Q&A for How to Test a Hygrometer

Salty solutions will create a specific atmosphere inside of the bag because of the modified vapor pressure. Just using distilled water, for example, would give you exactly 100% RH, and a saturated lithium chloride solution would give you about 11% RH. Keep in mind that this is true only at room temperature (20 °C), the whole process is temperature-dependent. Table salt (sodium chloride) tends to be very stable in that regard (less than 1 % change between 0 °C and 30 °C).

It doesn't sound like it's broken. More likely is that the humidor is dry.

It tells you that second hygrometer is accurate and your room is at 40% relative humidity. It also tells you need to calibrate the first one.

Rerun the tests. If your hygrometer is "high end" or under warranty or mechanical, it may or may not be possible or worth the cost to have it repaired. Otherwise, if it won't hold calibration it's not to be trusted. Time for a new one. Before you scrap it though, it might be worth checking its accuracy at a different RH and/or temp to see if it's accurate enough to be of use for something else.