On Sat, Jan 30, 2010 at 9:52 AM, Mark Watson <watsonm05@comcast.net> wrote:
> Dear All,
>
> Wow. Think about Tony's original statement "which I use around the shop
> checking AC in and out temps, motor temps, etc . ." and look at the chart
> Randall pointed to.
>
> So when a hot rodder (not me) points his IR thermometer at his polished
> aluminum radiator or polished aluminum thermostat housing he's certainly not
> getting anything like a meaningful measurement is he? "Aluminum Highly
> Polished" has an emissivity of about .05 versus what Randall claims of
> around .95!
>
> No. When I'm worried about that sort of thing, I stick a piece of
electrical tape on the target, and measure that.
>
> The other thing about IR thermometers is to remember that they don't
> really measure at a fixed size point but see a cone that increases in size
> with distance. Is there some optimal distance at which to use them?
>
> Depends on what you're measuring, and how big the hot spots are. Raytek
make a couple for automotive use that have two laser pointers, mounted with
a set amount of parallax. When they're focused on the same point, the
measured area is an inch in diamter. That's hand for measuring in a car.
> And yet another interesting and educational discussion!
>
> Mark Watson
> 1965 Ford Falcon hopefully short term repair project
> 1956 Daimler Regency Mk II '104' - in the middle of a slow restoration
> plus various other transportation pods
>
> ----- Original Message ----- From: "Randall" <tr3driver@ca.rr.com>
> To: "'Shop Talk List'" <shop-talk@autox.team.net>
> Sent: Friday, January 29, 2010 2:08 PM
>
> Subject: Re: [Shop-talk] IR temperature meter
>
>
> To put it another way, the accuracy of your meter depends greatly on the
>> emissivity of the object it's pointed at. In theory, to get an accurate
>> non-contact measurement, you must know and correct for the emissivity of
>> the
>> object of interest. Some thermometers have an adjustment for this, but
>> most
>> inexpensive ones just assume it's around .95 (which is reasonably close
>> for
>> most materials).
>>
>> http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
>>
>> But air has a really lousy emissivity and you aren't correcting for it.
>>
>> -- Randall
>>
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--
David Scheidt
dmscheidt@gmail.com
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