The Exo-Terra Hygrometer monitors terrarium humidity levels. It is easy to read and install. The Hygrometer reads levels for dry, moderate, and humid (0% to 100%) humidity.
Customer Rating: 
Summary: An Inexpesive Hygrometer for Practical Purposes
Comment: I am using three of these small (4 centimetres in diameter), inexpensive hygrometers to measure humidity in closets. I have checked their accuracy with a more expensive hygrometer, and they seem to be registering reasonable values.
The scale of relative humidity between 0 and 100 per cent is marked in 1 per cent intervals, which are quite small, but it is relatively easy to read by 5 and 10 per cent intervals. The scale is also colored from red (dry) through orange, yellow, green, blue, and violet (moist), to make the scale easier to read.
The hygrometer comes with a 2-centimetre dot of material that is sticky on both sides, so that the instrument can easily be attached to any surface and easily removed. This instrument is not at the standard of a meteorological instrument, but for practical purposes that do not require digital precision, it can be useful, at much less cost and size than those other instruments.
Customer Rating: Summary: Uncalibrated
Comment: I bought a set of 5 of these; each one gives a different reading, even if I leave them next to each other for hours.
Customer Rating: Summary: a seriously inaccurate measuring instrument
Comment: this hygrometer when placed beside two other hygrometers of different manufacture and of known accuracy, proved to be consistently off in its measurment by 20%. After disassembling and recalibrating it, it improved to 10% off of actual values, but then was no longer consistent in the degree of error. In other words, get back to the drawing board you designers of this mistake!
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