Each of these units is individually recalibrated at the workshop of David Burgess.
They are calibrated using two different saturated salt solutions, producing a
humidity of about 33% and 55% respectively, depending on temperature.
This method has been chosen because it verifies not just the reading at one level,
but also the "rate", and because it isn't prone to operator and equipment errors
inherent in some other methods. While it is generally assumed that the most
accurate method of determining humidity is by using a sling psychrometer
(wet/dry bulb temperatures), a US Army study found an average error of 9 percent
when using them.
Oddly enough, after recalibration, these cheap looking things are much more
accurate than any of the "certified", lab-grade hygrometers he has tested.
They are 2.75x2 inches, and will fit in almost any instrument case.
Accurate measuring range is 30 to 70%, guaranteed to be with 1%.
Unlike most hygrometers, these can be easily recalibrated as they drift over time,
as all hygrometers do. A hygrometer that isn't accurate is worse than useless;
it's a hazard to your instrument!
Due to calibration this item is shipped separately. Please expect an extra week for delivery.