Date: Fri, 11 Mar 2011 11:25:59 -0800
Reply-To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject: Re: IR Temp guns - use with caution
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good tip on the masking tape ..
or just paint flat black spots here and there ?
mainly I read the hoses ..
those should be fairly useful readings.
I have two temp guns ....they can differ by 20 degrees ..
so I use IR temp gun readings more as a 'relative' measurement than an
absolute one.
that's usually enough to know if things are making sense or not.
and it's always reassuring to note that the return hose from the radiator is
significantly cooler than the one going to the rad from the engine ..
that's all we really need often ....just relative readings.
----- Original Message -----
From: "David Beierl" <dbeierl@ATTGLOBAL.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, March 10, 2011 9:47 PM
Subject: IR Temp guns - use with caution
> At 11:20 PM 3/10/2011, Anthony Egeln wrote:
>>the wire. Still pegging max high with a steady flashing light, but
>>the thermal laser gun reveals a normal temp system.
>
> IR temp guns are not the be-all and end-all of temperature
> measurement. In order to obtain an accurate reading you have to take
> into account the field of view of the instrument, of course - the
> fancier ones will have a laser grid that directly shows the area
> measured, and the device being measured must fill this area. If it
> does not the reading will be low.
>
> In addition, however, is the factor of emissivity, the amount of
> infrared that a given substance will emit at a given wavelength for a
> given temperature. On expensive instruments you set the emissivity
> on the instrument, but cheap ones ordinarily assume an emissivity of
> 95% of what a theoretical black-body radiator would emit. This is a
> good average choice, but it fails on some materials we find
> interesting. Aluminum is one of them. Its emissivity under various
> conditions ranges from 0.02 to about 0.4. IR instruments assuming
> .95 emissivity will therefor read somewhere between moderately and
> ridiculously low.
>
> Polished metals in general have low emissivity and will read rather
> low on our cheap IR thermometers. I just checked a stainless-steel
> pot of boiling water - the shiny s/s lid of the pot measured about
> 105-108F unless there was a layer of condensed steam on it, in which
> case it went up to 125F or so. Rolling a kitchen thermometer across
> it demonstrated that the temperature was in fact above 200F. On
> substituting a sheet of aluminum foil for the pot lid I obtained a
> reading of 82F.
>
> Our aluminum engines aren't polished, and they typically are coated
> with oily dirt and such that has a higher emissivity than the metal
> itself, and that can be a saving grace; but their readings cannot be
> accepted uncritically.
>
> ONE WAY TO GET A MUCH MORE ACCURATE READING WOULD BE TO COVER THE
> MEASUREMENT AREA WITH CLOSELY ADHERED MASKING TAPE.
>
> Yours,
> David
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