Callibrating Thermometers

DOCARCH

This has been bugging me for quite a while now! I would appreciate your advice as I know some of you out there have quite professional setups.

I have three thermometers all within a couple of inches of each other - and all approx. 1.2m off the ground in the shade.

Now (at 12.20) they are reading the following:

1 The (cheap) mercury max/min. - currently redaing +7.0c
2 The Oregon Scietific remote senosr - currently redaing +6.5c
3 The LaCrosse WS2500 - currently redaing +7.4c

When temperatures are low the LaCrosse and the Oregan tend to roughly coincide - during daylight hours the LaCrosse tends to be approx. 0.5c above the Oregan.

I know that I have hit on the problem or answer there by that statement but the LaCrosse thermoeter is solar powered - does not need direct sunlight to power but needs daylight (so a screen semms to be out).

Any experiences of this problem or advice on how one should go about callibrating these or establishing which is the more accurate would be very useful Thanks

Snowbie

I have a similar local problem to. The dry bulb mercury thermo is probably the more accurate as is used to measure the temp at synoptic stations.
I find that on the clearest and stillness of night warmer air is trapped in my AWS chamber(as i dont have the 24hr fan) and my screened dry bulb and unscreened temp probe will coincide and will be generally lower than AWS.If a breeze is blowing well all three will coincide.AWS is more accurate during the day and has a constant wind is drawn up through the chamber followed by the screened drybulb which be a little higher etc.

Its also to do with placement of sensors around location.There would be slight difference in temp at different heights and distance,over concrete and grass even in your back garden or across a field.Air whether it be cold or mild is constantly moving but sometimes can pool. I have mine placed within a meter of each other so to eliminate some of the possibilites in the above.