Calibrating Digital Hygrometer by Salt Test Method
A guide to calibrating a digital hygrometer using the salt test method
It’s always a good idea to check the accuracy of your hygrometer. You can use the Salt Test method on both digital and analog hygrometers. Most hygrometers today can either be adjusted or calibrated to an accurate reading. The salt test method won’t fail you, and it’s very easy to do.
MATERIALS NEEDED:
- Small sandwich ziplock baggy
- Bottle cap from 2 liter soda bottle (works best)
- Table salt
- Hygrometer (whichever one you want to test, digital or analog)
Now that you have all of your materials handy, follow these steps and you’ll be on your way to effectively checking the accuracy of your hygrometer.
STEP 1:
Fill bottle cap with standard table salt; fill about 3/4 of the way up.
STEP 2:
Add tap water to the bottle cap to saturate the salt. If you see water floating on top of the salt, you’ve added too much water. Easy fix for this is to grab a paper towel, and soak up all of the excess water. You want more of a slurry consistency of water and salt. Again, if you see water actually floating on top of the salt, soak up the excess with a paper towel.
STEP 3:
Place both hygrometer and bottle cap (with salt/water mixture) inside of a small ziplock baggy, as pictured above. Wait 4 hours and come back for a reading check.
STEP 4:
If your hygrometer is perfectly accurate, it will read 75%. Most hygrometers will be +/- 3 %. If your hygrometer is digital and has a calibration button, follow the directions that it came with to calibrate to 75%. Digital hygrometers have a calibration button you push, while analog hygrometers have a screw which allows you to adjust the needle accordingly.
If your hygrometer is not adjustable, you’ll just have to make a note and remember how far off it is.
That’s all there is to it!
You’ve just calibrated/tested your hygrometer using the famous Salt Test Method.
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notes:
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Relative Humidity
The relative humidity 
Relative humidity is normally expressed as a percentage and is calculated by using the following equation:[1]
Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity is 100 percent, we feel like it's 80 degrees (27 C) out.
People tend to feel most comfortable at a relative humidity of about 45 percent.Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level.
Buildings
When controlling the climate in buildings using HVAC systems the key is to control the relative humidity in a comfortable range - low enough to be comfortable but high enough to avoid problems associated with very dry air.
When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hung on a line or rack dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink causing the paint that covers these surfaces to fracture.
When the temperature is high and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100 percent, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration.
Certain production and technical processes and treatments in factories, laboratories, hospitals and other facilities require specific relative humidity levels to be maintained using humidifiers, dehumidifiers and associated control systems.
What is the ideal humidity for electronics?
So unless you're in a sealed chamber where humidity can be controlled, you might want to consider things like room dehumidifiers or even central air conditioning in the summer to help control the humidity levels. In the winter months, humidity tens to be less in the colder climates and that's part of the reason that humidifiers are installed on furnace systems among other things. Humidity control is more for `creature comfort' than anything else.
But this leads to another problem, which is static electricity when the humidity is too low.That too, can be bad for circuit boards because a static discharge can reach potentials of several thousand volts.
According to CompTIA A+, a safe humidity is between 50 and 80 percent.
Read more: http://wiki.answers.com/Q/What_is_the_ideal_humidity_for_electronics#ixzz1E0kmT3DA
references:
1. wikipedia
2. http://science.howstuffworks.com/dictionary/meteorological-terms/question651.htm
3. http://wiki.answers.com/Q/What_is_the_ideal_humidity_for_electronics
