World Coins

Working with hygrometers

Conservators use saturated salt solutions to check the accuracy of a hygrometer. Shown is a calibration chamber. The sodium chloride saturated salt solution is in the plastic container next to the Arten thermohygrometer (small white rectangular device at right).

Image courtesy of Susan L. Maltby.

In my Sept. 24 column, I responded to a collector’s query regarding small bugs that had infested his safe. The bugs were psocids, commonly referred to as book lice. Their presence indicates high relative humidity.

Lowering the RH is the best solution for dealing with psocids.

The collector had silica gel packets in his safe, likely to lower the RH. I recommended that he determine the RH that the gel was conditioned to by placing it in a plastic bag along with a hygrometer that had been recently recalibrated.

You might wonder how you can check the calibration of a hygrometer.

Devices such as hygrometers, recording hygrothermographs and computer data loggers need to be recalibrated from time to time.

Checking their accuracy is fairly straightforward. Conservators use saturated salt solutions to do this. Saturated salt solutions create RH levels specific to the salt used.

Commonly used salts include lithium chloride (11 percent RH), magnesium chloride (33 percent) and sodium chloride (75 percent). Of the three, sodium chloride — table salt — is the one you are most likely to have at home.

Here is how to create your own calibration chamber.

First, make a saturated salt solution.

To do this, take a small dish or plastic food container. Add distilled water to the dish. Then, while stirring, continue to add salt until the salt no longer dissolves.

Once the salt no longer goes into solution, you have a saturated salt solution. The solution will look slushy.

Now, place the solution and hygrometer in a resealable plastic bag like the one illustrated above.

Seal the bag and set it aside for about 24 hours.

Saturated salt solutions are affected by changes in temperature. As a result, it is recommended that you put your calibration chamber in an area with a stable temperature such as a closed cabinet.

After 24 hours, check the hygrometer, reading it through the clear bag. Do not open the bag as this will affect the microclimate created inside the bag with the salt solution.

The hygrometer should be reading 75 percent RH. If it is not, you know it needs to be recalibrated.

The device you see illustrated with this column is a thermohygrometer from Arten. It measures temperature and RH.

One of the advantages to these devices is that you can recalibrate them yourself by turning two screws, one for temperature and one for relative humidity.

Many hygrometers must be sent back to the manufacturer for recalibration. This can be expensive, often costing close to the cost of a new hygrometer.

Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.

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