Why would I want to measure the Temperature of water?
Technically, temperature is a measure of the energy of the molecules comprising a substance.  For our purposes, it is how hot or how cold a water body might be.  The Centigrade scale is used most often in studies of water. 
 
Temperature affects natural waters several ways. Small changes in water temperature can affect the reproduction and growth rate of the aquatic flora and fauna. Chemically, temperature controls the speed of chemical reactions, including reactions that produce nutrients.  Physically, temperature determines the density of water.  For example, lakes “turn over” when the surface waters become cooler, and so denser, than the underlying waters.  Biologically, temperature affects the growth and reproduction of everything that lives in the water.
 
It is important to measure temperature carefully because it affects the performance of the conductivity, pH, dissolved oxygen, and specific-ion sensors.
 
Changes in long-term temperature trends can signal the need for more detailed chemical study of the water and its contamination sources.
 
How is Temperature measured?
Eureka instruments measure temperature with a thermistor - a small semiconductor whose electrical resistance varies predictably with temperature.  The instrument measures the resistance of the thermistor and converts that value to a properly scaled temperature reading.
 
The thermistor is mounted in a small, stainless-steel tube that insulates the thermistor from the thermal mass of the instrument.  That way, the thermistor responds quickly to changes in water temperature.
 
Eureka uses some of the highest tolerance interchangeable thermistors in the industry to achieve tight accuracy specifications.  In addition to choosing the best thermistors, each circuit is tested with precision resistors to minimize electronic error.  The length of the sensor was chosen to thermally isolate it from the body of the instrument, so that it quickly reaches water temperature after immersion.  Thermistors are very stable sensors.  The temperature sensor is not affected by other parameters, and no calibration or maintenance is required other than keeping the sensor free fouling.  A temperature sensor is required to use most other sensors (DO, conductivity, pH, etc.), since they often require temperature compensation for accurate results. For this reason, the Temperature sensor is standard on all Eureka multiprobes.
 
What should I know about temperature measurement in the field?
Temperature sensors will perform to specification for years, with no other maintenance other than keeping the sensor reasonably clean.