance scale in the water I get no reading on any scale up to 2 M ohm. On a more expensive meter the resistance appears to be 7 M ohm. Can any of you electronic experts expalin to me what is happening.
The circuit function would indicate that the resistance of water is in fact a few thousand ohms but why can this not be measured on an ohmmeter. This is important for designing the best probes for a holding tank and how far apart the two electrodes should be without doing a lot of trial and error. I am think of drilling the cap and putting through two SS bolts with rubber washers and a nut behind so that when the water level reaches the two bolt ends the circuit is made. Can any of you experts comment or help please.
The circuit function would indicate that the resistance of water is in fact a few thousand ohms but why can this not be measured on an ohmmeter. This is important for designing the best probes for a holding tank and how far apart the two electrodes should be without doing a lot of trial and error. I am think of drilling the cap and putting through two SS bolts with rubber washers and a nut behind so that when the water level reaches the two bolt ends the circuit is made. Can any of you experts comment or help please.