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How much is a 'hygrometer', and where...how to calibrate


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#21 Hippie3

Hippie3

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Posted 20 October 2006 - 10:13 PM

Hygrometer Calibration

Analog (dial-type) hygrometers are often shipped from the factory out of spec. To calibrate, you need plain table salt, a few drops of water and a see-through Tupperware container with a tightly fitting lid.

Place about 1 teaspoon of salt in a small shallow open container, such as the cap to a jar. Add a few drops of water to the salt. You DO NOT want a solution or to dissolve the salt, only get it damp. Place the container with the salt solution in the Tupperware container. Next, place the hygrometer in the Tupperware container face down. This will allow you to view and check the reading of the hygrometer before you open the container. CAUTION - do not spill any salt or solution on your hygrometer! Place the lid on the Tupperware container making sure it fits tight. Allow to stabilize for at least 6 hours. Without opening the container, check your reading. It should read exactly 75%. If not, note the deviation and or adjust the hygrometer.


Calibration of your Hygrometer (Humidity Gauge)

Analog hygrometers are manufactured with a factory tolerance of +/- 5% points of humidity (and sometimes more) through the normal range of 40-80%. Although humidity gauge is pre-set at the factory, its' calibration may be off within this range for various reasons.

A simple, easy to do at home means of calibrating hygrometers uses standard salt and a sealed container. Table salt, when used properly, will maintain an exact 75% humidity in a perfectly sealed environment. So, to do this, you'll need three items. First, obtain an airtight container such as a see-through tub with a tight fitting snap-on lid (the kind usually found in most kitchens). Alternately, a heavy-duty zip-lock bag will do. Second, you'll need about a teaspoon of salt. Third, any small, shallow open container is needed (to hold the salt in) such as a screw cap from a plastic bottle.

Place the salt in its small container and add a few drops of water to moisten it. Don't dissolve it yet. With just a few drops of water, you'll get what you need, which is damp salt. Next, carefully place the salt in its small container, along with your hygrometer, into the airtight container. Please do not get any moistened salt on the gauge. Also, check to see that the starting point (current reading) is anywhere in the 40% to 80% range.

Seal the container tightly. Note: if you have any doubt as to whether or not it is a perfectly sealed environment, double-it-up by putting it in a second container, or into a second baggie. Do not try to remove the remaining air trapped inside. Now, wait for at least 5 or 6 hours until the environment has stabilized. Do not open the container. Read the gauge's humidity % level. It should be exactly 75%. If it is not, note the deviation as being the amount your hygrometer is out of calibration. If for example, it reads 68%, the gauge is 7% low. If on the other hand it reads 80%, the gauge is 5% high.

Now, remove the gauge from the container and locate a very small jeweller’s flathead screwdriver. If it fits through the small hole in the center of the rear plate, it should work well. Put it in that hole, and turn it slowly while watching the dial on the front. If your gauge was low by 4%, turn the screwdriver clockwise and make the dial rest 4% points higher than it did previously. Conversely, if your gauge was high by 6%, turn the screwdriver counter-clockwise and make the dial rest 6% points lower than it did previously. That's it. Your gauge is now calibrated and you should feel very comfortable with its accuracy from this point on.


Checking your Hygrometer Calibration


Why should I check my hygrometer's calibration?
Even if you don't want to open its case and adjust your hygrometer, you would be well advised to check its calibration. These are not the precision instruments that some people think... Analog (dial-type) hygrometers are often shipped as much as 20% out of spec. Even the often-recommended Radio shack hygrometer is shipped with a factory tolerance of plus or minus 5 points of humidity through the range 40-80%. That means your unit could read anywhere from 65 to 75% while in a 70% environment and still be considered in spec! I've personally seen up to an 8 percentage point difference between units before calibration.

For a calibration source, we're going to use salt - NaCl - plain table salt... Why salt? The following is a brief explanation of the chemistry, originally posted to asc by Peter Shenkin of Columbia University. Don't worry too much about the technical details - the point is that plain salt will, when used properly will maintain an exact 75% humidity in a sealed environment.

