Calibrating Hygrometers

pawpaw

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Hygrometers are essential to anyquantitative approach to horticulture and especially to cannabis, its indoorproduction and related processes. They are, however, the leastaccurate of all the instruments that one is likely to encounter. Asimple mechanical instrument with a certified accuracy of +- 3% costsat least $150.00. I have need of at least two remote readinghygrometers so the issue of calibrating cheap wallyworld instrumentshas become important to me. During this thread I will cover several,increasingly accurate procedures for calibrating hygrometers fromfixed references.

Below are photographs of two suchinstruments in a jar that has a relative humidity of 75% +-0.5% Thefirst gives a reading of 80% the second 65%. At face value the firstis usable for some purposes the second is hardly usable at all and Iwon't even bother to calibrate it and will return it instead.

Hygro1.jpg


Hygro2.jpg
 

pawpaw

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At this point you are probably wondering how I know the RH inside that jar. Well, it's magic, which is to say it depends on matters of physical chemistry that I only vaguely remember studying. But the fact is that saturated solutions of various salts at a fixed temperature in a sealed volume maintain a fixed RH (given time to reach equilibrium). For table salt, NaCl, the relation between temperature and RH is mapped in the table below. In my next post I will begin to cover using this kind of data to calibrate hygrometers .

NaCl Eq. R.H..jpg
 

pawpaw

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How about that, you were wondering LOL
 

ozzydiodude

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What is the water to salt ratio to give you the correct saturation of the solution?
 

pawpaw

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ozzydiodude said:
What is the water to salt ratio to give you the correct saturation of the solution?
There is no such ratio for a saturated solution. I am not going to define and elaborate elementary terms of chemistry in this thread and will expect readers to look up unfamiliar terms and phrases for themselves. If after doing that there are still problems of understanding I will try to help.
 

ozzydiodude

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Then you are just wasting your time typing it in. Most ppl that come here want their questions answered not "you'll have to look it up"
There is no reason to do a DIY thread if you are not going to COMPLETELY give all the instruction on how to do something.
 

pawpaw

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Your point is well taken. I will take a more fine grained approach. In the next post I will give a detailed description of the preparation of a saturated solution and the magical humidity jar. Still I am loath to help those too slothful to help themselves and expect readers to look up unfamiliar terms before asking questions involving those terms.
 

pawpaw

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This post describes some basic features of solutions. More posts to come on creating the magic humidity jar.

A solution is a homogeneous mixture of a solute and a solvent. Solutions can be formed in any state of matter; that is they may be solid, liquid, or gas. A solution is prepared by dissolving a solute into the solvent. Solute is either the smaller component of a mixture or, when liquid solutions are considered, the gaseous or solid substance added to the solution. Solutions could be composed of either complete molecules - molecular solution, or ions - ionic solution. The latter usually is referred to aqueous solutions of salts. Fluids that mix or dissolve in each other in all proportions are call miscible fluids, lacking that property fluids are called immiscible. So gases are always miscible.

The number of grams of solute that can just be dissolved in 100 ml of solvent at 20°C is defined as the solubility (36g for NaCl). At the maximum solubility the solution is saturated and in dynamic equilibrium with the unsoluble part of solute. Such a solution is called saturated. Solution with less concentration is call unsaturated.
Solutions.jpg
 

pawpaw

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Prepairing a saturated solution.

Rinse a clean Pyrex cup with distilled water, then add ~4 Oz. of boiling hot distilled water to the cup. Stir in chemically pure salt until you can see an accumulation of undissolved crystals accumulating at the bottom of the cup. Continue stirring until you are certain that the crystals will not dissolve.

SaturatedSolution.jpg
 

nvthis

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Interesting.. Using an open salt test bed with electronic components.. And Walmarts cheapest unit to boot... Yeah, could have predicted those results

Basically, with the salt, you want to mix enough water to make it wet. NOT dissolved or with runoff. In an airtight space, the properties of the salt will produce a standard and reliable rh. Of course, it is hardly standard to plop in an eletronic device on top of that mess and not expect that bad things can happen. Such is the nature of salt corrosion.

Look, if you are gonna test equipment you've paid for, and intend to have a while, these are but a few dollars at Amazon. You get three, and each lasts 6 months.

2010_0630Jul30001-1.JPG
 

pawpaw

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nvthis said:
Look, if you are gonna test equipment you've paid for, and intend to have a while, these are but a few dollars at Amazon. You get three, and each lasts 6 months.
Thanks for pointing out the Humidipaks. I will order several RH's immediately.

What Hygrometer do you use?

Thanks again.
 

pawpaw

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dman1234 said:
I agree, its not a PH meter, if RH reading is off by 5 or even 1o% its ok.jmo

The accuracy is for preparing seeds for long term storage. Plusaccuracy is a personal preference.
 
D

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pawpaw said:
The accuracy is for preparing seeds for long term storage. Plusaccuracy is a personal preference.
No Problem, i hear ya, i was just offering an opinion, hope it all works out for you.
 

pawpaw

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. Of course, it is hardly standard to plop in an eletronic device on top of that mess and not expect that bad things can happen. Such is the nature of salt corrosion.
I am a little perplexed by that. What is your understanding of the mechanism by which the NaCl ions would be transferred to the device?

Also I do not believe that there would be any difference in the RH whether the saturated solution was slurry or a saturated solution that contained only a small amount of undissolved solute. But when I get the packs and calibrate a meter I will run the test and post the results. :)
 

pawpaw

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dman1234 said:
No Problem, i hear ya, i was just offering an opinion, hope it all works out for you.
Right, I was just explaining my motivations -- A pinch of reasonand a good handful of compulsiveness :)
 

pawpaw

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Sadly the closest values to 25% RH that Humidipak makes are 13% and 32%. Though I could get both and use interpolation, potassium acetate has an RH of 23% I think I will just diy with that. Oh well.
 

pawpaw

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On the other hand $399.99 will buy you +-2%

hxxp://www.calright.com/products/prod_id/854/
 

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