[topbobster]

I have a Polytunnel and this is the 1st winter. I have temperature sensors in place inside and out and have found something a little strange over the last month or so. The night time temps inside have been consistently a degree or 2 colder than outside. I have attached a graph to show the results. Any ideas why this might be? (Blue is inside, orange is outside)
Regards, Rob

[Snoop]

Might the sensors be inaccurate? Could be both sets or just one.

[BabbyAnn]

Might the sensors be inaccurate? Could be both sets or just one.

I might agree with that - I bought another sensor for a comparison project and noticed that the new one was about 2oC warmer than the other when both were in the same room.

[topbobster]

Might the sensors be inaccurate? Could be both sets or just one.

I might agree with that - I bought another sensor for a comparison project and noticed that the new one was about 2oC warmer than the other when both were in the same room.

Fair point. However, I hope not seeing how much I paid for the Logger and related extra sensors (Not solely for this purpose): http://www.metcheck.co.uk/acatalog/KLPRO.html

I will swap them over and see if I see corresponding changes or still the same difference.

Thanks for pointing out what might be the obvious. Will let you know what I find.

Cheers, Rob

[Snoop]

At that price, I can see why you expect accuracy.

Does seem odd that outside should be warmer than inside. Maybe it's to do with the position of the sensors. Might they be influenced by something - a thermal mass outside that has a very local warming effect, a metal pole inside?

Good luck, in any case.

[Headgardener22]

Where are the sensors relative to each other?

I wondered whether the tunnel was in partial shade and the outside one was not.

Alternatively the accuracy is only +/- 1C so actually being 2C different is within the allowable error. I'm thinking of building a system of my own based upon the Raspberry Pi and I've noticed that there's a wide range of different sensors available with significantly different temperature ranges and accuracy.

Also the early temperatures are outside the range of the sensors (lower than 0C).

[Trikidiki]

I have been doing some temperature logging under clear and black polythene. There are details on my Diary page:  http://chat.allotment-garden.org/index.php?topic=122319.0

I built the logger using an Arduino, an SD card shield and five DS18B20 sensors, also added a Bluetooth interface for an instant read out.

If you would like details send me a PM.

[Dr_Pepper]

I seem to remember a lecture at university about this one - it's all to do with physics and radiation:

On a clear night it is possible that the temperature in a closed glasshouse or tunnel can fall below the ambient outside temp because there is no influence of wind which moves and mixes warmer and cooler air masses around the surface layer outside. In the sealed (ish) glasshouse/polytunnel you just get the energy moving from the enclosed space out to the cold atmosphere (radiation). The polytunnel cools to the temperature that would be present if there was no wind.

Google it if you fancy reading some science papers about it