MIMF

Kyle, for control I have a humidifier and a dehumidifier. Controlled by yours' truly, by turning the controls on either device... <g> My shop is -very- tightly sealed, and is rather easy to control, summer or winter, but I do live in an extreme climate, and that means I can see an extreme 'swing' in RH within 24 hours or less. This evening, for example, saw me in the spray booth, which means the exhaust fan ran a good bit, and when I closed-up the shop a bit ago, it was 33% RH in there. It'll take another hour or two to recover back to 44%...

I'm a stickler for precision because it makes everything simpler. Now that I can trust my method for finding the true RH to a pretty precise figure, it'll be a breeze to recalibrate my hygrometers next time they need it, be it this spring or next fall. Until then, my mind is at ease, knowing everything's been checked and calibrated to the best of my ability.

BTW, when I say that I compare my 3 hygrometers against each other daily, it's not a big ritual; they're all clustered together and it only takes a passing glance to see that they're all reading the same, or nearly so. Only when one of 'em goes wonky do I even give RH a second thought.

I didn't see it in these posts, but did anyone think of using a thermometer like you would use to check a baby's temp? No, not the old ones that you more experienced fellows would suggest, but a newer one in which You push the button down and swipe it across the kid's forehead, or in this case some surface, then let the button go and you have a reading. I've used ours, not only on our son's head, but also just to check ambient air temp (holding it out and pushing the button). I don't see why you couldn't swipe it across a piece of cold glass/ metal.

Intrigued by Mario's quest for simplicity, I had to try this. I sent for some potassium carbonate from soapgoods.com ($11.53 including shipping for 1 lb.). At 20C, a potassium carbonate slurry draws the relative humidity overnight in a closed container to 43.17%. I cut the top of a small paper cup at an angle so a plastic bag draped over it wouldn't seal it. I half covered the bottom with water and dropped in enough potassium carbonate that some of it was not in the water. Overnight, I got the results you see in the photo. This tells me I was off by 1% on the face with my psychrometer calibration (it was a bit drier in the room than I had calculated). The cheap digital hygrometer was much closer than I expected it to be, however its reading varies with temperature versus the Abbeon so this reading could be coincidence. For me, this is easier than finding the dew point exactly and easier than the psychrometer.

Well done!

Can you try something else for us, please? Can you re-do the test, but with a slightly wetter solution one day, then a slightly drier one the next?

It's not that I don't trust your method, but rather to see if reasonable variations of "saturated solution" have any effect on the accuracy. That was always one of my issues when using the saturated salt test; what exactly is a "completely saturated, but not wet" solution? My thinking is that it will vary from user to user, so let's see if the amount of saturation is critical or not.

My wife's a soap maker, and when she gets home, I'll have to ask if we have any of this stuff, too; although 5 minutes with the GIT method is way quicker than 24 hours with the saturated stuff method, it'll be especially nice if we end up with no fewer than 3 acceptable methods for calibrating.

My understanding is that the solution must be supersaturated. My way of achieving that was to have the salt piled such that it extended above the water line. Thus, the dry salt could absorb moisture from the air while the wet could release it until the closed space reached equilibrium. So, that is what I did (and thus covered your two suggested tests all in one, I think). Is there a chemist reading who could clarify all of this? Before I ran my experiment, I discussed it with my brother-in-law who is a chemist. This whole topic was not his area of expertise. He had to do some research to tell me if the potassium carbonate would be safe to handle. He suggested adding it to the water in case it caused enough heat to spatter when it mixed. But, he couldn't comment on the calibration procedure. Another chemist?

That is not the way to get a supersaturated solution. Usually, you have to heat the water when adding whatever chemical and keep adding chemical and mixing the solution until no more dissolves. Then when the solution is cooled it becomes "supersaturated".

The saturated Ka carbonate solution has me wondering about what RH would stabilize at in a sealed container with silica gel which is widely available. I've read that it's supposed to get RH down to "about 40%". I wonder how close. Many silica driers are doped with other chemicals to increase their effectiveness so that might be a factor.

I wouldn't think a supersaturated solution would stay supersaturated for long especially if there are any seed crystals around. It's bound to go back to equilibrium at the temperature it's hanging out in. No?

Several years ago I had to measure air dew point. We wanted to make sure the air we used was dry, so we were looking for -30 C or so dew point.

The apparatus we used was home made, consisting of a mirror polished stainless stell tube, a beaker, a rubber stopper, and a thermometer.

The air was gently blown on the tube and we were using acetone and dry ice in the tube stirred with the thermometer to get to -30C.
As you reach the dew point, the fog could easily be seen on the mirror surface of the tube.

Such an apparatus could easily be made with a chrome tube (or even a square tube made of mirror, but metal is better being more conductive)
a jar and an aquarium pump to blow the ambiant air.
If you google dew point apparatus, you will see some variations of this device.
Here is a hand made sketch.
If you have a chrome tube of let say 3/4 of an inch in diameter, then you have the core of your device.

