Make your own Hygrometer...

[Vater Araignee]

Make your own Hygrometer...

Or more accurately psychrometer.

Its not a perfect method of determining relative humidity but it is close.
The advantages are no electronics to short out or tell you that it is raining when it obviously isn't.

You need:
2 thermometers
1 cotton shoe lace
1 water reservoir
something large enough to affix the thermometers to if not the reservoir

Attach the thermometers to your plank, plexiglas, pvc sheet or what ever.
If you are going to attach the reservoir then do it under the left thermometer.
Cut a length of shoe lace long enough to place one end over bulb of the left thermometer and have an inch or two on the bottom of the reservoir.
add water.

The right hand will be your dry bulb reading and the left hand is your wet bulb reading.
Calculate the difference between the two and consult the following table using your dry bulb reading and wet bulb differential.

°F		WET	1.8	3.6	5.4	7.2	9	10.8	12.6	14.4	16.2	18	21.6	25.2	28.8	32.4	36
	°c		1	2	3	4	5	6	7	8	9	10	12	14	16	18	20
DRY
35.6	2		84%	68%	52%	37%	22%	8%
39.2	4		85%	70%	56%	42%	29%	26%	3%
42.8	6		86%	73%	60%	47%	34%	22%	11%
46.4	8		87%	75%	63%	51%	39%	28%	18%	7%
50	10		88%	76%	65%	54%	44%	33%	23%	14%	3%
53.6	12		89%	78%	67%	57%	47%	38%	29%	20%	11%	3%
57.2	14		89%	79%	69%	60%	51%	42%	33%	25%	17%	9%
60.8	16		90%	80%	71%	62%	54%	45%	37%	29%	22%	14%
64.4	18		91%	81%	73%	64%	56%	48%	41%	33%	26%	19%	6%
68	20		91%	82%	74%	66%	58%	51%	44%	37%	30%	24%	11%
71.6	22		91%	83%	75%	68%	60%	53%	46%	40%	34%	27%	16%	5%
75.2	24		92%	84%	76%	69%	62%	55%	49%	43%	37%	31%	20%	9%
78.8	26		92%	85%	77%	70%	64%	57%	51%	45%	39%	34%	23%	14%	4%
82.4	28		92%	85%	78%	72%	65%	59%	53%	47%	42%	37%	26%	17%	8%
86	30		93%	86%	79%	73%	67%	61%	55%	49%	44%	39%	29%	20%	12%	4%
89.6	32		93%	86%	80%	74%	68%	62%	56%	51%	46%	41%	32%	23%	15%	8%	1%
93.2	34		93%	87%	81%	75%	69%	63%	58%	53%	48%	43%	34%	26%	18%	11%	5%

Thats it!
I hope it helps you.

[Brian Barnes]

Hello Vater. That's very interesting! Thanks for sharing that with us, I'll have to try that.
Happy Growing,
Brian.

[Aidan]

Try a whirling hygrometer (sling psychrometer). Not that expensive and accurate to +/- 2%

[Vater Araignee]

Jun 6, 2008 18:54:19 GMT Aidan said:

Try a whirling hygrometer (sling psychrometer). Not that expensive and accurate to +/- 2%

Aw, that takes the fun outta spending more money to make mine or better yet scrounging. I love scrounging, if it weren't for that then most of my magical devices would never get prototyped. No prototype then no final stage unit and the effect would die with me.

[ICPS-bob]

Vater, the only difference between your design and the sling psychrometer is the amount of air movement. The difference between the wet bulb and dry bulb reading is due to evaporative cooling of the wet bulb. This difference correlates to the relative humidity as your table shows. However, to be accurate, adequate air movement is required. If left in calm air, evaporation from the wet bulb becomes reduced because the water vapor (humidity) adjacent to the bulb is locally increased by the evaporation. There are two ways to increase air movement around the wet bulb: manually moving the thermometer through the air (sling psychrometer); or manually moving the air past a stationary thermometer (wind or fan). So a simple improvement on your design would be to add a small battery powered fan.

The other caution is to use pure water and a clean wet-bulb covering. Salts on the wet bulb changes the vapor pressure.

[Vater Araignee]

Ok, then I would gut one of those table top air fresheners. I don't remember what brand they are but I know them when I see them. The have an on/off switch, a small fan inside and a timing circuit 5 min on 15 off. The refillable air freshener pack contains the scented oil and two AAA batteries. I have used them for hydroponic control.
The price just went up. Curse mass production. Maybe the title should be appended with emergency.

[Aidan]

Sadly, they don't make them like this any more. A Negretti & Zambra Whirling Hygrometer circa 1950.





Vater - Fun to use, accurate and you could always try making your own with a matched pair of thermometers. ;D

[Vater Araignee]

That would be a cool one to make.
The picture also makes me nostalgic for old school storage boxes.
I'm just old enough to remember, some of my tools, most of my fathers and all of my grandfathers where in wooden boxes.
Heck even I still have a few sets that are, like my little man wood working set and my fist microscope.

I wonder what it will be like in the future when my great grandchildren are nostalgic for plastic and cardboard. ;D