I use these thermometers to measure and monitor my air temperatures in my system(s). They can be used for measuring air or water temperatures. They can be purchased for around $10 at Petco. They may not be 100% accurate but they do a pretty good job for the price. They can be left on all the time and the battery last a long time!
Here is an example of some data gathered using these thermometers!
Blue = Water Temp Red = Air Temp 1 Orange = Air Temp 2 Green = Air Temp 3
Observations:
March 5th - 10th you can see when our grow light ballast failed. Notice all of the air temperatures are pretty close.
March 19th -24th You can see the water temperature is very low. This is when we were mixing nutrients and water and adding it to our system. We did 3 gallons of the course of 5 days.
April 24th- You can see when we turned on a fan to help with the high temperatures. Notice the Range of temperature changed!
Overall Trend- We continued to try to get our temperatures down by making adjustments to light height and fan locations.