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can I trust my hygrometer?
http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=14827		 Page 1 of 2
Author:  J.R. Hunter [ Wed Dec 05, 2007 3:21 pm ]
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I bought a hygrometer at Menard's today. Nothing fancy, just an el cheapo thermometer/hygrometer combo. I'm well aware of the old adage "You get what you pay for", but my cheap Scotch blood got the better of me when it saw the $4.98 price tag. I did notice that they all had the same reading (which was reassuring to my skeptical Irish blood). So perhaps genetics won out over wisdom.  Sometimes it's good to live on the edge!


Anyway, how can you be sure that a hygrometer is working properly unless you have a proven one to compare it against?


Thanks in advance for your help y'all! Until then I'll be hanging on to the receipt ,


J.R.


 
Author:  James Orr [ Wed Dec 05, 2007 3:43 pm ]
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Bingo.
Author:  JohnAbercrombie [ Wed Dec 05, 2007 3:45 pm ]
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The 'salt test' will help you 'calibrate' your hygrometer to be correct at 75% RH, but won't guarantee that it will be accurate at 45%. However, it's a start, I suppose.

BTW, some (many?) digital hygrometers will be damaged by the 'salt test'.

Another idea is to calibrate your hygrometer with a psychrometer (aka wet-bulb thermometer), like Mario Proulx uses:
Mario's Humidity Measuring System

Cheers
John
Author:  James Orr [ Wed Dec 05, 2007 4:00 pm ]
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Mario's method seems fast and easy, but I'm confused by this:

The difference between the dry bulb and the wet bulb gives us the
relative humidity of that room.

By difference, does he mean high temp. - low temp. = humidity? Is he
averaging the two temperatures together and their quotient = humidity?
Author:  burbank [ Wed Dec 05, 2007 4:09 pm ]
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James,

Use two thermometers side by side, one with a wet bulb, the other dry. Take the difference between the two thermometers. That's it.

Author:  David Collins [ Wed Dec 05, 2007 4:34 pm ]
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I've never been a fan of the salt test, because as said above that can
accurately calibrate your hygrometer at the 75% range, but that should by
no means assure you that it will be accurate in the 40% range.

Just set up a wet/dy bulb system. It's easy. It's recommended that you
draw about 600 linear feet per minute, which if you put the bulbs in or in
front of a 2" vacuum hose means it would only have to draw about 50cfm.
A small computer fan works perfect for this.

As far as the cheap hygrometers, they're great to have around to monitor
at a quick glance. Just make sure you check their accuracy once a month
or so. In my experience they work well for a few years, depending on how
dirty your air is. The membrane sensors they use seem to fail as they get
contaminated. I've tried pulling them apart, but there's no good way to
clean them, and then it's just time to buy another.
Author:  David Collins [ Wed Dec 05, 2007 4:44 pm ]
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[QUOTE=James Orr] By difference, does he mean high temp. - low temp. =
humidity? Is he averaging the two temperatures together and their quotient
= humidity? [/QUOTE]

It's some logarithmic formula I've never bothered to learn. I have two
psychrometers, one at home and one at the shop, and they have a nice little
slide rule calculator with them. There are more advanced charts to
compensate for things like barometric pressure, but I don't need accuracy to
within 0.1%.   These psychrometers are essentially the same as Mario's
setup, just built in to a box.


Author:  Rick Turner [ Wed Dec 05, 2007 6:24 pm ]
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The real question is "Can I trust my customers to keep the guitar in the same climate in which I built it?"    We can all keep our shops at 46.75% RH at 71.235 degrees F. plus or minus 2%, and it don't mean sh.. if our customers abuse our creations.   Frankly, I think that as long as you keep your shop between about 40% and 55%, you're in the ballpark far better than your customers are likely to be.

I used to run a workshop at the Telluride Bluegrass Festival on "Instrument Care for Musicians", and one year a guy brought in a couple of really nice custom shop Martins.   On both guitars the tops had gone concave and the strings were laying down on the frets.   At that time of the year, the humidity in Telluride was about 10%; this poor guy lived in East Texas...swamp country...and he'd brought the guitar up from a home where the average humidity was maybe 80% in the summer.   Well, I know that Martin is very careful about humidity, but what good did that do?

