Date:         Fri, 11 Mar 2011 00:45:28 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: ECU & Coolant Temp Sensor
Comments: To: Anthony Egeln <regnsuzanne@YAHOO.COM>
In-Reply-To:  <985892.64328.qm@web39404.mail.mud.yahoo.com>
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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