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As the title says. . . I have two cheapo hygrometers (£15 ish) which display wildly different readings, looking to buy a decent one but under £100. Any suggestions?

Build or buy a psychrometer of some sort and use it to calibrate your cheap hygrometers. I was having concerns about humidity in the shop and the accuracy of my hygrometer so I found a used Psychro-Dyne, a high dollar lab grade psychrometer, on e-bay for like $40 - turns out my cheapo hygrometer was spot on. I checked everything against three other psychrometers ad David Collin's shop as well.

I still pull out the Psychro-Dyne about once a year to make sure my cheapo one is reading correctly.

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Saturated solution of potassium carbonate is an cheap and accurate way of checking calibrations of hydrometers - much less sensitive to temperature measurement errors. That's what I use to check my cheap and cheerful ones. http://www.mandolincafe.com/forum/archive/index.php/t-70693.html?s=e6d209efa46d7faac02b3abf21782ee7

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Thanks guys, think i'll do as Andy says as I won't have to buy anything Colin, funny I was reading that artice 5 mins ago ! 'suppose I could have googled "checking accuracy of hygrometer" but its nice to post once in a while.

Just make sure you have very accurately matched thermometers - one degree out can be 7 degrees error in RH.
I have used both methods.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Mario Proulx posted an easy, cheap, and quick method of measuring RH and all you need is an accurate temperature probe/thermomenter and a drinking glass. You might search for it here but I'm not certain which site the procedure was posted on.

Simple to use this once in a while to check the accuracy of your cheap Hygrometer.

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Formerly known as Adaboy.......

Grumpy did some work on using the dewpoint to determine RH. That discussion should be on here somewhere, the method was pretty simple. All you needed was a thin-walled glass, water, ice, and a pretty good thermometer. Accurate to within a couple of percent.
edit:Daryl beat me to it!

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

Found it:

Mario Proulx posted an accurate way to measure relative humidity over on the MIMF. This is simpler and more straight forward than the the drybulb/wet bulb method (my opinion) and all you need is an acurate thermometer. I can't describe the procedure any better than Mario so here is a link to his post on the MIMF:

http://www.mimf.com/phpbb/viewtopic.php ... 583#p15583

I made up a spreadsheet where you enter the 2 Fairenheit temps and it calculates the realtive humidity so if anyone wants it, go to the KitGuitarForum and you can download it (this site won't allow an Excel Spreadsheet to be uploaded). Here is a link:

http://www.kitguitarsforum.com/board/vi ... ity#p27397

Also, if you want a decently accurate, competitively priced, digital thermometer, here are a couple of suggestions. Both have 0.5C (0.9F) accuracy at room temps. Digital thermometers are notorius for taking forever to settle on the real temperature but these will go from ambient temp to freezing in 5 or 6 seconds. And bonus, these are great food thermometers! They are $19 (cheaper if you buy 2 or more). These would make great bending thermometers except they only go up to 302F (bummer). I own the first one below and batteries last a long time.

http://thermoworks.com/products/low_cost/rt600c.html

http://thermoworks.com/products/low_cost/rt301wa.html

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Formerly known as Adaboy.......

Pretty much all hygrometers have to be calibrated, and the wet/dry bulb process is pretty accurate if you have good thermometers, or use the same one for both tests. Nothing short of $1,000 is going to get you much closer than 3% +/- which is close enough. If you know you are between 40 - 45% you should be good to go. Inexpensive digital hygrometers are a waste of money. Nearly always off by 10% unless you are just lucky, even the ones that claim 1% accuracy.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Not sure how accurate my approach is...perhaps some of you can provide feedback. On several occasions I have gone online when the outside temp is room temperature to see what The Weather Network has posted as RH. I had three cheaper hygrometers outside as well. Two were within 5% which I further calibrated. The other was way off and I don't use it anymore. Now I have those two cheaper units in the shop and as long as their readings are similar I feel fairly confident of the shop RH. Reasonable? Bogus?

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Doug

Don't let fear or common sense stop you from trying to build something

Probably somewhere between, the assumption that online values and actual values is at least a little suspect.
Check them using the dewpoint method in the link above.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

Thanks everyone. You have saved me from the throw money at a problem solution which is rarely ever necessary !

I use two (for good measure) cheapo Acurite hygrometer/thermometers. Salt tested them to and adjusted them at 70% RH and they both read within 5% of each other at low humidity levels.

I think I paid about $5 each for them

I use the same approach.