Hygrometer calibration

[Taylor814]

Has anyone tried to calibrate their hygrometer by the salt method? I recently purchased an Oasis OH2 and Googled how to calibrate it. Cigar aficionados seem more obsessed with humidity control than guitarists so I found a number of essentially the same procedures for calibrating humidor hygrometers. I combined 4 tsp table salt with 1 tsp distilled water in a small open container, and then placed that and the hygrometer in a sealed sandwich bag for 24 hrs. Supposedly the internal RH will be exactly 75% under these conditions. However, the RH reading on the Oasis (as well as a decades old hygrometer from Radio Shack) appeared to both equilibrate at 65%. So either the method failed to produce the proper RH or both hygrometers are reading exactly 10% too low. That seems excessive. Anybody actually try this successfully? I know that Boveda sells a calibration kit, which I assume uses something similar, but I thought this would be easier and faster. Perhaps not...

[BT55]

I smoke cigars and use cigar humidor hygrometers. They’re probably no more accurate than other ones but using two (they’re close to each other’s readings) I average them out. My humidifiers have humidity controls and they’re sort of accurate but if you maintain about 40% you’ll have no worries.

[Tim McKnight]

Originally Posted by Taylor814

Has anyone tried to calibrate their hygrometer by the salt method? I recently purchased an Oasis OH2 and Googled how to calibrate it. Cigar aficionados seem more obsessed with humidity control than guitarists so I found a number of essentially the same procedures for calibrating humidor hygrometers. I combined 4 tsp table salt with 1 tsp distilled water in a small open container, and then placed that and the hygrometer in a sealed sandwich bag for 24 hrs. Supposedly the internal RH will be exactly 75% under these conditions. However, the RH reading on the Oasis (as well as a decades old hygrometer from Radio Shack) appeared to both equilibrate at 65%. So either the method failed to produce the proper RH or both hygrometers are reading exactly 10% too low. That seems excessive. Anybody actually try this successfully? I know that Boveda sells a calibration kit, which I assume uses something similar, but I thought this would be easier and faster. Perhaps not...

It may be helpful to search Sling Psychrometer method.

[Simon Fay]

The salt test NaCl isn't that useful for guitars. However, K2CO3 Potassium Carbonate is -- a saturated salt solution maintains a RH of 43%.

The next time I calibrate my hygrometers, I'll do a pictorial as I see this question asked a fair amount. However, rather than going to the trouble of doing all this -- just buy anew Caliber IV hygrometer every 2 to 3 years and you're set. They are calibrated very well and the calibration will be suitable for a couple of years.

If you want to use Potassium Carbonate, you can also buy that off of Amazon. And that way you can get about 5 or so years out of your digital hygrometer by calibrating every year or so.

If doing a salt test, you want to use a sealed container - Tupperware works great. Place a tbsp of the salt and then add water until you get a slurry. You want to see water but with undissolved salt -- that way you know it is "saturated". I let it sit for a day and check 24 hours later.

[jseth]

The salt test works fine - just be sure to have the hygrometer and the salt/water slurry in a CLOSED container or bag... and to leave it in there for the 24 hour period recommended.

Gives you a good reference for your own hygrometer's readings...

[JayBee1404]

Originally Posted by Tim McKnight

It may be helpful to search Sling Cyclometer method.

Sling Psychrometer!

A cyclometer is a device used to assist in encryption/decryption, which was designed in WW2 to decrypt messages created by the Germans' Enigma machine.

Apologies for my pedantry - just can't help it.....

[ljguitar]

Originally Posted by Taylor814

Has anyone tried to calibrate their hygrometer by the salt method? I recently purchased an Oasis OH2 and Googled how to calibrate it. Cigar aficionados seem more obsessed with humidity control than guitarists so I found a number of essentially the same procedures for calibrating humidor hygrometers. I combined 4 tsp table salt with 1 tsp distilled water in a small open container, and then placed that and the hygrometer in a sealed sandwich bag for 24 hrs. Supposedly the internal RH will be exactly 75% under these conditions. However, the RH reading on the Oasis (as well as a decades old hygrometer from Radio Shack) appeared to both equilibrate at 65%. So either the method failed to produce the proper RH or both hygrometers are reading exactly 10% too low. That seems excessive. Anybody actually try this successfully? I know that Boveda sells a calibration kit, which I assume uses something similar, but I thought this would be easier and faster. Perhaps not...

H T814

I've seen people throw away perfectly good hygrometers because they 'failed' the salt test. And I've seen debates by 'experts' (self-proclaimed) use amounts of salt and water from a cap-of-a-water-bottle to ½ cup and everything from damp salt to a full-on-slurry). I've seen them suggest they be put in sealed hard containers and zip-lock-bags. The point is nobody agrees on which salt test is the 'right' or 'best', and I suggest you don't need a salt test to keep guitars safe.

Most inexpensive hygrometers cannot be adjusted. The only adjustment any of ours ever needed was either a new battery if it got wonky, or pull the battery and let it sit over night and then put new batteries in it the next day. And if it was still wonky, we pitched it and bought a new one at Walmart for under $15.

First of all, we only need to know the guitar is between 35-45% to be safe, and a $11to $14 hygrometer from Amazon will let you know that. We have three hygrometers in our house (in different rooms) and they are all within 3-5 points of each other most of the time. Close enough for me (as I never let the humidity drop lower than 35% on the lowest registering one). And many people don't realize that the humidity in the kitchen may not match the humidity in the guitar room.

