Ambient Weather Sensor Humidity Calibration, Saturated Salt Test

Overview

The saturated salt solution test is an easy method for testing the accuracy of your humidity sensor. Most electronic sensors are accurate from +/- 3 to 5% humidity, and some weather stations or weather centers can be calibrated from the display console by applying a bias.

First a discussion about humidity accuracy and drift, please visit the FAQs at the bottom of this page.

Calibration Test Procedure

1. Mix common table salt with tap water until you have a completely saturated solution with no standing water.
2. Place about 1 inch of the saturated salt solution in an open container or bowl that is approximately 4 inches high. The 3 inches above the solution will contain air with a predictable humidity value. You can use a cut away container as shown in the figure below. One edge of the container is cut slightly lower to rest the sensor on the lip.
3. Place this container and the sensors in a waterproof zip lock plastic bag, as shown in the figure below. Leave one of the corners open and remove all of the remaining air with a straw. Close the air tight bag.
4. Leave the sensor in the bag for four days or 96 hours to completely stabilize.
5. The sensor should read 75% plus or minus the sensors rated margin of error. For example, if the sensor is rated at +/- 5%, the display should read between 70 and 80% relative humidity.
6. If the sensor is adjustable or calibrateable, set the humidity on the display to 75%.
7. Repeat this procedure once per year to account for electronic drift (sensor contamination).

The image above show a table salt slurry of 1″ height mixed in a container about 3″ tall. This solution is then placed in a plastic bag with the sensor.

Frequently Asked Questions

1. My humidity reads different than an official weather station, like National Weather Service, Weather Channel, etc. Why is that?
   This is a poor method for comparing your humidity since you are in a different location and the reporting station may update only once per hour. In addition, sensor placement is very important. You will find the humidity on the north side of your house (mostly shade) is much different than the south side of your house (mostly sun). In addition, grass, trees, structures and houses and have a large impact on humidity. The National Weather Service requires very strict guidelines in installing sensors. However, they do not always follow their own guidelines. SurfaceStations.orghas done a wonderful job of documenting the poor quality of official NOAA weather stations.Again, compare to a psychrometer or calibration kit (see above) in a shaded, well controlled area, like the inside of your house.

2. Why do digital hygrometers have a margin of error  ( +/- 5%)  ? If they are more expensive than a dial hygrometer, shouldn’t they be more accurate?
   Digital hygrometers use a humidity sensitive capacitor. The electronics measure this capacitance and it is converted to a humidity. All resistors have small variations which result in variations in humidity. It is impossible to integrate a bi-metal hygrometer (dial hygrometer) into a digital device.

3. Can the humidity sensor be calibrated?
   Only a few weather stations have calibration capability. They tend to be higher end weather stations with computer connectivity.

4. At the humidity extremes, my sensor is less accurate. Why is that?
   There is a linear relationship between capacitance and humidity within a specified range, and then becomes non-linear at the extremes. Thus, the accuracy can not be guaranteed at extreme conditions. Also, make sure you use lithium batteries in the remote sensor, which operates better during cold weather conditions.

5. My humidity is slow to respond. Why is that?
   The humidity is inside a housing and the housing has a time constant associated with it.

6. At low humidity, my sensor is innacurate. Why is that?
   Humidity is a very difficult parameter to measure below 20%. Expect larger variations in humidity at low humidity levels. Some weather stations limit humidity measurements to 20% because below this level, the accuracy is out of specification.

7. Will my humidity sensor drift over time?
   Yes, humidity sensors are subject to drift over time. The National Weather Service wil calibrate and/or replace the humidity sensor once per year.

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