Worry more about educating your customers than whether you've got a Radio Shack battery operated unit or a sling hygrometer and a degree in meteorology.
Author:  Hesh [ Wed Dec 05, 2007 11:09 pm ]
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JR you can trust your hygrometer once you understand how your particular one will respond to change and you calibrate it.

There is a great thread in the archives about hygrometers where some "guest" guy takes us through a pictorial sequence of a digital hygrometer and the salt test.  In that case it took longer then a day to reach the desired 75% RH.

This is my set-up and each of the three hygrometers is accurate if you know how they individually respond.  For example, the white digital has been calibrated to be approx. 2% low in the 42 - 48% range that we seek.  The black "Caliber III" that was recommended to me by Tracy is and has been dead-nuts accurate for the entire time that I have had it.  And the Aberon reads typically 1% high.

What is important to me is that all three form a consciouses  once you factor in their individual and known biases.

In any event if there is a change in my shop I have 3 opportunities to know that something is happening.





This morning it is 12 degrees F outside so my place is dry and both humidifiers are running constantly as a result.  My shop is at the low end of the desired spectrum also as a result of the furnace being on more frequently.  And all three of my hygrometers reflect this AND agree with each other regarding the changing conditions.  That is good enough for me and I know that my shop has a stable, acceptable RH.

I am also freezing..........
Author:  Hesh [ Wed Dec 05, 2007 11:17 pm ]
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Here is a link to the archived thread:

http://www.luthiersforum.com/forum/forum_posts.asp?TID=6795& amp;KW=digital+hygrometer#forumTop


Author:  J.R. Hunter [ Thu Dec 06, 2007 12:55 am ]
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Chris, I did the salt test and it was right on the money. By the way it is an analog hygrometer not digital.


Thanks for the education, guys! I'm going to be making Kiwicraigs hair hygrometer and asking Santa for a digital. (And rereading this thread a number of times.)


I'll take the Old Testament on this one ..."Let every matter be established by two or three witnesses."


P.S. I'm thinking the soda cap of salt slurry will make a convenient carrot dipper.(My Scotch blood again not wanting anything to go to waste! 

Author:  Barry Daniels [ Thu Dec 06, 2007 2:04 am ]
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I don't care how many hygrometers you have and how closely they agree with each other. If you don't calibrate them with a sling psychrometer then you are deluding yourself that you have an accurate measure of relative humidity. I learned this lesson when I got a sling phychrometer a couple of years ago and discovered that my two meters were about 12% off.
Author:  J.R. Hunter [ Thu Dec 06, 2007 2:25 am ]
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Barry,


I just checked out Mario's psychrometer on his web site.(Which is an education in itself!)


 We homeschool so it looks like a great science project for the kids. "Hey kids, today you're going to make me- er,ah, make yourselves a psychrometer."


Hesh, Thanks for the pics. I noticed a little balsa airplane hanging out with your hygrometers. Is that for the day your RH is so uncontrollable you'll have something to do?   


J.R.
Author:  Hesh [ Thu Dec 06, 2007 2:33 am ]
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JR Tim McNight who is a member here and a fantastic builder gives those out to kids at trade shows so they can go off and dent the competition's instruments.........
Author:  Grant Goltz [ Thu Dec 06, 2007 3:12 am ]
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I would agree with Rick, plus or minus 5 or 6 percent or so should be fine. Unless you are dealing with flatsawn maple, wood does not move much within a 10 or 12 percent humidity range. It is the extremes beyond that that can get you into trouble.

I would spend more time learning to understand just exactly what to expect for the woods that you are using relative to humidity changes. Read some books, do the math on the wood, and then you will know what may be a problem and what is not. Hoadly is a good place to start.

So lighten up a bit, get it close, understant what is happening with your wood, and not to worry

Grant
Author:  grumpy [ Thu Dec 06, 2007 3:28 am ]
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I don't agree with Rick on this one.

the reason we shoot for the middle ground of 42 -47% is BECAUSE our clients won't live I that perfect world. Because guitar number 777 may ship to Arizona and #778 may go to Louisiana.

