A glass thermometer is an important part of your water sampling kit. Once the sample is collected, immediately immerse the thermometer into the sample for 30-60 seconds and read the temperature directly from the thermometer (match the top of the red line to the nearest 0.5 degree). This ensures that the temperature reading reflects the temperature of the sample site, not the air temperature. The temperature of the sample will start to change as soon as it is collected, so make sure to get the temperature reading as quickly as possible. Bring the thermometer to eye level to get the most accurate reading possible. If you read the thermometer at an angle, the scale will look different; this is called parallax error.
Thermometers are fragile and should be handled gently. Because they can break, it is preferable to use thermometers filled with alcohol or kerosene (usually dyed red), rather than mercury (silver liquid metal), which is highly toxic.