Be aware that there is a difference between a visibly black object and a true blackbody (an object with an emissivity of 1.0 at all wavelengths.) Normal IR thermometers assume an emissivity of 0.90 or 0.95 or such; high quality IR thermometers allow you to dial in an emissivity. Of course you need to know what value to dial in for that to work, but it does help. Normal IR thermometers are most useful at finding relative temperatures more than anything else. And require calibration against the material under consideration for "real" applications.
Donald.
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----- "James Bishop" <
bishopaj@...> wrote:
> From: "James Bishop" <bishopaj@...>
> To: "Homebrew PCBs" <Homebrew_PCBs@yahoogroups.com>
> Sent: Thursday, July 8, 2010 11:40:09 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [Homebrew_PCBs] interesting
>
> I have an IR thermometer and experience the same thing.
>
> I believe this is related to the concept of 'black body radiation'. The IR
> thermometer is calibrated to measure the radiation of a pure black object
> (absorbs light). If it's not pure black, then it will give a false reading.
>
> That's my vague understanding, try googling 'black body radiation'.
>
> If you use a thermocouple you won't have this problem! Or you can try
> putting a black object on the hotplate, and measuring the temp of
> that.
>
> On Fri, Jul 9, 2010 at 12:56 PM, rlspell2000
> <rls@...>wrote:
>
> >
> >
> > Bought a GOOD lab hot plate to do solder reflow. Half inch thick aluminum
> > top, temp setting dial.
> >
> > When I first fired it up I very carefully found the settings on the dial
> > that corresponded to 160C and 230C, the two temps I was interested in, for
> > Toner Transfer, and for solder paste reflow.
> >
> > The IR thermometer said the dial was WAY off. By a factor of like 1.5.
> > But this is OK, I just marked the spots on the dial and went about my
> > business.
> >
> > Just did some toner transfer with the plate on the bottom and the normal
> > iron on the top. I.E, the board and paper sandwiched between the hot plate
> > and the iron. Set the hot plate for where the IR thermometer had found the
> > setting for 160C
> >
> > After being on the hot plate for like 15 seconds the paper started to
> > turn brown and smoke.
> >
> > WTF?
> >
> > I took the temp again. Yep, says right at 160C. But paper doesn't burn
> > at that low a temp... ????
> >
> > I turned it down to the setting on the dial for 160, and tried again.
> >
> > I after it had cooled off I put a piece of paper on top of the hot plate
> > and took ∗it's∗ temperature. Just about exactly 160C.
> >
> > After cleaning the board up and scraping off the burnt paper I ran it
> > through again and it worked fine a the dial setting of 160C.
> >
> > Odd, no? IR non-emission coating on the plate or something?
> >
> >