I found some info about these pumps, they have an internal diode so that the pump only uses half of the AC cycle. Mechanical internals is basically a solenoid that fires each time it gets a forward half wave and then closes. I had been thinking that maybe there was a tiny leakage current through the triac and SSR I tried earlier, so I tried using a DPST relay. First I switched just the hot wire, then both the hot wire and the neutral wire. The pump still hums and slowly leaks when the relay is not engaged. I checked with my multimeter and measured 0VAC across the pump when it is not engaged.

I just tried switching both hot and neutral. I was switching the hot with the triac and snubber that I used earlier this morning and switched the neutral with a known working SSR. It woks, but the pump still hums and leaks when not being activated. I had been under the impression that just switching the hot wire should work, and switching both hot and neutral should definitely work. Does anyone have any insights on this phenomenon?

I tried using a triac with snubber circuit this morning and the pump is now controlled by a 5V push button . However the pump still has an electrical hum when turned off and leaks a little when off. Should I maybe be switching both hot and neutral wires?

Just to clarify, I meant an electrical schematic

I'm having a lot of difficulty finding a schematic for the Ulka pumps. I metered across the terminals and conformed that there is indeed a internal diode. Would I be correct to think that the power to the pump is rectified, but not filtered, AC?

Thanks for your reply Chris, I have tested the pump after each attempt and it works as expected. I think I may need to buy another electromechanical relay tomorrow. To confirm the wiring is correct I tried shorting the leads on the AC side of the relay to each other and the pump works, however the humming still occurs. I think you're right in that I will need to do a lot more reading about these types of pumps.

I need to lengthen the thermistor leads, would a shielded twisted pair be the way to go?

I'm now using a RC filter with a 100K resistor and a 1uF capacitor giving a time constant of 0.1 seconds, then a voltage follower using a MCP6001 op-amp. I'm getting nice stable results. Thank you for your help @SpehroPefhany

Would something like a 741 op-amp be sufficient, or should I be looking for some specific characteristics in a op-amp?

And just to be clear, I should be using an op-amp set up as a unity gain differential amplifier, correct?

Might I need, instead of a unity gain op-amp, an op-amp set up with enough gain to fill the "window" of the total range I expect to see? Or should I just use a reference voltage on the ADC for the maximum voltage that will be produced by the voltage divider ?

Thank you. I've found a thermistor that has a resistance of 80.7K ohms at my temperature of interest. Can you tell me whether I would be better off using an instrumentation amplifier rather than an opamp as the unity gain buffer?