I have been hell bent on finding out the real scoop on what the RH is in my shop. I have a number of hygrometers and I understand that digitals usually suck in terms of accuracy. My Abbeon is my gold standard and I have been sending it to Abbeon in California for calibration. But that takes about 2 weeks and costs $35ish so I wanted to learn to calibrate my hygrometers myself.
I purchased 4 lab thermometers in C scale from Physics.com. These things were inexpensive (about $4 ea.) and IMHO they suck.... Why do they suck? Because I can't see the stinkin mercury without a stinkin magnifying glass...
Anyway I am digressing....
Once I had the 4 thermometers in house I stuck them all in 1/2" of what was left of my morning tea and picked the two that read exactly the same. The others were only off by 1/2 a degree but I thought it would be best to have a matched pair of thermometers.
Next the thermometers were taped to a bench and a 2" X 2" piece of linen was completely wet and wrapped around the bulb on the down wind thermometer. This is a wet/dry bulb test that I am setting up here.
I decided to use my vac to draw air from the shop over the bulbs of both thermometers. This takes about 2 minutes for the readings to stabilize and it worked quite nicely.
I repeated this experiment 3 times a couple of hours apart.
The results were compared to this chart: http://www.novalynx.com/reference-rh-table.html and that is how my results were obtained.
My dry bulb read 26.5C and my wet bulb read 19.5C for a difference of 7.
My read of the chart is that the RH at the times of the tests (all three had the same results) was 50-51%
A glance at my hygrometers told me that my Abbeon is only off in this temp/RH range by perhaps 1-2% which is excellent. Unfortunately though I don't get to experience taking the thing apart and calibrating it.
The Caliber III's were 9% low and I wrote that on the back of them with the date. The Stew-Mac hygrometer was 10% low and that was noted too.
My understanding is that when calibrating RH the temp and RH "range" makes a difference. For example the results that I see in the 50% range could be correct but in say the 75% range my dry/wet bulb test may show different discrepancies with the hygrometers.
Since I am not a scientist or an engineer I would appreciate it if the willing here who read this would check my results and method and let us all know if this looks like a valid test and probable results?
Here is the set-up:
Here is the wet and dry bulb. Note that the vac is sucking so the dry bulb is up wind of the wet bulb:
And here is what the various hygrometers read at the time of the test(s):
Conclusions: Again I am not a scientist so it's very possible that I am fooling myself so please let me know?
Digital hygrometers suck! Being off 9-10% in the range that we are looking for is too far off. But I expected this.
The Abbeon is pretty accurate and in this case did not need calibration.
Based on the results my shop is too humid and needs to come down at least several %.
Let me know if you think that I did this test correctly and what I may need to do differently?
Many thanks!
If it's in the 50% "Smiths" range then a nice onion or Garlic dip...It won't cure the curl but will make the spruce taste better. 
