Thermometer Placement Tips for Small Greenhouse

PaulTandberg · May 23, 2018

I have some plants in a small 10X10 greenhouse (translucent cover spread over a tube frame). I place a wireless thermometer sensor inside. Should I take care that the sensor is in a shaded off the ground location (as I would if using one for accurate readings outdoors) or do I place it next to the tomatoes exposed to the same diffused sunlight the tomatoes are exposed to? Air temperature or leaf temperature?

When I place the sensor in a shaded (by box) location, I get a reading 4-8 degrees cooler than if sunlight is allowed to hit it.

bower · May 23, 2018

Paul, you must have a better thermometer than I do. When sunlight hits mine, it goes off the charts. So for that reason I always do position it in shade.
I do have it off the ground too, usually. When you move it around, you discover how much the temp varies in different parts of the greenhouse. Coldest is next to the ground (or the windows/vents!). For the sake of keeping daily records, I just try to be consistent and keep it in one place all season, knowing that there is a difference between shade and sunlight and also at different heights, it gives me a consistent measure of relative conditions.
The tomatoes are pretty reliable in response to excess heat, by dropping their blossoms I assume at or above about 95 F. I know if my shade temperature goes over 85 F they may start to wilt and show signs of stress, and if it reads over 90 F for very long, there will likely be losses.

Oh, I will mention I once had a fancy thermometer that remembered day max and night minimum for you. One day in full sun in the greenhouse, it was toast.

PaulTandberg · May 24, 2018

I put up something like one of these things https://express.google.com/product/1...m_campaign=gsx

I wonder if the opaque fabric allows less radiant heat than glass? Regardless, I will create a shaded, off the ground location, one that won't trap or hold heated air.

bower · May 24, 2018

Yes, semi-opaque fabric and even plastic will diffuse the light in a way that plants love - and thermometers too, no doubt.

Glass is brutal when it comes to heat and light. Mine is double glazed and I don't seem to get the temperature inversion effect on still clear nights which can make your greenhouse colder than outdoors.. that's about the only advantage though.
The fabric type in your link probably has some insulative value too, nice choice.

In your case, I wouldn't worry about thermometer placement. If you want the highest temperature around leaf surfaces, you could hang the sensor above them where air temp will be max, and get a reasonably true reading that isn't as high as thermometer goes.

Gardeneer · May 25, 2018

Definitely, thermometer should not be hit by direct sun or close to something that generates heat.
Additionally it should be where the air moves around it. Otherwise you will get false reading.
I, myself, have a remote temperature sensor, place on the outside wall facing NNE that never sees the sun.
Even then on hot day with no breeze I get higher readings.

Worth1 · May 25, 2018

Just stick the probe in a cardboard tube or pipe of some sort to keep the IR light rays off of it.

May 23, 2018	#1
PaulTandberg Tomatovillian™ Join Date: Feb 2018 Location: North Dakota Posts: 77	Thermometer Placement Tips for Small Greenhouse I have some plants in a small 10X10 greenhouse (translucent cover spread over a tube frame). I place a wireless thermometer sensor inside. Should I take care that the sensor is in a shaded off the ground location (as I would if using one for accurate readings outdoors) or do I place it next to the tomatoes exposed to the same diffused sunlight the tomatoes are exposed to? Air temperature or leaf temperature? When I place the sensor in a shaded (by box) location, I get a reading 4-8 degrees cooler than if sunlight is allowed to hit it. Last edited by PaulTandberg; May 24, 2018 at 12:31 AM.

May 23, 2018	#2
bower Tomatovillian™ Join Date: Feb 2012 Location: Newfoundland, Canada Posts: 6,794	Paul, you must have a better thermometer than I do. When sunlight hits mine, it goes off the charts. So for that reason I always do position it in shade. I do have it off the ground too, usually. When you move it around, you discover how much the temp varies in different parts of the greenhouse. Coldest is next to the ground (or the windows/vents!). For the sake of keeping daily records, I just try to be consistent and keep it in one place all season, knowing that there is a difference between shade and sunlight and also at different heights, it gives me a consistent measure of relative conditions. The tomatoes are pretty reliable in response to excess heat, by dropping their blossoms I assume at or above about 95 F. I know if my shade temperature goes over 85 F they may start to wilt and show signs of stress, and if it reads over 90 F for very long, there will likely be losses. Oh, I will mention I once had a fancy thermometer that remembered day max and night minimum for you. One day in full sun in the greenhouse, it was toast. Last edited by bower; May 23, 2018 at 07:27 PM. Reason: add

May 24, 2018	#4
bower Tomatovillian™ Join Date: Feb 2012 Location: Newfoundland, Canada Posts: 6,794	Yes, semi-opaque fabric and even plastic will diffuse the light in a way that plants love - and thermometers too, no doubt. Glass is brutal when it comes to heat and light. Mine is double glazed and I don't seem to get the temperature inversion effect on still clear nights which can make your greenhouse colder than outdoors.. that's about the only advantage though. The fabric type in your link probably has some insulative value too, nice choice. In your case, I wouldn't worry about thermometer placement. If you want the highest temperature around leaf surfaces, you could hang the sensor above them where air temp will be max, and get a reasonably true reading that isn't as high as thermometer goes. Last edited by bower; May 24, 2018 at 08:59 AM.

May 25, 2018	#5
Gardeneer Tomatovillian™ Join Date: Feb 2015 Location: NC - zone 8a - heat zone 7 Posts: 4,919	Definitely, thermometer should not be hit by direct sun or close to something that generates heat. Additionally it should be where the air moves around it. Otherwise you will get false reading. I, myself, have a remote temperature sensor, place on the outside wall facing NNE that never sees the sun. Even then on hot day with no breeze I get higher readings. __________________ Gardeneer Happy Gardening ! Last edited by Gardeneer; May 25, 2018 at 10:30 PM.

May 25, 2018	#6
Worth1 Tomatovillian™ Join Date: Feb 2006 Location: Den of Drunken Fools Posts: 38,539	Just stick the probe in a cardboard tube or pipe of some sort to keep the IR light rays off of it.