I know the humidity

Finding out you’re wrong stinks, but not as much as being wrong and not knowing it.

As mentioned in a comment to my earlier post, StewMac sent me another hygrometer to see if it’d work better than the first one, which I’d concluded was junk. I expected one of two things: either the first one was a lemon and the second would be good, in which case I’d get a reading in the expected range, or both were junk and the second would give a crazy reading unrelated to the first’s.

I really wasn’t expecting what did happen: They agreed.

They agreed to within a percent RH or two, so it seemed likely that in fact both were working as intended — whether that intent was correct or not. It seemed hard to believe the manufacturer would carefully calibrate both units to give the same completely wrong reading. Yet it also seemed hard to believe they were right and every single other hygrometer I had — including the ostensibly idiot-proof wet bulb psychrometer — was reading much too high. So now what?

I tried repeating my previous test, doing a more careful job (or I thought so at least) with the wet bulb psychrometer. Results were similar: the Calibers read lower than the analogs, and the wet bulb was at least roughly consistent with the analogs, not with the Calibers.

Then yesterday I bought two things:

I bought (wait for it…) another hygrometer. Actually two more. Actually a Radio Shack Wireless Weather Forecaster. Yes, it gets lousy reviews, and I bought one. I’m an idiot, maybe, but that’s not what we’re discussing here, is it? It has a remote temperature/RH sensor and a main unit that also senses temperature/RH.

And I bought a Humidipak refill. The Humidipak is a system for maintaining RH in a guitar case, based on packets of, erm, stuff that absorbs or releases water to keep the RH at 45%. (They sell similar things to woodwind players to keep their reeds at somewhat higher RH.) The system’s not designed to work with a uke, though I suspect just keeping a single packet in the uke case would work, but what I wanted it for was calibration.

At home, first thing I did, I set up the Radio Shack equipment, with the remote unit sitting next to the main unit. After several minutes settling time they were reading the same temperature and RH to within a degree/percent or two. That gave me at least some confidence in that system. Moreover…

… they were reading about the same as both of the Calibers! First hygrometers ever not to be substantially higher.

Next, calibration with the Humidipak. I put the Calibers, the three analog hygrometers, and the remote sensor from Radio Shack in a zip bag with the Humidipak and waited several hours. Readings:

        Big analog: 62%
        Medium analog: 60%
        Small (frog) analog: 68%
        Caliber III #1: 47%
        Caliber III #2: 45%
        Radio Shack remote: 45%

Oh my.

Next, salt test. The Caliber III instructions, remember, said “DO NOT do the salt test”, but that seems silly, and anyway I’d pretty much decided by this point I was going to keep one of the Calibers which gave me the right to do what I want with it. I took the Humidipak and one of the Calibers out of the bag, put in a soda bottle cap full of wet salt, and waited several hours. (I put the Humidipak in my uke case; why not?) Readings:

        Big analog: 80%
        Medium analog: 82%
        Small (frog) analog: 89%
        Caliber III #1: 66%
        Radio Shack remote: 67%

Meanwhile, outside the bag:

        Caliber III #2: 36%
        Radio Shack main: 38%

It’s supposed to be 75% RH, but that’s if you do it right. This was a slapdash setup which I could easily imagine was producing only 65% RH. Maybe I didn’t use enough water. Notice the analog hygrometers all read just about 4% RH higher than the last salt test, which suggests the humidity was 4% RH higher — if so the RH is not precisely 75% each time. (And maybe this is why they say not to do the salt test: Not reliable when done by amateurs.)

Point is, the Calibers agree within 2% RH with the Radio Shack, both at the low and the elevated humidities.

So then. Looks like I had it backwards. Revised conclusions:

  1. The Calibers are fine.
  2. So is the Radio Shack.
  3. The analog hygrometers are junk. (Including the one being sold to clarinetists to keep their instruments from cracking, and the one being sold to pet owners to keep their reptiles and amphibians alive.)
  4. Somehow my wet bulb psychrometer was either built or used wrong, in both its incarnations, despite its seeming idiot-proof nature. I don’t know what was wrong, I being the idiot in question. Wait, I said we weren’t discussing that here.

Anyway, guess I’ll send back one Caliber, keep the other, and use the analogs for… well, I don’t know, steampunk modding the ink jet printer or something.


4 thoughts on “I know the humidity

  1. The old laptop called dibs on “cool shit”, since it enjoyed your screen transplant so much.

    Maybe the froggy hygrometer works best in appropriately moist conditions like inside your terrarium with your frog?

    They’re just out to get the clarinetists. It’s a conspiracy by reed manufacturers. You have to replace them more frequently if they crack faster.


  2. One last (I think) note on this topic: I offered to keep and pay for the second Caliber hygrometer, since it was my mistake thinking the first was defective and they would not be able to sell it as new if I returned it, but StewMac said they’d still be happy to let me return it at their expense. Which I did. They’re a class act.


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