A Simple Humidity Verification Test

Since we sell many data logger products that can measure temperature and humidity customers often ask us how they can verify accurate humidity measurements. That’s a good question. Verifying temperature is easy, but humidity is an entirely different matter. Fortunately, ordinary salt possesses a valuable characteristic that we can use to our advantage. It turns out that table salt (preferably non-iodized), properly moistened and maintained in a sealed environment over time, will generate a predictable humidity of 75% over a very wide room temperature range. Here’s how it’s done:

What you’ll need:
(1) zip-lock plastic bag, large enough to comfortably fit a shallow glass container and your data logger
(1) small, shallow open glass container that easily fits in the zip-lock bag
(1) tablespoon table salt (preferably non-iodized)
(1) oz. of distilled water, or less

• Pour the salt into the glass container and add just enough distilled water to make a paste. You’re looking for a consistency similar to a sand and water mixture that you could use to sculpt a sand castle.
• Place the glass container containing the water/salt mixture and the data logger (in its actively logging state) into the zip-lock bag, ensuring that the salt mixture does not come in contact with the loggers.
• Firmly seal the bag making sure that plenty of air is trapped inside it.
• Place the sealed bag in a reasonably controlled temperature environment of about 25 °C, not in the sun or near a HVAC duct.
• Wait eight hours (don’t cheat), remove the logger, and upload the data. Humidity should have moved from ambient to 75% near the end of the eight hours while the bag was still sealed.

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