David King wrote:I wouldn't think a supersaturated solution would stay supersaturated for long especially if there are any seed crystals around. It's bound to go back to equilibrium at the temperature it's hanging out in. No?

Yes, it can grow crystals. My genius brother used to grow crystals like this. But it is not a fast process. A crystal of any size can take weeks.

Barry, you're very right. I should have used the word "saturated" instead of "supersaturated." Equilibrium is what we're after in the salt calibration method. What I described above ensures the saturated part, I think.

When all is said and done, the instruments are going to go out to a customer who is probably savvy enough to keep the guitar in an RH between 40% -55% but probably no more accurate than that, whether in a humidified room or in the case with an in-case humidifier.

Is it really necessary to get all OCD about accuracy to within .1% ...??

Maybe it is, but I don't really see why ...

Murray, it's important, for me, to know that my hygrometers are reading correctly. I allow my shop to range from 42-47%, which is the industry standard. I had grown very weary of the wet bulb method giving me results that were barely within 4-5% between repeated tests. With that method, I always had to guess as to whether to calibrate my hygrometers to the low mark or the high mark. Since I don't like doing it more than twice per year, that meant that my instruments could vary by as much as 8-10% between batches. Totally unacceptable!

As I said in a few posts back, the more precise I can be in determining the true RH, the easier and quicker the re-calibration of the hygrometers. What we've come up with here is both precise, dead-simple, and takes all the guesswork out of it all. If it's precise and repeatable to 0.1%, great!

All you weekend warriors who build only for yourselves, and maybe for friends and family, need not be overly concerned, but those of us who do this for a living, and are striving to be the best at it, need to be precise at everything. To me, this is no different than being as precise as possible when laying-out fret slots or setting the saddle's position. If I begin to get lax on any one thing, I may as well quit completely. As I've always said: the Devils in the details!

But really, a simple, precise, and above all else, repeatable method like we have come up with here, simply makes everything else simpler.

Mario, what is your reasoning for taking the dew point reading as the formed dew disappears rather than at the beginning of dew formation?

I was taking it both ways to be sure I was getting the true dewpoint. Now, I just take it when it first forms....

thanks for this idea...i don't have to squint to read my sling psychrometer any longer!

Wow, used that online calculator to figure the RH in my shop this morning: found out that my little Caliber unit reads like 10% high! Time to invest in a real hygrometer.

Mario, I have the results of the experiment you requested and they are as expected. When there is more water in the cup than the potassium carbonate can absorb, the humidity in the bag goes up several per cent. I let it go overnight to get a reading of 48%. I'm sure that that was limited by the amount of water that I put in. When dry potassium carbonate was within the sealed bag, the humidity dropped to 38% within 4 hours. So, the 43% level is achieved only when there is water and dry potassium carbonate in the same cup so that equilibrium can be achieved between the dry and the wet. It sounds like we could use potassium carbonate as a dehumidifier if we had enough. When I discussed this at a party, asking if anyone knew where potassium carbonate was routinely used so I could get some, one of the guys told how his dad used to hang a bag of something, he thought potash or maybe potassium carbonate, over a bucket to dehumidify a basement shop. Water would drip continuously off the bag into the bucket.

Very cool, Bob. Another tool for the toolbox!

I've been repeating the dewpoint test every day or two, and it's bang-on consistent. When I'm doing soemthing that lowers the RH, it follows perfectly, as well as at higher RH levels. I've tried it with mason jars and regular glasses, and while not quite as sensitive, the results are still within less than 1/2 of a percentage point, and that's plenty fine. Haven't tried the shiny can yet, because I don't have any cans kicking around. We eat fresh food, mostly...
And I'm only taking the dewpoint reading when dew initially forms. I no longer repeat the test by warming the cold glass... Tests with the IR temp gun are all over the place, so that method was not the answer; they're just too flaky. At least, the one I have is. I also tried a piece of thin masking tape on a mirror, cooled it, and then took readings off the back of the mirror and on the tape, and the tape does act as an insulator, giving me a temperature consistently well above what the back of the mirror gives. In the end, a simple thermometer is all that's necessary.

Edit to add: All told, it takes well under 5 minute to perform the test, though that doesn't include coming back to the house to go on-line and crunch the numbers. I haven't looked yet, but I'm sure that a chart exists that I can print and keep in the shop. I keep my beer glass in the fridge, and my shop's water comes straight from the well, so it's already very cold, too. Just two ice cubes do the trick, though come summer, it may take 3 or 4. As with any task, once we've done it a few times, it become very easy and quick.

Potassium Carbonate is AKA pearlash. Potash is any of several compounds containing potassium.

Mario, you can make your own chart by crunching the numbers around the area of interest. For the wet bulb/dry bulb humidity, I put the equations in a spreadsheet. I keep a laptop in my shop. That makes it easy.