Accuracy of humidity sensor

Question

I just had a new Carrier Infinity HVAC system installed in my home, which included the Infinity thermostat. I have a couple of old household thermometer/humidity gauges - one mechanical, the other electronic (a Radio Shack unit from perhaps 15 years ago). They don't quite agree on the humidity level in my home. For example, as I write this, Infinity says 45%, Radio Shack says 46%, mechanical says 65%. Subjectively, the air "feels" humid, making me inclined to believe the mechanical gauge over both the Carrier and the Radio Shack despite their close agreement. It matters to me because, among other things, I have solid hardwood floors and don't want to risk ruining them due either excess or inadequate humidity.

So, I have to ask: how accurate is the humidity sensor in the Infinity system i.e. +/- up to how many percentage points could it be in error from the true value?

Answer 1

Digital relative humidity sensors are typically accurate to plus/minus 3% relative humidity throughout the entire 0-100% RH range, but closer to plus/minus 2% at 50% RH. At the bottom are some specs from the datasheet of a typical digital humidity sensor.

The simplest way to calibrate a relative humidity sensor is with table salt and water in a airtight container. Salt water calibration instructions. The basic concept is that an airtight container that has undissolved salt mixed with water at normal room temperatures will come to an equilibrium over time to exactly 75% relative humidity. You use the known 75% RH environment to calibrate your instrument.

I've had experience with mechanical hygrometers for cigar humidors, and I've found that they need to be calibrated but can be pretty accurate after calibration. I would recalibrate your mechanical hygrometer to the average of your two digital readings if you're unable to do the salt and water calibration. It would be difficult to put your thermostat into an airtight container, but maybe your Radio Shack or mechanical units will fit?

Answer 2

You could make a definitive measurement using wet and dry bulb measurements and that measurement should enable you to validate the humidity sensor in your HVAC system.

You may find plenty of advice on how to perform the wet/dry bulb tests, so I'll mention just briefly what's to be done. Get a thermometer. You might consider a lab thermometer, which is only a few bucks on eBay, for instance. You can validate it's calibration if you'd like (out of scope for this answer). You probably should run the fan on continuous for a while, then, near the air intake, swing the thermometer, tied onto a string, in a circle around your head. Goofy, yes, but it's fast. Alternatively, you could just wait. Write down the measurement. Now take a bit of wet paper towel and secure it to the bulb with maybe a rubber band or something. Repeat the goofy swinging motion until the temperature stabilizes. Now write down that measurement. Get on the Internet and type those two values into one of the calculators wet bulb / dry bulb calculators, such as this one. Now you know what the true value is, and will know if your HVAC is accurate or not.