PROBLEMS WITH MINIMUM THERMOMETER

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PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 13, 14:26

Any suggestions anyone.

about a week ago my minimum thermom' had a broken thread and hence i have been using a spare since.

When I tried to 're-set' the original, it was slightly awkward and after several attempts the thread would keep on separating once it got back out to approx 20c
BUT finally i sorted it and the mercury thread is complete - a quick dip in the freezer confirmed this.

So two night ago i put it back out and it apperas to be reading about 1.0c lower than it should - two checks this p.m. show it about 1c below the dry bulb - but the thread is still intact.

tonight i will have both min' thermoms' in the screen to compare.

But anyone have any ideas/help.
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
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Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Martin Rowley » 2012 Sep 13, 20:40

... not sure if this is relevant, but from the Observer's Handbook ...

" If, after being set, the thermometer reads consistently low, the fault is probably due to spirit condensed on the walls of the tube beyond the end of the column. It can usually be cured by carefully warming the upper end of the thermometer, thus causing the condensed spirit to evaporate off the walls and recondense on the spirit column."

Martin.
Martin Rowley
West Moors, East Dorset [ altitude: 17 m/56' ]
NGR SU 082 023: postcode BH22 0GB
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Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 14, 08:46

Martin - thanks and have now just looked at my observers handbook.

I presume they mean the bulb end when they say upper end?

in a few mins I will know how they both read in the screen last night.
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
Darren Rogers
 
Posts: 145
Joined: 2011 Nov 27, 20:22
Location: Cumbria - half way between Shap and Appleby in the Eden valley

Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Martin Rowley » 2012 Sep 14, 09:17

Darren Rogers wrote:Martin - thanks and have now just looked at my observers handbook.

I presume they mean the bulb end when they say upper end?

in a few mins I will know how they both read in the screen last night.


... You've got alcohol filling the bulb and extending up (or 'along' perhaps more correctly if the thermometer is set quasi-horizontally) the tube - which rises/falls as the temperature rises/falls (dragging the index downwards when it falls) - with an alcohol-fumed 'free space' above the top of that column. The problem occurs when condensates of the alcohol collect at the 'top' end of the thermometer (i.e. the end away from the bulb); if left exposed - that end of the thermometer should be covered with some sort of black shield ... see page 124 of the Handbook (1969 edition).

Martin.
Martin Rowley
West Moors, East Dorset [ altitude: 17 m/56' ]
NGR SU 082 023: postcode BH22 0GB
50deg49min15secN, 01deg53min03secW
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Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 14, 14:09

Well in short - Russell scientific have recommended the deep freeze.

It was still doing it and following the handbook, tipping it vertical actually brought some spirit out of the safety chamber - you would never have known it was there, but i can still see some bubbles in the chamber.

further tipping and introduction to heat have not done the job - so the deep freeze it is - hope that -22c is enough as the themom' goes down to -35c
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
Darren Rogers
 
Posts: 145
Joined: 2011 Nov 27, 20:22
Location: Cumbria - half way between Shap and Appleby in the Eden valley

Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Stephen Burt » 2012 Sep 14, 16:10

I certainly wouldn't recommend putting it in the freezer - that'll just amplify any condensation problems.

Martin is right - gentle warming, taking the spirit column right up to the safety chamber (don't worry - it won't pop, or not until upwards of 60 °C at least!) and then let it cool down gradually. That'll pick up any condensation on the sides of the capillary tube as the spirit rises, leaving the safety chamber the only place for condensation. Then as it cools, the column should rejoin nicely, and after settling it should read the correct temperature once more. Very occasionally this process needs to be done more than once, but it's never failed me in 40 years.

(The Weather Observer's Handbook, page 226 ... !)

SB
-----
Stratfield Mortimer, Berkshire - central southern England
51.4°N, 1.0°W, 60 m AMSL, station grade A - AAAA47R
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Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 14, 22:18

Thanks for the replies.

the gentle cooling was what I did at first, several times and it was still about 0.7c below the DB

have tried the gentle cooling again tonight and there are still plenty of bubbles in the safety chamber, although as before the main thread is intact.

Have left it sitting vertical for tonight, but is starting to look a little bleak.
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
Darren Rogers
 
Posts: 145
Joined: 2011 Nov 27, 20:22
Location: Cumbria - half way between Shap and Appleby in the Eden valley

Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 16, 20:09

When, regardless of multiple efforts to remove the bubbles from the safety chamber, they are still there - is that the time to announce RIP?
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
Darren Rogers
 
Posts: 145
Joined: 2011 Nov 27, 20:22
Location: Cumbria - half way between Shap and Appleby in the Eden valley

Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Martin Rowley » 2012 Sep 16, 20:18

Darren Rogers wrote:When, regardless of multiple efforts to remove the bubbles from the safety chamber, they are still there - is that the time to announce RIP?


... have you tried the *warming* method several times? As Stephen says, I've not experienced any problem with these if that procedure is followed - but it can require several applications (but gentle). If you have, and still no joy, I'd put it aside for a while in an environment with a small temperature range ( a week or so ) then have another go .... then and only then if it still refuses to behave it would have to be shown the door.

Martin.
Martin Rowley
West Moors, East Dorset [ altitude: 17 m/56' ]
NGR SU 082 023: postcode BH22 0GB
50deg49min15secN, 01deg53min03secW
Martin Rowley
 
Posts: 154
Joined: 2011 Nov 25, 12:54

Re: PROBLEMS WITH MINIMUM THERMOMETER

Postby Darren Rogers » 2012 Sep 18, 19:21

Martin - have tried those thing, but will do as you suggest - thanks
Darren Rogers
Maulds Meaburn
Cumbria (Half way between Shap and Appleby)

http://www.mauldsmeaburnweather.co.uk
Darren Rogers
 
Posts: 145
Joined: 2011 Nov 27, 20:22
Location: Cumbria - half way between Shap and Appleby in the Eden valley


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