MIMF

My three hygrometers began to read differently from one another last week, by as as much as ten percent, telling me it was time to calibrate them. I use a dry bulb/wet bulb arrangement to arrive at the "true" RH, and calibrate the 3 units from that result, but each time I do this, I get frustrated by the lack of repeatability; even with multiple wet bulb tests, with even as little as a one degree difference in either thermometer, we're talking four to 5 percent difference in RH. So, +/- 1 degree C, can still lead to as much as 8 to 10% variation in RH. To get around this, I take multiple tests, over a period of days, and take not the average, but the most common figure, always allowing for the slight day-today variation, which I still track at the time of each test with the wall units. Tedious, to say the least....

So I searched around for other methods, and discovered "chilled mirror hygrometers". Not cheap! But reputed to be the most precise and repeatable method of measuring RH. They do this by precisely measuring the dew point.

Neat, thinks I! So I chilled a mirror, and a piece of plate glass, in my shop's fridge yesterday, and upon removing either one, they instantly fog up. Perfect! I then set them upright on a bench where there's as little air movement as possible, and watched what happened. Nothing for a minute or so, but then, as expected, as they warmed-up, the fog lifted, beginning at the edges, and rather quickly, toward the center.

Bingo! All we need to do is find a VERY accurate method of quickly measuring the temperature of the glass(or mirror) precisely at the moment the last of the fog clears. I tried the various thermometers and such that I had, and came very close to repeating the results from the wet-bulb tests. The biggest challenge is getting a reading at that precise moment. Bulb thermometers are too slow.

So, friends, I turn to the unmatched knowledge resources we have here. Even if it entails using a computer or laptop, what would be the simplest, yet most precise(within difficulty and financial reason, of course), to read the temperature of the glass surface, instantly, or even in real time? Is there a film or sensor we could fix at the very center of a piece of plate glass that could give us real time readings?

The commercial chilled mirror units use refrigerant to chill the plate, and lasers and what not to detect the dew)fog), etc...., Such that they give a constant, precise and real time RH figure, but for our use, as a simple, foolproof, and ultimately precise method of calibrating/checking our day to day, month to month hygrometers a few times per year, all we need to do is accurately measure the air temperature, and the dew point(the temperature of the plate glass at the instant that it fogs, or clears), so for us, a fridge to chill the glass, a chair to sit and watch the glass clear, and a pencil to record the temperature at the moment it clears, would suffice just fine!

What say?

IR (infrared) temperature gun?

I have a digital IR temperature gun. Picked it up from Canadian Tire. It has a laser pointer and gives an accurate instant reading. I think it's around $25. Goes on sale from time to time.

That was my first thought, but, they apparently can have an inaccuracy issue with shiny surfaces, and while it may have a pinpoint laser, that's for aiming purposes; the IR beam itself is rarely less than an inch in diameter for the inexpensive models. I'll keep an eye open for one and grab it the next time they come on sale; been meaning to do so for a while now, but I still think we want more accuracy than that, especially since we're looking to read temps to at least a tenth of a degree.

Although....!

I had an idea just about as soon as I walked into the shop a moment after writing the first post, here. I took a thin-wall beer glass, topped it with cold water and 3 ice cubes, and added my best lab thermometer. Within minutes, the glass was all sweaty, so I scooped out what was left of the ice cubes, then kept an eye on it while I went about working on other stuff. The moment it was clear, I took the first photo. I then added a couple of ice cubes back into the glass, and stirred it slowly until the glass suddenly fogged-over once again. I grabbed the camera and took photo number two. The difference appears to be less than 0.3(°C)! So, accounting for the fact that the second photo is taken moments -after-reaching the dewpoint, and because there was still ice in the glass, it was still cooling, I think it's safe to assume the dewpoint was at least 0.1(°C) higher, giving me a range of 2 tenth's of a degree Celcius for the true dewpoint. With the air temp at 21(°C), and a dewpoint of 8.3(°C), it calculates to 44.08% RH. Spot-on with all the wet bulb tests of the past week, and spot-on with all 3 hygrometers I calibrated yesterday.

Pretty damn snazzy, methinks!

It'd still be slick to have a precise surface temp reading, so let's keep thinking....

Oops! I just looked at the photos and realized the dewpoint was 7.3(°C), which gives a calculated RH of 41.17%. Though if the air temp had cooled to 20(°C)(I read the thermometer before adding it to the glass, and at least 30 minutes have passed since the furnace ran, so 20° might well be the actual air temp), that takes the RH back to 43.8%.

Obviously, I need to get another matched pair of lab thermometers, too(broke the match to this one recently)....

And here's the on-line calculator I'm using:

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

Mario,
back up a little in the thread here, to your first post...I really like your first idea.

The digital infrared temperature readers are accurate to a tenth of a degree, at least all three of mine are. You are correct in that they don't read off shiny surfaces such as metal flashing, some glass, etc. This is easily remedied by giving it something non-shiny to read off of, in your case a piece of tape stuck onto part of your mirror or glass will solve that.

As our KISS oversight committee chairman on this forum, you have a duty to practice what you preach, and your first idea sounds the most simple.

I say go back to your first idea of chilled plate glass, and a chair to sit in. once the fog clears look up at your infrared reading, and Bob's your uncle.

