Calibrating Temperature, Hygrometer,
and Barometric Pressure Sensors
Here's how to calibrate these sensors in your home weather station. It requires a bit of explanation but it's really very easy.
Calibrating the temperature:
You need to compare the reading to a thermometer you know to be accurate. The Ambient Weather Wiki says,
"Red Spirit Thermometers are very accurate. You can purchase them at most hardware stores for a few dollars. Here is an example on Amazon:
Remember to place the thermometer in the same location as the sensor and stabilize for a few hours. The sensor should be within the margin of accuracy."
What they don't tell you is that before you do the calibration of your weather station you can, and should calibrate the red spirit thermometer itself. This is easy -- fill a large bowl with ice cubes and crushed ice, and add just enough cold water to connect the cubes and crushed ice. Let that sit for at least 20 minutes until it's at equilibrium. Remove the metal mounting bracket from the red spirit thermometer and push it into the ice water at almost a horizontal angle, so that the bulb at the bottom is submerged and surrounded by ice, but flat enough that you can read the temperature on the thermometer. Watch the temperature go down. The rate of decrease will slow when it gets near 32º - watch it until it stops and write down that temperature. Mine stopped at 33.5º, which means the thermometer reads 1.5º too high. Whatever this value is for your thermometer, is the "offset".
Now you can calibrate your Ambient Weather system. Bring the outdoor sensor indoors. Place the red spirit thermometer next to the sensor, near to the weather system console, let them come to equilibrium for an hour, and look at the thermometer reading versus the indoor and outdoor reading on the weather station. Apply the offset to the thermometer reading and use the calibration process for the weather station to adjust those readings to match the thermometer.
Calibrating the Humidity:
For this you need to get a Hygrometer Calibration Kit like this one, which is very inexpensive:
It is based on a chemical principle that a saturated salt solution inside a closed container, maintains a 75% humidity level. Follow its directions.
Calibrating the Barometric Pressure:
As Ambient Weather recommends, find a professional weather station as close to you as possible and use that for your calibration. Note however, that pressure DROPS by 26 millimeters (mm, about 1 inch) for every additional 1,000 feet above sea level. So if your source is significantly different in elevation than you are, you'll need to add an elevation correction. If you have a GPS this is not difficult - GPS units receive elevation as well as longitude and latitude. So use your GPS to record your elevation where the weather station is, then drive to the location of your calibration source and write down that elevation, and correct your barometric pressure according to 1 inch pressure drop per 1,000 feet increase in elevation. Unless you live in a mountainous area this correction is likely to be insignificant, especially since it is the changes in barometric pressure that are most significant.
Frequently Asked Questions (FAQs)
An Introductory Guide