What I would do is get a good medical grade thermometer. Either digital or bulb kind. I use both. (have a 40 year old bulb mercury rectal thermometer) Place the medical thermometer in a large cup or bowl of water, suspended so it does not touch sides or bottom of the container. Use that thermometer to ensure that the temperature of the water in the container is 100*F. Then add an easy to read bulb type of thermometer, such as this one: https://www.zoro.com/taylor-analog-thermometer-40-to-120-degree-f-5132fc/i/G1345976/
Ideally, you should use a thermometer that will give readings in tenths of a degree.
Allow that thermometer to level out at the temp of your water. Now compare the two. If your medical reads 100*, but your large thermometer reads 98, you know it is reading 2* low. So, in order to be at 100*, you need that thermometer to read 102*. You can then place this thermometer in your incubator, plug the bator in, and compare the reading of the known accuracy to what the bator thermometer reads.
When you test your bator prior to setting eggs, take the time to test all areas of the bator. Even with a fan, you will be surprised that there are low and high temp spots that you need to be aware of in order to have a successful hatch. I take 2 days or longer working with the temps in my bators, using filled water bottles as heat sinks before committing eggs to them.
I use a wet and dry bulb. You calibrate them both in ice water and boiling water so you know they're accurate (I use a dial meat thermometer). Then you cut the hem out of a cotton t-shirt, wet it, and put it over the stem of one of the thermometers and put the loose end into the water reservoir or a reservoir of it's own, like a shot glass full of water. Once the incubator is at 100 dry, I check the wet. I like my wet to be at 88-89. https://goo.gl/images/q8EDxf - there's a chart.
It's a bit of an involved process, but I know it's accurate, so I go with that.
OOPS, my bad... I did not read that you were talking HYGROMETER!!! I'd do the salt test on a separate one, then put it in the bator, and do a comparison between the known one which you have calibrated, and the one that is built into your bator.
BTW: common misconception between these two instruments.
an instrument for determining the specific gravity of a liquid, commonly consisting of a graduated tube weighted to float upright in the liquid whose specific gravity is being measured. ... The hydrometer is used for measuring the specific gravity of water and other liquids.
The true hygrometer definition is an instrument which measures the water vapor of the atmosphere. This is also referred to as the “relative humidity” in the environment. All weather meters that measure relative humidity are also known as hygrometers.