Hi all,
I have one of these IR thermometers that you just point at things to find out how hot they are, great for checking for blocked rad's, binding brakes etc.
My young son noticed that blue sky is very cold but clouds are only 'a bit cold' so a question that I am hoping someone can answer.
If I point the thermometer at a cloud, is it telling me the dewpoint? It seems that it may be, the shade temp at the time was 22 and the cloud temp 13, the cloud base looked about 4000ft
Pete Diffey
infrared thermometers and dewpoint
Moderators: John Dean, Moderator
infrared thermometers and dewpoint
Peter Diffey
029340
029340
-
Trevor Harvey
- Posts: 190
- Joined: Mon Feb 04, 2008 1:20 pm
- Location: Scotland
-
Ian Melville
- Posts: 1000
- Joined: Mon Feb 11, 2008 7:21 pm
-
rans6andrew
- Posts: 63
- Joined: Fri Jul 17, 2009 3:39 pm
er..... the infra red "point and shoot" thermometer has unlimited range as it is a "receive only" device. It is like your eye in that you can see the sun easily and that is 93 million miles away. The field of view of those thermometers is a cone shape with the pointed tip at the unit and the base at whatever the device is pointed at. So, a couple of inches from the thing it "sees" a circular patch maybe an inch diameter and reads the temperature of the hottest region within that circle and, if you point it at a cloud a few miles away it "sees" a circle of cloud a mile diameter and reads the temperature of the hottest area of the circle.
You have to exercise caution when you use the devices as they can be fooled very easily. If you point it at a surface that reflects IR it will read the temperature of whatever is reflected, usually the nearest filament bulb or your face!
Good on mat finish items though, like tarnished exhaust stubs.
Rans6........
You have to exercise caution when you use the devices as they can be fooled very easily. If you point it at a surface that reflects IR it will read the temperature of whatever is reflected, usually the nearest filament bulb or your face!
Good on mat finish items though, like tarnished exhaust stubs.
Rans6........
Andrew Cattell
Rans S6 Microlight.
Rans S6 Microlight.
-
JimCrawford
- Posts: 59
- Joined: Tue Apr 20, 2010 2:23 pm
Pete,
The infra red thermometer views the scene through transparent windows in the atmospheric spectrum generally ~4.7 microns and ~10-11 microns. Pointing at the clear sky the thermometer will be viewing the temperature of space, about -270 C however the signal is so low that you will measure a mixture of the noise floor of the thermometer, which would probably be ~ -30 C, and the radiation from water vapour, CO2 and other atmospheric constituents - the window isn't that transparent. The 'blue' of the clear sky is. of course, in the visual spectrum and has no meaning as far as the infra red thermometer is concerned. Looking at clouds you will measure the temperature of the cloud base, which will be the dew point. If you know the surface level temperature you will get the approximate cloud base height from the dry aidiabat. The cloud has to fill the field of view of the thermometer otherwise you'll get a mixture of cloud and clear sky.
It gets more interesting if you set your thermometer to view at different spectral regions. For example if it is responsive at ~ 3.5 microns it will see CO2, have a range of just a few metres and give you the true air temperature. Even better if you look down from orbit, although not a likely LAA project, and look at different parts of the CO2 spectrum you can 'see' to different depths into the atmosphere and derive the vertical temperature profile.
Good spot by your son!
[ used to be the day job
]
Jim
The infra red thermometer views the scene through transparent windows in the atmospheric spectrum generally ~4.7 microns and ~10-11 microns. Pointing at the clear sky the thermometer will be viewing the temperature of space, about -270 C however the signal is so low that you will measure a mixture of the noise floor of the thermometer, which would probably be ~ -30 C, and the radiation from water vapour, CO2 and other atmospheric constituents - the window isn't that transparent. The 'blue' of the clear sky is. of course, in the visual spectrum and has no meaning as far as the infra red thermometer is concerned. Looking at clouds you will measure the temperature of the cloud base, which will be the dew point. If you know the surface level temperature you will get the approximate cloud base height from the dry aidiabat. The cloud has to fill the field of view of the thermometer otherwise you'll get a mixture of cloud and clear sky.
It gets more interesting if you set your thermometer to view at different spectral regions. For example if it is responsive at ~ 3.5 microns it will see CO2, have a range of just a few metres and give you the true air temperature. Even better if you look down from orbit, although not a likely LAA project, and look at different parts of the CO2 spectrum you can 'see' to different depths into the atmosphere and derive the vertical temperature profile.
Good spot by your son!
[ used to be the day job
]Jim
-
rans6andrew
- Posts: 63
- Joined: Fri Jul 17, 2009 3:39 pm
Hi Jim,
That's what I was thinking, I figured it as an easy way to get an approximate cloud base. These low cost thermometers are probably just as accurate as a wet/dry thermometer and easier to use. I seem to get fairly repeatable measurements with a reading spread of less than one degree allowing an estimate of cloud base better than +/- 500 ft.
This could be a useful tool when we have grey overcasts that can be difficult to judge and there is no local weather station.
Pete
That's what I was thinking, I figured it as an easy way to get an approximate cloud base. These low cost thermometers are probably just as accurate as a wet/dry thermometer and easier to use. I seem to get fairly repeatable measurements with a reading spread of less than one degree allowing an estimate of cloud base better than +/- 500 ft.
This could be a useful tool when we have grey overcasts that can be difficult to judge and there is no local weather station.
Pete
Peter Diffey
029340
029340