Call me old fashioned but a well-trained hand is a highly reliable source of
rad temp information!  ;)

Jake

On Thu, Mar 10, 2011 at 9:47 PM, David Beierl <dbeierl@attglobal.net> wrote:

> 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
>

--
Jake

1984 Vanagon GL 1.9 WBX - 'The Grey Van'
1986 Westy Weekender/2.5 SOHC Subie - 'Dixie'

Crescent Beach, BC

www.thebassspa.com
www.crescentbeachguitar.com
http://subyjake.googlepages.com/mydixiedarlin%27