From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: IR Thermometer Date: Tue, 09 Mar 2004 00:53:40 -0500 Message-ID: <vemq405ib7qoqnvbuq835ve3i5mhu9rknm@4ax.com> The pyrometer sensor generates an output proportional to the difference in its substrate (background, reference junction) temperature and the temperature on the microthermopiles inside. The sensor is therefore temperature compensated by means of some sort of sensor, usually a diode, mounted near or in contact with the pyrometer sensor. It takes awhile for the change in ambient temperature to filter through to the diode sensor while the thermopiles sence the change in radiation immediately. Only when the two are again equal will the compensation be correct. Fancy ones like my Wahl have a "zero" function in which one half-pulls the trigger which raises a black target in front of the pyrometer sensor. The processor logs that as "zero". A complete pull on the trigger moves the flag past the sensor and opens the optical path from the infrared telescope, enabling a reading. Even on the fancy one, a zero'ing is necessary every few minutes after a big ambient temperature change, until the instrument temperature equilibrates with the new ambient. On Mon, 08 Mar 2004 23:22:21 -0500, bill horne <redydog@rye.net> wrote: >Peter Pan wrote: > >> How far away from the bulbs are you? To be PC we should probably standardize >> on distance. I'm playing with mine on the lights here, and the closer I get, >> the more the temp goes up. > >About 2-3 inches. > >And I ran into the abrupt temperature change problem last night. I went >out to try to repeat the previous nights results, and I got >not-believable results from van body, tires, and woods - off by 20-30 >degrees. So I left it outside for about 30 minutes, and tried again - >back within 2-3 degrees of another thermometer. > >The manual apparently means what it says - that the device itself must >be cooled or warmed to near-ambient temp before it'll read correctly. >And no, I don't know what "near" means yet - but I know that going from >a 75 deg house to a 40 deg yard is way too much for it to handle. > >A quick web crawl indicates that this true of even more expensive ones - >until they get expensive enough to have some form of ambient temp >compensation. >
From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: IR Thermometer Date: Tue, 09 Mar 2004 12:52:01 -0500 Message-ID: <010s40d3o3tr3o6gv6k4eh5v9me3aushng@4ax.com> There ya go, Bill, twisting my arm and making me take one apart... Actually I'd been looking for an excuse. :-) Since a picture is worth a thousand words:
A shot of the internals. First thing I notice is the Raytek logo. This is super very good stuff! Raytek and Wahl are the Big Two in the IR thermometry field. No wonder the thing works so well. To the right is the pyrometer attached to an isothermal mass. This mass does two things: It slows the change in temperature the pyrometer sees and it provides the pyrometer a nice even thermal background. Note that the pyrometer is a 4 lead device. More on that later.
Here is the pyrometer removed from the isothermal block. Looks like a zinc selenide unit. I paid several hundred $$$ for a bare sensor perhaps 5 years ago. Yeah for Moore's Law!!
Back side of the PCB showing several switches and connectors. The LCD interface is to note. IT connects to the LCD display via a conductive rubber strip. If your thermometer starts dropping segments or gets dim, take the unit apart, clean these contacts with PGA, dry and reassemble. Do not touch the conductive rubber unless absolutely necessary. I've never had much luck getting the thing to conduct after molesting one.
