I wasn't sure where to put this, so I'll place it here.
I was reading Inferalis' post on his poor Monitor who ended up with gout due to low humidity.
I have purchased nearly 30 (yes 30) brands of hygrometers, and this is BY FAR the most reliable, you can find them for ~$20-30 online.
I wanted to let you all know about a little problem many of you may not realize, and may be affecting you!
One of my other hobbies is smoking a pipe (tobacco) and occasionally cigars (don't make this about health stuff, I guarantee I'm healthier than you
(you wouldn't believe the flak i get for referencing that hobby in some forums, so i feel a need to mention it) ... just take it as experience with hygrometers)
I have seen many pictures of enclosures and noticed a trend.. ONE hygrometer!
Why is this bad, you ask? Well, the cigar hobby is much more picky about humidity than you would imagine. the difference between 68% and 72% humidity could be the difference between a cigar that is too dry and has lost its oils (and thus flavor) or a cigar that is COVERED in mold and destroys your entire collection.
Because of this, many cigar smokers utilize multiple hygrometers to monitor different drawers, sections, etc. of their humidors (some are rather large).
People wiser than I, a while ago, realized that their humidors were uneven in humidity and trying to find the problem, decided to place all their hygrometers in one area and see if they might be off. You would be horrified as to how inaccurate some of the "high quality" hygrometers are, never-mind the cheapo ones.
I will use my own experienced from here on out to explain/apply this to reptiles.
I purchased a 5 pack of hygrometers (about $40 each if purchased retail) from a high-end cigar company, CAO. Upon opening the hygrometers, I laid them all out and let them acclimate (they take some time to read a change of environment) and they ranged from 20% humidity to 55% humidity. (it was 51% humidity in the room, measured by an accurate hygrometer) Yup, a 35% disparity between 5 identical, $25 hygrometers. This means
your expensive hygrometer may be as off as 30% or more without you realizing it. 
Luckily, my buddy who has owned a cigar shop for longer than I have been alive showed me his little trick to ensure his hundreds of thousands of dollars in cigar stock don't turn into bad cheese over night.
It's called the salt test, and many good hygrometers (the xikar ones) are sold as "adjustable" and have a "reset" button, which sets the hygrometer to read whatever the current humidity is as 75%. The reason it does this is because of the infamous (around cigar people) "salt test"
What is a salt test?
A salt test is an easy test that basically puts your hygrometer into an isolated 75% RH (relative humidity) environment, in which you can then determine how inaccurate the reading is and adjust accordingly.
How to do the salt test:
First off, grab some materials. Luckily, you should have all of them already in your house.
Small sandwich ziplock baggy
- Bottle cap from 2 liter soda bottle (works best)
- Table salt (sodium chloride)
- Hygrometer (AVOID ANALOGUE, think of them like the "hot rocks" of hygrometers. that's how bad they are!)
STEP 1:
Fill bottle cap with standard table salt; fill about 3/4 of the way up, not too hard!
STEP 2:
Add filtered or distilled water (not tap) to the bottle cap to saturate the salt.
You want more of a slurry consistency of water and salt.
If you see water floating on top of the salt, you’ve added too much water. Easy fix for this is to grab a paper towel, and soak up all of the excess water. This is a bit harder than it sounds, but you'll get it down in a few tries. just keep dumping it out and retrying until you get the right consistency, salt isn't expensive.
STEP 3:
Place both hygrometer and bottle cap (with salt/water mixture) inside of a small ziplock baggy. Note how sloshy the salt mixture looks:
(credit: cigarpass.com)
STEP 4:
WAIT. Wait 24-48 hours actually. I would probably go do something else, 24 hours is a long time to stare at a bag. Depending on the size of your ziplock bag, it can take between 24 and 48 hours for the bag to reach a stable humidity. It should be pretty close in 4-6 hours, but better safe than sorry.
Step 5:
If your hygrometer is perfectly accurate, it will read 75%. If your hygrometer is digital and has a calibration button or adjusting button, follow the directions that it came with to calibrate to 75%. Digital hygrometers have a calibration button you push, while analog hygrometers have a screw which allows you to adjust the needle accordingly. The push are ideal, as its annoying to try to adjust within the bag (as soon as you open it, the humidity will plummet), pushing one button is much easier.
If your hygrometer is not adjustable, you’ll just have to make a note and remember how far off it is. Mark -11% or +5% or whatever with a sharpie on the face so you know.
Now that you have a perfectly calibrated hygrometer, you're set for life, right? WRONG! Your hygrometer will slowly drain the battery (and depending on the model, burn through a battery within a year) and from personal experience, once a battery has reached its mid-way point, the reading will begin to read lower than it should.
Retest your hygrometers IDEALLY every 4 months, at LEAST once a year.
I hope this helps at least one person out there who went out and bought an good-quality hygrometer expecting it to function as advertised, but is actually off and could be causing health problems you don't even know are happening! Your pet will be happy you read this as well!
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