Accuracy of salt test calibration

All I know is I have had a Walmart digital running in a salt test since before Xmas along with a couple hygrometers. The digital RH moves more depending on the ambient outside temp. Colder outside the lower the humidity in the test container. The mechanical hygrometers have a little less change but they still change. So, I don't know what ambient temp is best for the salt test. Maybe it is 75 degrees?