This is one of the most important feature and what sets this unit apart from the other cheap ones. An infrared Fresnel lens that focuses the field onto the pyrometer. Most of the cheap units I've seen have no lens and simply expose the pyrometer sensor to the world. With no focus, the sensor responds to everything in the field of view. This lens narrows the field of view and screens off-axis IR radiation. Now to your question, Bill. The 4 leads emitting from the pyrometer device indicates one of two things to me. I'm not sure which, as I haven't kept up with this field in a couple of years. One possibility is that the temperature compensation sensor is built in the pyrometer package. The other possibility is that the sensor can be electronically "blinded" so that it can read its own temperature. In either event, the actual pyrometer package is where the reference temperature is measured. The package is contained within that isothermal metal block. Ergo, the whole block must reach the new ambient before the unit will indicate accurately again. Swinging the unit hole forward would indeed help, as the isothermal block extends to the face of the unit. Drilling some small vent holes in the case near the sensor would also help, at the loss of some amount of water resistance. Seeing how well this thing is designed I think I'm going to go snarf up the remaining stock at my local Rat Shack :-) John On Tue, 09 Mar 2004 01:38:57 -0500, bill horne <redydog@rye.net> wrote: >Neon John wrote: >> >> The pyrometer sensor generates an output proportional to the difference in its >> substrate (background, reference junction) temperature and the temperature on >> the microthermopiles inside. The sensor is therefore temperature compensated >> by means of some sort of sensor, usually a diode, mounted near or in contact >> with the pyrometer sensor. It takes awhile for the change in ambient >> temperature to filter through to the diode sensor while the thermopiles sence >> the change in radiation immediately. Only when the two are again equal will >> the compensation be correct. >> >> --- >> John De Armond > >Does that mean that swinging the thing eyehole forward through the air >might make it ready sooner?
From: John De Armond Newsgroups: rec.outdoors.rv-travel Subject: Re: IR Thermometer Date: Wed, 10 Mar 2004 01:03:17 -0500 Message-ID: <uiat40p6treb5gfbmbacoaa0c5fgv3hf5g@4ax.com> I can't imagine any universal formula. Assuming the instrument had equilibrated to the van's temperature, at the instant you step outside, you should be able to just add or subtract (depending on which way the temperature is inside vs out) the difference between inside and out. If this works it would only work for a few seconds, as the pyrometer chip immediately starts cooling or heating as the case may be via IR radiation. This problem exists even on my fancy unit. If I aim it at something fairly hot, the IR radiation will heat up the sensor more rapidly than the compensation can work. With an open shutter, this constitutes itself as long term (minutes) drift. Similarly, if I take it outside into the cold, the sensor cools by radiation faster than the heat can be pulled from the compensation sensor. It drifts down. The only solution I've found is to half-trigger and zero immediately before each reading. This thing has a 1mv/deg analog output but the drift renders it useless for long term data logging. My unit is >10 years old so maybe they've improved things in later models. For a really fancy one, check this one out:
This is a hand-held reflector telescope with a pyrometer sensor mounted in the focus. Also a Raytek product. The rifle scope aims coaxially with the telescope via a periscope arrangement. No parallax error. As long as the target can fill the little square in the cross-hairs, the instrument can read the temperature, regardless of the distance. One can measure an individual wire clamp on an overhead transmission line, even on the next block! This instrument addresses the problem you're having the old fashioned way - it reads in degrees above ambient and has a little mercury thermometer to show ambient temperature. One does the math in his head :-) BTW, you ought to see the looks I get as I walk along a street looking at the overhead lines while holding the raygun out and up like a rifle... John On Tue, 09 Mar 2004 16:19:45 -0500, bill horne <redydog@rye.net> wrote: >Thanks, John. I knew you'd come through. And before you lose your >momentum, here's one more: > >Is there a table, chart, graph, formula or something for this thing, so >that I could adjust it on the fly by adding or subtracting a number >based on the difference between ambient and phaser temp? > >For example, say that I know the the temp of the phaser because it's >been riding in the van with me, and I know the temp in the van. I either >know - or can measure with a regular thermometer - the outside temp. So >I'm busting down the road and decide to stop and check my tires - and I >don't want to wait 30 minutes. it's 75 in the van, and 35 outside. I >stop, jump out, zap a tire, and get a not-believable reading. Then I >apply the magic fudge factor to the reading, and get a much more >accurate reading. Possible? Oh. Sorry. That makes it two more questions. > >Kind of ironic that this thing can measure a temp in half a second, but >you might have to wait 30 minutes for it to get its ducks lined up if >you don't plan ahead.