I am trying to make a wire wave gauge by measuring the resistance between two stainless steel wires. Theoretically, the output voltage reading should change the deeper the wires are in the water. If you look at the attached photo, the graph shows a nice wave when I move the gauge up and down but when I leave it stationary in the water the signal decreases very very slowly. I am hoping someone with very specific knowledge can shed some light on why this is happening. I need to have repeatability with this test and make a calibration curve of various depths and I can't really do it if there isn't a clear voltage value for each depth.
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$\begingroup$ It's tough without the time scale. But... Round up the usual suspects. Batteries wearing down? Electrodes corroding? Water changing temperature? Water out-gassing residual dissolved air? Something sliding down/up/off you have not noticed? $\endgroup$– puppetsockCommented Jul 9, 2019 at 16:42
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1$\begingroup$ It's not surprising that the resistance will vary with time over long time scales due to the factors that "puppetsock" mentioned. But the purpose of your device is to measure water waves, so you're only interested in time-varying signals with periods up to a few seconds at most. $\endgroup$– user93237Commented Jul 9, 2019 at 19:31
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$\begingroup$ So the time scale is actually very short, I was misleading there. The voltage changes instantly when I move the gauge up or down and immediately decreases significantly. I don't have exact numbers but for example: say the depth is 0 to 10 cm and the voltage ranges between 2 and 2.4 volts. I'll put the gauge all the way in and it shoots up to 2.4 and within a few seconds it's decayed down to like 2.2. We're powering it through a power supply because we thought that it might be the batteries and replaced the wires with stainless steel (they had copper in them before and were clearly bubbling). $\endgroup$– Camilla JastrzebskiCommented Jul 9, 2019 at 19:51
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$\begingroup$ Are you sure that a voltage of around 2 volts is appropriate for this? Electrolysis of water occurs at about 1.23 volts (en.wikipedia.org/wiki/Electrolysis_of_water ), and I would think that one would want to remain below that voltage for water wave measurements. In any case, I would expect that the time dependence of the resistivity depends on the voltage used for the measurements and that working at lower voltages may be better. $\endgroup$– user93237Commented Jul 10, 2019 at 0:47
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2 Answers
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To measure resistance, your multimeter injects a small amount of precisely-known current through the circuit and measures the resulting voltage, then converts this to a resistance reading. In the absence of stirring action in the water, that current flow is probably electrochemically growing a (very thin) chemical layer on the surface of one of your electrodes which affects the transport of electrons or ions through it, or disassembling the native chromium oxide that normally coats stainless steel parts when immersed in water.
Many sorts of stainless steels are available. If you are using 304, try switching to 316 instead, or another grade that is specifically intended for use in salty and slightly acidic conditions.
Try manually adjusting the range of the metering device to a higher resistance setting, as this will reduce the injection current and slow down the formation or degradation of that film. this will require you to choose a more sensitive detection range, which might degrade your signal-to-noise ratio.
Also note that in general, electrochemical cells like this are notoriously affected by the amount of stirring action, which is going to make life hard for you.
answered Jul 9, 2019 at 16:51
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$\begingroup$ It could be as simple as the electrolysis of water and the formation of (very small) gas bubbles on the electrodes? Changing the type of stainless steel might not changes the situation? $\endgroup$– FarcherCommented Jul 9, 2019 at 18:46
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$\begingroup$ best to try some experiments! $\endgroup$ Commented Jul 10, 2019 at 6:04
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Other answers are really good. I would only suggest you have two meters, one inside an enclosure with a small hole at the bottom, so water would flow in and out slowly.
Then look at the difference between the two meters.
answered Jul 9, 2019 at 19:33