Put it where your plants are assuming that is what you're concerned about. And shade it somehow. Mine is about 5ft off the floor, shaded by al foil, and about 2/3 the distance from the intake, 1/3 from exhaust fans. My fruit crops are about 8.5 ft tall.

In my sunroom, about 10 ft tall, ground level is 10-20F cooler than near the roof. So hot air near the top enters the house and cool air drains out lower down. I have upper and lower thermometers.

Are you referring to a thermostat? You can take temperature readings at as many locations as you like. Plant level is ideal but as plants grow the upper air temperature is most important for controlling fans, etc. You need to place a thermostat away from doors and also out of your way- they hurt when you bang your head against them. I prefer a central location about 1/3 of the structure length from the main device being controlled and just about as high as I can reach.

If you want to use a remote thermometer to measure temperatures at plant level, at eye level and outside the structure at 10 minute intervals you could publish your findings but what really matters is that your plants don't die from frost or cook. Readings in full sun aren't reliable and without any form of air circulation within the structure you might want to be looking for a corner as a possible cold spot. However, with the crops you mention, if they are well established you will only have setbacks in growth with cold temperatures so The excessive heat would be your greatest concern. Since that can change drastically in 15 minutes once the sun shines you likely want to know when any area starts to get above 90F. Much higher you might have some losses.

Steve,

I think if you temperatures are quite different, depending on where readings are taken, are you will want to consider a circulation fan along the top of the structure. Shielding the sensors from direct sunlight is also going to help you.

Stephen

All you are interested in is what temps your plants are growing at (for thermostats, my answer would be different.) Since they are on the ground, that temperature is going to be more important than at bench level because yes, there is going to be a difference. If your foliage is up, why not just buy a shove-in soil thermometer and nest it in the foliage of the crops. You will see what they are experiencing and those are cheap and water-proof. When you have plants at all levels, then you become more concerned about temps at each level, and may have to monitor at each level and essentially they are different zones. So you place the plants you are growing in the zones they like best. I know when I was doing bedding plants certain benches got different light exposures and temps and I was able to accomodate a variety of them with different environmental requirements.

I'm starting to see that the most useful thing would be to have a system that would allow me to monitor a variety of different temperature locations. I have plants in the ground and at bench height and it would be really nice to be able to monitor soil temps for germination and thermal mass water temps as well as air temps. I have an idea to build a monitoring system using the Arduino micro controller. I'll let you all know if I get a good multi-node system working.

I have a window between my greenhouse outside and my bathroom inside - so I put my thermometer where I can
read it in the bathroom. It's much nicer that having to
put your coat and boots on to figure out the temperature in your greenhouse