The answer:Which hygrometer reading? None. ( I would add the 3 readings and divide by 3 to get the average reading.) but in reality you need to calibrate hygrometer to a reference standard.Below is a DIY method which is accurate.
Measuring instruments are calibrated or set to a reference standard before it is sold but given the mechanical structure
of analogue hygrometer( those that have needle) and digital type.The digital type require you to know which adjustable trimpot (potentiometer or adjustable resistor to adjust).I have copied the summary of this webpage,you can go there to see the pictures and step by step instructions.
Summary of Steps to Calibrate - for Printing
For needle type you can do an accurate calibration but need time and if you have more than one then you need to duplicate the setup and then average the readings and adjust accordingly.8-12 hours to do a calibration.
How to Calibrate a Hygrometer
From Lianne McLeod, DVM, former About.com Guide
.8 of 8Previous NextPrintable Summary of Steps
.To calibrate a hygrometer you will need:
•1/2 cup table salt
•approximately 1/4 cup water
•coffee cup
•hygrometer
•large re-sealable freezer bag
1. Place 1/2 cup of salt in the coffee cup, and add the water. Stir for a bit to totally saturate the salt (the salt won't dissolve, it will be more like really wet sand).
2. Place the salt/water mix in a re-sealable plastic bag, along with the hygrometer, and seal the bag. Note: make sure none of the salt/water mix comes in direct contact with the hygrometer.
3. Let this bag aside at room temperature for 8-12 hours, in a location where the temperature is fairly constant.
4. After 8-12 hours, check the reading of the hygrometer. It is best to read it while still in the bag.
The relative humidity in the sealed bag with the salt/water mix should be 75 percent (mine read about 72 percent).
5. For adjustable hygrometers, adjust to read 75 percent. You will have to do this very quickly, or remember how much you need to adjust the setting (e.g. mine read 72 percent rather than 75 percent, so I would need to adjust the dial up 3 percentage points).
If yours is not adjustable (like mine), simply make a note of how "off" your hygrometer reads. If it reads below 75 percent, you will need to add the difference to your actual readings. If your hygrometer read above 75 percent on the calibration, you will need to subtract the difference from your actual reading.
In my example: after sitting in the bag, my hygrometer read 72 percent, when it should have read 75 percent -- a difference of 3 percent. I now add 3 percent to the readings I take on the hygrometer (e.g. in a tank) to get the actual relative humidity.
Remember: always give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.
Edit: Additional information:From Wikipedia
http://en.wikipedia.org/wiki/Hygrometer
Difficulty of accurate humidity measurementHumidity measurement is among the more difficult problems in basic meteorology. According to the WMO Guide, "The achievable accuracies [for humidity determination] listed in the table refer to good quality instruments that are well operated and maintained. In practice, these are not easy to achieve." Two thermometers can be compared by immersing them both in an insulated vessel of water (or alcohol, for temperatures below the freezing point of water) and stirring vigorously to minimize temperature variations. A high-quality liquid-in-glass thermometer if handled with care should remain stable for some years. Hygrometers must be calibrated in air, which is a much less effective heat transfer medium than is water, and many types are subject to drift[2] so need regular recalibration. A further difficulty is that most hygrometers sense relative humidity rather than the absolute amount of water present, but
relative humidity is a function of both temperature and absolute moisture content, so small temperature variations within the air in a test chamber will translate into relative humidity variations.
From:
http://www.veriteq.com/download/whitepaper/catching-the-drift.pdf
Hysteresis:
Hysteresis is the tendency of measuring
devices to not return completely to their
original state after a change has been
measured. It’s also a major source of error.
Unfortunately, despite its ubiquity, too few
data sheets include hysteresis as a factor in
their accuracy values.
Unfortunately, if hysteresis appears at all,
it’s often de-emphasized by being placed
far apart from the total accuracy
specification. Hysteresis unmentioned or
disconnected textually from an accuracy
value could be considered product
misrepresentation to a discerning
purchaser.
Calibration by saturated salt solution
http://www.omega.com/temperature/z/pdf/z103.pdf
