Electronic air frost

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Len W

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Dec 7, 2020, 11:23:00 AM12/7/20
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Min. temperature measurements in my Stevenson screen last night:

Standard sheathed alcohol in glass thermometer 0.0°C
Electronic thermometer -0.3°C

What to believe and what to think?

As my 36-yr record relates only to conventional manual readings,
no air frost will be logged.

Len
Wembury, SW Devon, 83 m asl


jack.h...@gmail.com

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Dec 7, 2020, 11:53:20 AM12/7/20
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I have had several electronic thermometers over the years and while they might give a reading to a precision of 0.1C they don't give an accuracy of better than +/- O.5C. 
So your alcohol in glass is likely to be the more accurate.

I always check any new electronic thermometer by using ice in water (previously boiled to make as realistic as possible to distilled).  Stirring the water/ice mix should eventually give 0.0C.  I then note the error on the electronic probe (stirred but not shaken in the water/ice mix) and apply as a rough guide that correction throughout the temperature range.

Jack

Graham Easterling

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Dec 8, 2020, 4:34:04 AM12/8/20
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I think it's more a case of consistency of equipment than right or wrong. That's why at the end of this year I will have a 30 year record in Penzance, using the same equipment. In the case of temperature sheathed thermometers in a wooden screen. I've not even changed to a plastic screen, I can't believe it behaves in exactly the same way as a wooden one (which I paint each year). 

I've calibrated my receiver so the electronic thermometer in the screen (I've got a couple of others around the garden, the front garden is frequently colder in winter, it collects cold air when it's still.) so it agrees with the sheathed thermometers under stable conditions. Under rapidly changing conditions they are frequently 0.3C or so different.

When I see these very precise (as opposed to very accurate) figures quoted for global warming (of which there is no doubt)  I do wonder how they can claim to be so accurate considering all the instrument & location changes. As for a traditional rain gauge & a TBR!

I help out with the COL returns sometimes, and I get some very precise figures, sometimes to 3 decimal points for mean temperature, 2 decimal points for mean wind speed.

Graham
Penzance - Glorious start - no ground frost this morning, minimum 1.8C late yesterday evening, lowest of the autumn/winter so far. Overnight shower dropped 0.3mm

Brian Wakem

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Dec 8, 2020, 5:15:17 AM12/8/20
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On Monday, 7 December 2020 at 16:53:20 UTC jack.h...@gmail.com wrote:
I always check any new electronic thermometer by using ice in water (previously boiled to make as realistic as possible to distilled). 


I hate to break it to you but boiling it will make it less pure as you will lose some water but none of the impurities.  Distilling is a very different process where you capture and condense the water vapour, leaving the impurities in the original vessel.

-- 
Brian Wakem
Lower Bourne, Farnham, Surrey
Live obs @ 10:14:08 : 0.6C, DP -0.1C, RH 95%, 0.0 mm


 

Tudor Hughes

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Dec 8, 2020, 11:21:42 AM12/8/20
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It is only soluble substances that will lower the freezing point.  The soluble impurities in tap water are at a very low concentration (mainly calcium bicarbonate) so it is hardly worthwhile trying to remove them.  Boiling the water will nevertheless remove calcium bicarbonate by decomposing it into calcium carbonate, which is deposited, and CO2 which the boiling removes.

Tudor Hughes, Warlingham, NE Surrey

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