Thermometer Differences

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Thermometer Differences

Postby Mike Brown » 2012 Apr 26, 12:21

I am using Met office spec thermometers in a new plastic type screen. The thermometers are the sheathed type with a yellow background. My problem is that when taking the readings I notice that the level of the mercury in the maximum thermometer gives a 'current' temperature value some 0.5 deg C different to the normal dry bulb. I am aware that there may be differences perhaps due to lag with the maximum thermometer constriction. I am unsure if this is a normal type of response or if indeed there is a difference between the two thermometers. If the latter is the case which should I use as the correct version in the absence of a calibration certificate?

Any advice would be appreciated please.

Thanks

Best wishes

Mike
Mike Brown
 
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Re: Thermometer Differences

Postby Stephen Burt » 2012 Apr 26, 16:28

Not to be flippant, but there is a saying amongst metrologists (i.e. instrument scientists - not atmospheric scientists):

“The person who has only one watch knows what time it is, but the person who has two is … not sure.”

This is also a common problem with thermometers, and to be sure you need to know a reasonably accurate calibration for at least one of them. With the dry bulb, that's straighforward enough to do using an ice-water mixture to get you 0.0 °C. Clearly if the dry-bulb reads +0.5 or -0.5 in the ice-water mixture, then there's your answer.

It may be lag in the maximum thermometer, but 0.5 degC is rather a lot. Is it difficult to shake down? That may indicate a 'stiff' constriction, although you don't say whether the 0.5 degC is above or below the dry-bulb temperature. If it's above, and the dry-bulb is correctly calibrated, then the error is in the maximum thermometer.

A good way to check is to borrow the COL Tinytag calibrated logger and run it in your screen for 3-4 weeks, and compare readings. That should help you pin down which is closer to true air temperature. I think the logger is still with Ian Currie, although John Goulding was going to take it over I think. An e-mail to John would find out. There's a nominal charge - I think £10 for a month - plus postage/packing, but it will answer the 'two watches' problem.

Hope this helps.

SB
-----
Stratfield Mortimer, Berkshire - central southern England
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Re: Thermometer Differences

Postby Mike Brown » 2012 Apr 27, 12:27

The difference is negative, i.e, the maximum thermometer is always below the dry bulb. I do take you point about two watches though. I think I may well investigate using another sheathed thermometer, and another maximum thermometer I have in stock and run comparisons for a few weeks. I could run against the electronic ones but I am not really comfortable about there accuracy.

Thank you so much for your really helpful reply.

Best wishes

Mike
Mike Brown
 
Posts: 49
Joined: 2011 Dec 03, 18:04
Location: Bolton le Sands, North West Lancashire

Re: Thermometer Differences

Postby Mike Brown » 2012 May 26, 13:30

I know seem to have four watches. I have been experimenting with two additional thermometers, one ordinary the other maximum. Again both of these were to the same Met Office spec. All were made by Casella. Sadly, both Max thermometers agree as do both normal thermometers, which still leaves me with the dilemma. I have now installed an electronic sensor which records within 0.1 deg C of the normal thermometer. Once that has settled in I shall compare the values then, hopefully I won't acquire another watch.

Best wishes

Mike
Mike Brown
 
Posts: 49
Joined: 2011 Dec 03, 18:04
Location: Bolton le Sands, North West Lancashire

Re: Thermometer Differences

Postby Darren Rogers » 2012 Dec 10, 09:53

Did you ever resolve this?

reason being is that my Max' thermom' would appear to have started to read about 0.1 - 0.2c below the DB.

i can't see any bubbles or breaks in the thread, etc and upon checking, both the min' and DB thermoms tally with each other - any suggestions?
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
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