Guitars don't experience dangerously low humidity side-effects overnight, or in an evening of playing at the local open mic, or church on Sunday or a camping trip. They go off the rails if exposed to extremes for weeks and months (unless you are sticking them in a kiln).

Back in the day (when I was a kid) people tossed ½ apple or ½ potato in the pick drawer, and when it dried out they replaced it. That's a little primitive for me.

As I got older, Dampit (the company) bundled a little chemically treated chart with different colored bars on it with each guitar humidifier which told you when you were in the danger zone or OK, (and they still make and sell them, and they still works just fine).

Only when I got older and starting to listen to people who loved to speculate (instead of builders who make guitars) did I start to worry. And I learned not to listen to the guys at the music store - especially the ones who sell humidifying systems - they are out to make money by selling us on the idea of unsafe systems.

I'm not advocating going without humidification, just suggesting moderation in measuring is beyond safe (unless you own a 300 year old Stradavarius guitar).

Hope this stirs the pot a little…

[Shuksan]

Calibrating to 75% RH with NaCl is not that useful for a hygrometer used for determining humidity for keeping your guitars happy. As mentioned above, you want to use potassium carbonate salt which produces an RH value of 43% at equilibrium in a closed container.

I recommend reading this thread on the Australian/New Zealand Luthiers Forum for a good "how to". http://www.anzlf.com/viewtopic.php?t=6123

I use this method to annually calibrate the two digital hygrometers I have in my shop where I build guitars. It works. Both of the hygrometers I have check out with readings of 43% and 44% using this method.

[BradleyS]

Originally Posted by Tim McKnight

It may be helpful to search Sling Cyclometer method.

I agree. I use a sling psychrometer and then adjust the hygrometer reading (+/-) .

[Mandobart]

The commonly performed "salt check" is a single point calibration check. A real calibration sets the instrument zero, span, linearity (if applicable) and confirms tolerance throughout the indicating range using 5 cardinal points. This provides confidence that the instrument is providing accurate readings throughout its range. Calibration needs to be repeated at some frequency since all measuring instruments drift over time. Calibration returns the instrument to standard performance and accuracy by actually adjusting the response from sensing element to output indication/control function.

All a single point cal check tells you is how accurate the instrument reading is at one point (75% RH for the standard salt test). There is no reason to expect the same accuracy throughout the instrument's range. There is no way in most consumer electronics to adjust the instrument output if the reading differs from the standard. If you keep your relative humidity around 75% then this cal check may be helpful. If you prefer an RH of 40 - 50% you still don't know how accurate your hygrometer is in that range. It would be incorrect to assume that if your hygrometer read 78% in the salt check you should then subtract 3% from all readings throughout its range - you don't know enough about the instrument's circuitry, designed response, linearity, quality of your salt solution, etc. to make that assumption.

Here is a simpler way to do a single point cal check - I checked my Extech hygrometer by putting it outside. Per my phone local weather app and weather underground the RH was 64%. The Extech read 62%. Being as reported weather data usually comes from a local airport which is ~4 miles from me and reported no more frequently than hourly, I consider this reasonably accurate. This can be repeated over a range of actual outdoor RH values to get a better idea of how your hygrometer performs.

[mercy]

You only need to know one point and that is when it is dangerous for your guitar. That varies depending on what RH your guitar was built at. I think a lot of factory guitars are built at 50% so you need to know when its 40%. 35 is dangerous but when it reads 40 you can start doing something about it. If your guitar is built at 40 as mine are I dont have to get concerned until the hygro reads 30%. Different salts give diffferent saturations but table salt is a worthless test.

[Taylor814]

Thanks to everyone for the replies. I repeated in a Tupperware box and the overnight RH was about 73%, so it appears there was an issue with the ziplock bags I initially used. I fully agree that a single point calibration at a point far from interest is not very useful, and although the Oasis unit does allow resetting the reading based on calibration, changing 73 to 75 tells me nothing about the effect on accuracy at 45%, which as several pointed out, is the number that matters. The only reason I engaged in this silly exercise in the first place is that I recently bought an OH2 with the expectation that it included a Boveda test kit, but that only applied if I had purchased directly from Oasis, which I did not. That sparked my interest, since I had never considered calibration checking my hygrometers in the past. That turned into a series of Google searches, a post in AGF and a day of my life I'll never get back. Thanks again for the responses.

[Shuksan]

Originally Posted by Mandobart

Here is a simpler way to do a single point cal check - I checked my Extech hygrometer by putting it outside. Per my phone local weather app and weather underground the RH was 64%. The Extech read 62%. Being as reported weather data usually comes from a local airport which is ~4 miles from me and reported no more frequently than hourly, I consider this reasonably accurate. This can be repeated over a range of actual outdoor RH values to get a better idea of how your hygrometer performs.

Assuming that the humidity measured at a location miles away from your location is the same or very close to the same as at your location is a pretty big assumption. Maybe one could get away with that in some parts of the country, but I bet that RH values vary significantly over short distances in many geographies, similar to local temperature variation. For a single-point check in the humidity range that matters for guitars, a potassium carbonate salt test in a closed container doesn't involve making assumptions.

[fazool]

I have and had good luck

(deep in HERE you will find info/cautions on the salt test)

[Inyo]

Originally Posted by Mandobart

Here is a simpler way to do a single point cal check - I checked my Extech hygrometer by putting it outside. Per my phone local weather app and weather underground the RH was 64%. The Extech read 62%.

Way too complicated a procedure.

Just throw your hygrometer into a bucket of water. When the Relative Humidity reading reaches 100 percent, you know the hygrometer is calibrated accurately.