A saturated solution at constant temperature & pressure has a fixed composition. [[ Explanation in terms of the Gibbs Phase Rule omitted. ]] Therefore, such a solution has a fixed vapor pressure. Thus, at constant temperature, no matter how much solid NaCl and how much water are present, the concentration of the NaCl in the water is fixed, just as long as both the solution and the solid phase are present. Therefore such a solution has a fixed vapor pressure.

Now, it just happens to turn out that the vapor pressure of a saturated solution of NaCl in H20 is about 75% of the vapor pressure of pure water at any temperature close to room temperature. This means that at equilibrium, if there were nothing present except this solution and a vapor phase in contact with it (no air), the pressure above the solution would be about 15 mm_Hg, in round numbers, using 20 mm_Hg as the vapor pressure of water near room temperature, again in round numbers. As mentioned earlier, the only gas providing the pressure above the solution would be water vapor. We'll see in a few minutes that the same thing happens even when air is introduced.

OK, class dismissed - you've got it now, right? :-) For more details about the science behind this calibration procedure, check out the excelent report available at: http://www.pip.dknet...85/gemarkus.htm

Now to procedures. You'll need a zip-lock baggie or other see-through container, about a teaspoon of salt, and a small, shallow open container for that salt (I use the screw cap from a bottle of milk) Gee, I love this high-tech lab gear.... ;-)

Place a teaspoon or so of salt in its container and add a few drops of water to get it wet. You don't want to dissolve it, just get a good damp pile of salt in your bottle cap. This is important! Just a few drops! you want damp salt, NOT a solution. As the technical explanation above says, you want the salt present in a wet solid phase. Place it in the baggie, along with your hygrometer. CAUTION - DON'T SPILL ANY SALT ON YOUR INSTRUMENT! Be sure the Hygrometer isn't in its "min" or "max" mode, BTW.

Seal the baggie with some air trapped inside (so it's not tight against the hygrometer) and let it sit. Allow this to stabilize for at least 6 hours. (don't rush it!). After the internal "system" has stabilized, check your reading WITHOUT OPENING THE BAGGIE. It should be exactly 75%. If not, note the deviation - this is how much your hygrometer is out. Don't be surprised if your reading up to 5 points out - unfortunately, that's the factory tolerance of this instrument.

Note that several readers have had occasional problems with poorly sealing baggies. Use a small tupperware container, or double-bag your experimental setup will help avoid inaccuracies.


6.2 Radio Shack #63-855 Calibration

Ok! Feeling "scientific after testing the calibration of your hygrometer? Well, the next step for the ambitious among us is to actually callibrate our hygrometers. Many cybersmokers use the Radio Shack 63-855 Hygrometer, (also known as the "Airguide" Hygrometer). The following procedure was written specifically for that instrument. Please read these instructions very carefully before proceeding - If you don't feel comfortable tinkering with sensitive circuitry, DON'T DO IT!

Now that you know how far your instrument is out, you might want to adjust it. CAUTION! Mess-up here, and you could damage your hygrometer! There is no simple calibration knob to turn. To adjust this instrument, you'll be re-adjusting the bias current of the sensor, then comparing it to your calibration reference - the salt in the baggie.

Remove the battery door, the batteries, and finally the back of the unit. along the top edge of the PC board inside, you'll see two round, flattish metal disks (micro-potentiometers). don't touch the left one, you'll be working only with the one on the right. First, look at it closely. You'll see a tiny indent for a tiny screwdriver blade across the middle. Sketch its exact position carefully - this procedure might take several days, and you will forget the original orientation if you don't write it down!

You'll be turning this right potentiometer (pot) a tiny amount to change the humidity reading (1/8 turn might change your reading by 3 or 4%). DO NOT turn it more that 1/4 turn! Use a very small screwdriver (like a jeweler's screwdriver), and don't change it by much! If you were reading above 75% in the calibration test, turn this pot clockwise, again only a tiny amount... If your reading was low, turn it slightly counter-clockwise...

You can leave the back off to save some effort, and put the batteries back in now. Be sure the "min" and "max" functions are turned off, and place the hygrometer back in your baggie with the salt. Again, let it stabilize for 6 hours, and DON'T LET IT TOUCH THE WET SALT! Check your reading, and re-adjust if necessary. Be patient, you may have to try a few times to get it right. If you feel you're getting nowhere, set the pot back to exactly where it was when you started.

When you're satisfied with your setting (set it to +- 1% of 75%) remove the batteries, re-assemble the unit, and give it one final check. Once you're done, light up one of your best cigars - you'll deserve it!

Radio Shack #63-1013 Calibration

The Radio Shack model 63-1013 hygrometer can be calibrated similarly. The potentiometer (pot.) can be found on the upper left hand side of the circuit board once the back is removed. The pot. is small -- about half the diameter of a pencil eraser. The pot. has a small slot in it to allow for adjustments. Clockwise turns increase the humidy percent reading and counter clockwise turns decrease the reading.
6.3 Older Radio Shack units
contributed by Steven Banks

I would like to add some new information. I have one of the older Radio Shack hygrometers #63-844, but do you think that stopped me? You give me an opportunity to take something I know nothing about apart, I'm there! Plus, my Dad was a quality assurance engineer and he wouldn't have put up with that much of a error factor.

Well the salt portion of this whole procedure is sweet indeed - so simple, I felt like I was watching "Mr. Wizard" again. After doing this I found that my hygrometer was a whopping +8% off. Now... I'm not great with math, so I decided to correct the problem instead of just knowing how much it's off by. The problem was, which "pot" screws to adjust, since the instructions were for a #63-855. Well for those of you that have the same model as I do, there are two circuit boards in it, the top one is for the temperature and the bottom is for the humidity. The "pot" screw CLOSEST to the battery compartment is the one you need to adjust. And yes, as was mentioned in the first posting, it does take some fiddling!!! There were times I'd adjust the screw, and see no change. But I finally got that puppy within + -1%. Note that this hygrometer's reading will INCREASE as you turn the control CLOCKWISE - exactly the opposite of the 63-855 model...

Let it be known that I tinkered where no person in their right mind should, and this is no guarantee that it'll work for you (especially knowing Radio Shack stuff - yours could be laid out differently?) But if you're bold, go where no man has gone before and make it right!


6.4 The Airguide Digital Hygrometer

This is the same unit internally as the Radio Shack #63-855. See section 6.1 and 6.2.


6.5 The Bionaire Digital Hygrometer

This is a fine looking digital hygrometer available from several suppliers for about $17.95. It's similar in operation to the Radio Shack #63-855, buy without its memory features or internal adjustments. Its factory rated accuracy is +- 5%, but tests show its linearity (accuracy over a broad range) is considerably less. Since this unit can not be re-calibrated, I recommend testing its accuracy with the salt calibration test, to allow for differences in readings.


6.6 Analog Hygrometers

Surprisingly, both the most expensive and inexpensive classes of hygrometers are the dial-type (or analog) hygrometers. Many are sold in fancy brass or chrome housings, but their accuracy in general is not very impressive. Being a mechanical device, rough handling in shipping can often knock these gauges as far as 20% out from an accurate reading. The calibration of this type of instrument should always be checked.

Many of these gauges come with the simple instructions to "wrap the instrument in a damp towel for at least 20 minutes, then adjust it (either by set-screw or rotating the outer housing) to a reading of 95%." As this technique does not address the non-linearity of these gauges, I recommend checking them in the general RH range cigar-smokers need by using the above mentioned "salt-standard" calibration technique.

Don Meyer (donme@primenet.com) has recommended a very inexpensive ($4.86) Sunbeam thermometer/hygrometer, available at Wal-Mart. On inspection in the store, he found that of the 10 or so on display, all read within 3%, whereas others (in the same store) displayed a broad range. Of course, consistency doesn't always mean accuracy, but it's a good sign. It should be noted that this unit is mounted in a rather large case. Users should consider re-mounting this unit for use in their humidors. The model # BTW, is 90113. (old #1803)


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