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I attempt to regulate the humidity in my piano room. I use a room humidifier and dehumidifier, and a hygrometer to monitor. I keep the room at 40%. In winter, this generally means the humidifier is on when the heating system is on. Ditto in the summer, with the AC. Overall, I seem to need the dehumidifier less than the humidifier.

Anyway, all appeared well until about 2 months ago, when I began to suspect my hygrometer was malfunctioning, showing values that were way too low. This was causing me to turn up the humidifier more than usual. I ordered 3 new hygrometers, same brand as the original: ThermoPro. The new hygrometers all show the opposite: the humidity is too HIGH, nearly 50%. This is with the humidifier OFF for many days now. The home heating doesn't seem to affect their reading much.

Anyway, these new hygrometers are suggesting that I should keep the dehumidifier on most of the time, to get levels down closer to 40%. I've never seen these new hygrometers register below 45%. I haven't used the humidifier since I got them.

I'm unsure what's going on. Based on past experience and common sense, I feel like I should be humidifying some when the heating system is on. But according to the new hygrometers, the humidity in the room is almost always too high. Any ideas about what might be going on? Don't most rooms need some humidity during winter in New England?

I am in NJ. This spring is very humid here IMHO, with a lot of raining. And because it is warm outside as well, the heater is not as frequent as during winter. So we get a lot of humid ambient air in our house. May be, the same happens in your area.
Regarding hygrometers, the cheap ones are calibrated very poorly, show correct numbers only in near range around their calibration point, and can be extremely inaccurate in measuring low humidity. Also, digital hygrometer reading accuracy would suffer from exhausted battery.

Can you recommend a higher quality hygrometer? I was also wondering about battery replacement on the original one, but I did try that and it didn't change anything. It still shows about 20 points below the new ones. Just FYI: I'm not interested in installing a dampp-chaser.

I've used the Govee brand hygrometers, but shop for any brand that can be calibrated.

I calibrate every 6 months or so using the bags available online. Leave it in the bag for 24hrs to stabilize and then calibrate to the bag reading.

Good luck!

Ron Koval

Without humidification there is no way that indoors humidity can be higher than the outside humidity, when outside air is heated to comfort level of say 68F indoor temperature.

Just look for the current temperature and relative humidity from your weather site.

Open a psychometric chart and find the cross section point for the outside temperature and relative humidity.

Now move horizontally to right until you reach your room temperature and find the relative humidity curve that intersects it. That relative humidity is about your indoors humidity without any humidification.

IMO your old hygrometer might be the correct one.

Buy five cheap Hygrometers and keep the one(s) that read the average?

This might sound a flippant comment, but I think there is a lot of wisdom in it. Years ago I went into a DIY store, they had a cheap hygrometer (with a needle, not digital), of which they had maybe 15 in stock. These covered a wide range of readings. I bought four which read roughly in the centre of the range. I keep them in various positions around the piano room, and they have served me well over the years. My tuner checked them and pronounced that their readings were quite close to his hygrometer.

I take it you are assuming here that the absolute humidity (kg water / kg air) is the same inside and outside. I guess that would be a good assumption.

You can test the hygrometer's accuracy by the potassium carbonate test, which produces an environment that has 43% RH at 77 degrees. Just put your hygrometer in the same environment (i.e. a sealed bag) to see how close it measures.

No Potassium Carbonate on hand? You can use salt which will produce 75% RH at 77 degrees. The downside is your house if probably no where near 75% and a hygrometer that reads accurate at 75% RH may not read accurate at a lower RH indicative of your house.

Here are some accracy tests I did on 7 hygrometers years ago. Household hygrometers suck:

While this is a logical conclusion, its not true in my experience. I got an Ecobee thermostat which reports both indoor humidity and the outdoor humidity. I also have a Thermpro which generally agrees with the Ecobee.

On the Ecobee website, you can graph the dips and rises in indoor and outdoor humidity and the results are surprising. You can have a huge dip from 30% to 7% outside in the desert with only a little dip from 40% to 35% indoors. The building envelope essentially retains yesterday's humidity with only a little leakage.

In a small home, doing the dishes or taking a shower can change humidity for about 2 hours too.. so in that sense a resident is inadvertently humidifying. We also exhale liters of water when we sleep and presumably when we are awake too.

That is why I said without “humidification” but not “humidifier” . It included any type of humidity source, humans, dishes etc.

Humidity sensors go out of calibration pretty fast.

There is something called a sling psychrometer. It's a manual device, so you can't leave it on and recording, but it is a good check on your electronic ones.

I can't recommend from my own experience - I suspect that accurate hygrometers may tend to be expensive. Alway remember you get what you pay for.