John is right, just about any sensor you find should be able to work with the Mayfly, though you might have to add some interface circuitry or adapters depending on the power requirements and communication protocol of the sensor. I’ve got Mayfly board working with digital output sensors (either TTL, RS232, or SDI-12 outputs) and analog output sensors, and I’ll be posting code examples soon. It’s also possible to use 4-20ma output sensors if you have an interface, but we don’t currently use them in any of our active projects so maybe someone else can chime in with their experience with using 4-20ma sensors.
As for the types of sensors, we normally use research-grade high-quality sensors for turbidity, conductivity, pH, dissolved oxygen, etc. While the cost is relatively high compared to the low cost nature of the Mayfly, we prefer them because of their precision and dependability. We are also experimenting with different low cost sensors for measuring those parameters and hope to have some results to share later this summer.