 A guitar built at 45% will live relatively happily at 35%, and at 75%. One built at 55% will be happy at 45 to 85. Way different.

And if you're building half your guitars at 40, and the other half at 55, even your own work is all over the map. Wood changes dimension measurably in that range.

Rick builds a very solid instrument, we've learned that. Light, loose tops, but a very ridgid structure otherwise. There's nearly as much carbon fiber in there than in a F1 car <bg> his necks have more truss rods and CF than I could imagine. I'm positive that helps, but Rick, since you're all but obsessed with warranty issues, I'm floored that you take that stand on RH conditions. Now I understand why you build so much stiffness and adjustability into your instruments; they must need it if you allow a range of RH like that in the builds.
Author:  James Orr [ Thu Dec 06, 2007 4:52 am ]
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[QUOTE=burbank] James,

Use two thermometers side by side, one with a wet bulb, the other dry.
Take the difference between the two thermometers. That's it.

[/QUOTE]

Thanks Pat. I understand the method, but I'm not sure what "difference"
means. On his page, Mario gives 21C and 14C as the two temperatures
he got with the thermometers. Convert to Fahrenheit and that gives you
69.8 and 57.2.

The sum difference between 69.8 and 57.2 is 12.6.

The average difference between 69.8 and 57.2 is 63.5.

Mario said the difference gave him 44% relative humidity.

It seems like a great method once I understand the math, but I'm not sure
what kind of difference he was finding to get 44?
Author:  grumpy [ Thu Dec 06, 2007 4:59 am ]
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The logo's a cartoon....

 James, the chart. Follow the chart....
Author:  J.R. Hunter [ Thu Dec 06, 2007 6:03 am ]
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Wow!!


Thanks everybody for your input. I love this forum!


It's kinda like being in school again except:


a.) I'm actually learning something.


b.) If I fall asleep I won't get detention.


C.) I don't have to take a shower with the kid that beat me up at the bus stop that morning!


Thanks again and God bless you all!


J.R.
Author:  WaddyThomson [ Thu Dec 06, 2007 6:15 am ]
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Here is a link to a build your own Sling Psychrometer pdf file that has a simple chart included.  You can get the thermometers add the wick to one, wet it pull some air across them until they are both stable, read the difference in temps, then read the chart to get the relative humidity.  Duck Soup!   Go Here
Author:  James Orr [ Thu Dec 06, 2007 6:53 am ]
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I get it now. Thanks, Mario.
Author:  Michael Dale Payne [ Thu Dec 06, 2007 9:42 am ]
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I am going to blow every-one's except maybe Grumpy's mind here.

The war of 45%-50% RH is as Einstein put it relative

I live in an area of the country where winter and summer the average RH outside is 35% but we have jumps and drops that can be drastic but tend to only last for very short periods , a day or two. Therefore I maintain my shop at 35%-40% Rh. This allows me to deal if my humidifier goes out on me.

Now I ship out all over the country. I look at each clients climatic situation and give instructions how best to acclimate their instrument to their environment. If they follow this is up to them but coming from a dry climate going to a damper to begin with is not as bad as coming from a damp climate to a dry climate. This is when most of the after fabrication nightmares occur.

To me right or wrong, the ideal that a guitar must be kept at 45% is not only ridiculus but also unrealistic. It is important to maintain a relatively constant rh in the shop during fabrication but mostly for our (the builders) convenience to avoid bad brace glue-up, warped plates, bad finishes and such. Once the guitar is in its living environment it will, if proper care is taken, acclimate.
Author:  J.R. Hunter [ Thu Dec 06, 2007 11:17 am ]
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Waddy,


 Thanks for the link. They explained it so well that even I was able to grasp it.


J.R.
Author:  grumpy [ Thu Dec 06, 2007 11:50 am ]
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We're not expecting the completed guitar to live at 45% all the time. Of course that is nonsense. But it's the very fact that it won't live there that we need to -build- there. Rick's example of the Martins at Telluride is perfect. They survived, though the change was incredibly drastic. If they'd have been built at 55% or higher, instead of Martin's predicable 45 or lower, that short stint in Telluride could have proven deadly, instead of a mere nuisance.

I don't understand why this is even being debated.
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