Hygrometers, Humidors, and a bit of model cannon stuff.

by John

I like to think that I have very few vices outside the workshop, where there are quite a few.

But, one of the vices in which I indulge occasionally, is a good Cuban. And I make a point of indulging no more than 3 per week.

And, actually, that does add to the number of vices in my workshop, because that is where most of the Cubans (and Bolivians, and other central Americans) meet their destiny.

SWMBO does not encourage cigar smoking in the house for some reason, but I quite like the stale cigar smell in the workshop. Come to think of it, maybe that’s what is keeping the tigers outside.

To cut to the story, my son in law, James, gave me the Humidor pictured above, for Christmas. A great gift, which I greatly appreciated. Mind you, I have a sneaking suspicion that he might have had an ulterior motive, because James too enjoys an odd cigar, and he likes his cigars to be at the perfect humidity of 75% when he visits the workshop, “for a chat” or whatever.

The humidor box is very nicely made, with a Spanish Cedar internal lining. Made in Indonesia. It shuts perfectly tightly and just the size for about 50 coronas. And it has a hygrometer to measure the humidity.

But…… it comes with instructions to calibrate the hygrometer. WTF! If I buy a Mitutoyo gauge, I assume that the calibration is correct. What is this “calibration”. And then there were the instructions for calibration….

  1. place a tablespoon of salt in a plastic dish, and add JUST ENOUGH water to make it damp.
  2. Place the dish, and the hygrometer in a zip lock sealed bag and let them stand for 6 hours.
  3. After that time the hygrometer should read 75%

Well that all sounded Mickey Mouse to me, so I asked my fellow model engineers at our second last meeting.

As usual, in our group of 15-20 participants, one person clearly knew ALL ABOUT humidity measurements, because he had worked in the munitions experimentation industry.

Next meeting, he brought the following hygrometers……

An electronic, high accuracy instrument.
2 thermometers, one connected to a well, and one in the air. The well contains water, and the difference between the temperatures in the thermometers is read off a table, giving the humidity of the air.
And this hygrometer……. see next photo for the reverse……
…which uses human hairs which contract when exposed to moisture, moving the indicator needle. The hairs in this instrument need replacing. Frank was looking for volunteers, but no members had enough hair to spare.

But! Do you know which method the explosives experts used?

The salt dissolved in water method!

Apparently that method is accurate to less than +/- 0.5 %.

So that is what I did. And after the hours of waiting, when the humidor hygrometer should have read 75% it actually read 65%.

There was a screwdriver slot at the back, which enabled me to adjust the calibration on the hygrometer.

The figures in the columns refer to percentage humidity at different temperatures with different dissolved salts.

Apparently the explosives scientists carried a kit which contained a selection of the above salts, and they used them to calculate the humidity of the air before conducting their experiments.


But I have been working on finishing Armstrong RML number 2.

Not much to show but I have been working on the gears, brake, and chassis……

This photo shows the brake drum on the big gear, the lever cam, and the stainless steel band bent into position, and pinched ready for silver soldering. Easy to make, but difficult to get exactly right. Too loose and the actuating handle bottoms out. Too tight, and the gear wheel refuses to rotate in reverse. So I ended up making 2 steel bands. The flaps were silver soldered. Top right, and I also made a dog clutch support bracket. Not physically necessary IMO, but consistent with the original.