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?
Best Answer
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?