The only problem I see is that once you get your infrared temperature reader you aren't going to accomplish any meaningful work for three days because your going to be going around taking the temp of everything within 10 miles of you.

You could shine the IR thermometer on the back of the mirror.

I'm also wondering if there's a change the fog on the glass might be enough to allow for a good laser reading? Once the glass is clear, you're past the dew-point, right? Maybe you'll be able to get valid readings until you don't need them?

As another for another method to read the temp: Maybe CA-Glue a Thermo-Couple to the glass and use a digital thermometer?

paint a small matt black square on your mirror surface for your ir to read off. we have ir cameras in my work and if you point it at a shiney surface you see your reflection as heat on the camera you dont even need to be that close to the shiney surface and it doesnt even need to be overly shiney although they are pretty expensive bits of kit

I like how this is headed... Bob may have it simplified to the nth degree. Back of the mirror....
But you have to admit, that it doesn't get much simpler than a glass, some water, some ice cubes, and one decently accurate thermometer, now, does it? No batteries, no nuthin'... <bg>

Tell me more about these thermocouples. The only ones I know of are in my furnace and water heaters so's they know when the pilot light is lit or not.

My $35 Extech multimeter came with one. I don't know if it's good to a .1 degree but I could pull it out and see.
The $5 digi meat thermometers all read to .1º but the two I have can't agree about much.

Well, I've been reading up on IR temp guns and such.

As I suspected, they're not all too accurate. Don't confuse resolution with accuracy. While it may have a resolution of 0.1 degree, that doesn't mean it's accurate to within 0.1 degree. Pretty much any device under $1,000 will have a accuracy rating of, at best, +/- 1.5%, with most doing no better than +/- 2%.

Repeatability follows accuracy, so these things are out of the question for this use.

A better hygrometer ...at least it might be, once it's calibrated ...

I've made a lot of those, for friends. They work fine for maintaining a wide range, but not worth squat for what I'm after here...

My multimeter's thermocouple only reads to within one degree. In playing with the calculator that Mario linked, it kicks out an answer to two decimal places when you input to the nearest one degree. doesn't appear that more precision will be of any value on the input end. accuracy of those numbers is what is needed. i'll do a comparison of my multimeter/thermocouple and the bulb thermometer on my sling psychrometer and report back.

One of the cheap electronic thermometer/hygrometer devices (Acurite from Walmart) that I have has a wired probe (thermocouple) for outdoor temperature (and an internal thermocouple for indoor temperature. The device is next to useless as a hygrometer but the temperature part is pretty good. It has a resolution of .1 F. For accuracy, you'd have to calibrate it against something you trust. When the device and its remote thermocouple are in the same atmosphere, they read the same. I have used it in a wet/dry bulb configuration to calibrate my good hygrometers in the past. If you sanded the plastic off the probe and attached it to your mirror (maybe with silicone heat grease?), it might get as close to what you want as you can get.

The best you'll do with thermocouple accuracy is about +/- 2 F. RTDs are better but we are starting to talk about some fairly specialized scientific type probes at that point. IR probe readings will depend on the emissivity of the surface they are pointed at. For example, black paint has an emissivity of about 0.9 while polished stainless steel has an emissivity of about 0.1. Good IR probes allow you to set the emissivity but are still a course reading at best.

I don't intend to move this topic entirely away from its present direction, but I'd like to offer an alternative approach to the mirror/temperature method currently under discussion. Saturated salt solutions in a closed volume will equilibrate to a known and very constant RH within the temperature ranges typically found in our shops. Simple sodium chloride, table salt, will maintain a RH of 75.5% at 0 C and 75.0% at 30 C. Closer to the RH range we are interested in is potassium carbonate which has a RH of 43.13% at 0 C and 43.17% at 30 C. Typical uncertainties are +/- 0.5%.

I must admit I haven't tried this myself, but it would be pretty easy I think to try it with table salt and see if it's viable.

So, where can we get some potassium carbonate and what does it cost? Does it require any special handling? I've done the deed with table salt in a ziploc bag but calibrating a hygrometer at 75% humidity doesn't really tell you if it's accurate around 40%.

I've done the salt deal, also, and when the hygrometers return to the normal RH range, they all read differently again. We -want- to calibrate at or very near where we want the most accuracy, for us, being 42-47%. I also found the air-tight-ness of the baggie to be a factor, as well as how much water I used to saturate the salt.

Really, guys, the dewpoint calculations are the simplest, quickest, and possibly THE most precise method, so far.

Calibrating a thermometer, or any temperature measuring device, at a given temperature is simple enough, but that doesn't assure accuracy at all other temperatures.

For this use, the more precise we can be, the simpler it will all get, because precision also means repeatability, and if I can trust 3 readings taken within an hour, then I'm done for another 6 months or so. But if I have to take dozens of readings over the span of a week, and still, in the end, not feel 100% certain I got it right, that leaves me a bit frustrated.

Yes, the range offered by the wet/dry bulb tests are all within reason for our work, but I'm being fussy because, well, I'm fussy!

Thermocouples might be okay, but since they are all encased in a metal probe or sleeve or something, then would require adhesive to attach it to the glass, we have added thermal mass to the equation. What part of the thermocouple does the actual reading? Is it a tiny component within the probe/sleeve/?...? If so, that tiny component is what we're needing!

What about piezo film? Does temperature affect it? Anything similar?