What type of sensing element is used in Water Quality Monitoring meters such as those pictured below? I might not be using the correct search terms.
Some of these meters monitor Total Dissolved Solids (TDS) or pH levels in water.
I suspect there is an electro-chemical sensor. I have not been able to locate any information on the sensing element or the mechanism. I intend to build a simple water quality meter using a simple microprocessor and respective sensing element.
Hand-held sensors measuring multiple properties may require more than one type of sensor element; it's important to know what exactly you need to measure. There is no one property (or fixed set of properties) that defines "water quality" though some (including the two you mention, pH and TDS) are more commonly used than others.
In the case of pH and TDS, either can be determined by passing a current through the sample and measuring the voltage of the circuit. Rather than using a specialized sensor, the unit may simply use a battery with some fixed electrodes (so the distance traveled through the sample is known); your microprocessor board can handle the voltage measurement on its own. This water quality monitoring text gives the following expectation for commercial devices:
Pocket-sized, battery-powered, portable meters that give readings with an accuracy of ± 0.05 pH units are suitable for field use.
This should be achievable with a "DIY" device, though calibration may be tricky.
The source above also points to an electrode-based method as being acceptable for dissolved oxygen (DO) measurements, though you haven't mentioned that as one of your target measurements. The electrodes required may work for pH as well, or you may require separate circuits; this probe measures pH and electrical conductivity (EC) with what looks like a single electrode. It's used in this device, which also measures TDS, and I'm guessing it does so by using the EC measurement.
Your best option may be to find a pet store that specializes in aquarium supplies; they sell many different affordable sensor devices for home monitoring. You should be able to find manufacturer contact information if none of those devices can be modified, cannibalized or reverse-engineered to meet your needs.
You should expect that TDS in the sample may affect pH readings, and vice versa, since you'll be measuring them in nearly the same way (conductivity and voltage).
Related Reading
I'm not sure if you've already come across these in your research since you didn't elaborate in the question, but readers may get some useful information from the following sources:
