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Salt test mechanics

preembargo

Sleeping not so peacefully
Joined
Feb 27, 2005
Messages
2,686
-RELATIVE HUMIDITY-
SATURATED SALT SOLUTION TEST

(For Wilkeyites)


Short description

This validation method measures relative humidity in a range of 0 - 100% using saturated solutions of various salts at various temperatures. This method relies on the principle that the partial vapor pressure of water above a saturated salt solution in equilibrium with its surroundings is constant at known temperatures.

In other words, by placing a saturated salt solution in a sealed container, the relative humidity of the air column above the solution can be precisely measured at specific temperatures. A list of saturated salts and their humidity values at different temperatures is shown in the following table.

Note that these temperature values are useful for the purposes of precise calibrations but the variations in RH due to temperature are negligible and therefore are of little consequence regarding RH values within your humidor.

table.jpg


(By the way, if you have Potassium Iodine lying around, you clearly have bigger concerns than calibrating your hygrometer. Just make sure your Geiger Counter is calibrated)




Calibration procedure:

- Place about 1 inch of saturated salt solution in an open container that is approximately 4 inches high. The 3 inches above the solution will contain air with a predictable humidity value.

- Place the container with the saturated salt solution in a sealable container. The hygrometer must be suspended in the saturated air column above the salt solution. Do not get the salt or salt solution on the hygrometer.

- Seal the container. Allow 6 to 8 hours for the air inside the hygrometer to reach equilibration.

-Compare your readings to the values in the above table.

- If you’re obsessive compulsive, repeat the above procedure using various saturated salt solutions from the above table.




The problem with using salt solutions for measuring relative humidity

It is known and accepted that relative humidity is one of the physical quantities most difficult to calibrate. The primary difficulty is to generate humidity with a high degree of stability for the purpose of precise calibration. All classical methods require either temperature stability and uniformity or accurate measurement of the temperature.

The water vapor concentration, and therefore the relative humidity over a salt solution is less than that over pure water. This is because water is present in both the gas and the liquid phase, whereas the salt molecules are only present in the liquid. During this process the saturated solution begins to absorb water from the air. This dilutes the solution at the surface. The less dense surface solution is gravitationally stable and does not mix with the bulk of saturated solution, so the RH continually drifts upwards from the value for the saturated solution towards 100%. If the room temperature is unstable the salt solution behaves quite unpredictably, causing considerable errors when such solutions are used to calibrate your hygrometer.

The situation can be improved by heaping up the solid salt in a mound, with just enough water to dampen it. The solution formed by water absorption from the air will roll down the outside of the mound, continually renewing the crystal surface enabling it to grab more water from the air. This process is known as convective mixing. Convective mixing can also be achieved by enclosing the saturated solution in a polymer sachet that is permeable to water molecules but not to charged ions. Thin silicone sheeting works well for this purpose but then the test has become so complicated that it is better to use another RH control method altogether.

This argument applies to all salt solutions, saturated or not. The reason for using saturated solutions, in contact with a heap of the solid crystals, is that the concentration theoretically remains constant even if water enters or leaves the solution from the air.



How To Use Saturated Salt Solutions For Calibrating Hygrometers
–English Version-

(For the rest of us)


The dry salt is placed in a mound about 1 inch deep in a small tray. Water is added to moisten the salt. Do not add more water than is needed to make the salt look damp. The tray is placed in an airtight bag or box. The hygrometer is then placed in the container with the damp salt. In about 6-8 hours your hygrometer should read 75%.


edited to add: Mostly plagiarized from various sources, I ain’t that book lerned.


________________________________________
 
Thanks for the low down...

I think I’m going to go home and suspend my hygrometer in the column of RH created by the sodium chloride solution.

Or maybe I'll just smoke a Padron '64...
 
(For Wilkeyites)
WTF ??? :D

(By the way, if you have Potassium Iodine lying around, you clearly have bigger concerns than calibrating your hygrometer. Just make sure your Geiger Counter is calibrated)
It's potassium "iodide" denoting a single atom anion (negatively charged) as in sodium chloride. Sheesh. :rolleyes:

Calibration procedure:
Convective mixing can also be achieved by enclosing the saturated solution in a polymer sachet that is permeable to water molecules but not to charged ions. Thin silicone sheeting works well for this purpose but then the test has become so complicated that it is better to use another RH control method altogether.
In fact, I'm pretty sure that the Boveda packs and the humipacks that come inside Fuente boxes use a saturated salt slurry inside a high moisture vapor transmission sachet made from ether polyurethane. I've taken a few apart and the toughness of the membrane suggests that it is more likely urethane instead of some other thermoplastic elastomer.

How To Use Saturated Salt Solutions For Calibrating Hygrometers
–English Version-

(For the rest of us)


The dry salt is placed in a mound about 1 inch deep in a small tray. Water is added to moisten the salt. Do not add more water than is needed to make the salt look damp. The tray is placed in an airtight bag or box. The hygrometer is then placed in the container with the damp salt. In about 6-8 hours your hygrometer should read 75%.
Very nice. I would add that using a plain old Ziploc is probably not as good as using Tupperware which is again not as good as using a Mason or other glass jar.

A good read, Mike. I thoroughly enjoyed it! Keep 'em coming.

BTW, here is a pretty readable short article on the technology of breathable films.

Wilkey
 
Good read Mike, looks like you've done your homework.

Thanks a bunches
 
Has anyone used one of these to calibrate instead of the mound of salt method?

I'm just wondering how accurate these Boveda one-step packs are. For only $4.95 it seems like an easy way to go.
 
Has anyone used one of these to calibrate instead of the mound of salt method?

I'm just wondering how accurate these Boveda one-step packs are. For only $4.95 it seems like an easy way to go.
Those kits are dead on. The only issue I had is the bag it comes with is two small. I moved the whole test to a tupperware and was able to test several remote hygrometers at the same time but it took 48 hours to get a accurate result. According to Tim @ Boveda anyway.
Frank
 
Here's a picture of my salt test set up. I use a shot glass. The wet salt shouldn't be real smooth; it should look a little coarse. This is visible if you look closely.
Madeleinesalttestsetupsmall.jpg


I put the salt and hygrometer in 2 closable bags; it seems to work fine. Squeeze out most, but not all, of the air.
Madeleinesalttestresultsmall.jpg
 
Very nicely done. My only question is possible damage from salt testing your hygrometer considering electronics hate salt. I've heard of hygros coming with warnings saying to not salt test them. Any ever have damage from the salt test?
 
Very nice post. Sodium Bromide is not in that list, the numbers are 57.6 at 25C and 57.4% ar 30C. One of my friends wanted a dry box around that humidity and I sent mine to him, but with the introduction of the 60% beads, I don't think he bothered.

As a point of clarification, KI isn't normally radioactive. It can be, of course, but you would have to buy or make it. The stuff yiou buy from Fisher is not radioactive. I-131 has a shelf life of 8 days and is used sometimes for imaging in labs. Buying, handling and storage are problems as this is a gamma emitter. I-129 has a half life of millions of years and is a major problem when nuclear reactors go bad. There are traces of this compound in the environment, thanks to the above ground testing/use of nuclear weapons 40-60 years ago.

One additional point related to the use of saturated salt solutions for testing hygrometers. In theory, you should use distilled water for the saturated salt solution. In practical terms I doubt that this is necessary. Very high water hardness would be around 1000mg/L, which represents 0.1% total salts (a mixture of different salts in there). Given that saturated NaCl is north of 20%w/v, it seems to me that the difference would be seen at the third or fourth decimal place. We have 200mg/l around here, so the total salts are at 0.02%.

Although the temperature does not affect the humidity of saturated NaCl solutions much over the range posted above, it will affect the level of salt needed to achieve saturation. Not much relevance, though, as most people have a great excess of NaCl when doing the test.
 
Jeese, I wonder if ionizing radiation would kill Chuckie and his bretheren.
 
preembargo,

Great post on salt test mechanics :)

I wonder though, the "table salt" widely available is iodized. I have never been able to find out what affect that has on the RH reference numbers published for NaCl.

I see seangars thoughts on percentage of salt in non-distilled water. But that is a weight basis and I wonder if the interaction effects of the various salts in hard water have a greater effect on RH than their weight bases.

Another consideration, almost everything I have seen online regarding proper storage of cigars and RH is based on empirical observations by "laymen". Therefore, if in general the salt test is not entirely accurate and the hygrometers are not truly calibrated does it really matter? If people think they are calibrated to 70% (for example) but it is really 65%, and if everyone calibrates the same way, the "observation" of 70% RH would be technically incorrect but the result produced/desired for the cigars would still be achieved.

Just my $0.02 worth. Good job guys.

antaean
 
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