A question we often get from customers who are using our dehumidifier is “Why isn’t it working?” or “Why hasn’t it stopped running?”. More often than not, they have a humidity meter in their hand that’s reading at a relative humidity (RH) level that indicates that the dehumidifier should have either stopped running already, or should still be running but isn’t.
The real question is: Which is more accurate – the dehumidifier’s humidity sensor, or your humidity meter?
This post aims to explain why your dehumidifier and your humidity meter are reading the humidity in the same room differently and disagreeing with each other.
Our Ionmax dehumidifiers have built-in humidistats, which enables it to stop running once it reaches its target RH level, and to start running back again once the RH level goes back up.
This works the same as a thermostat on a radiator turning the radiator on or off as the temperature goes up or down.
So customers are correct in thinking that their dehumidifier will turn on and off at some point.
However, measuring relative humidity is not as simple as measuring temperature. Here’s why:
1. Accuracy range
Thermometers are fairly simple measuring tools and can usually measure within an accuracy range of +/-0.5°C and remain stable for several years.
On the other hand, relative humidity sensors struggle to measure better than +/- 3%RH and are susceptible to drift over time.
With that accuracy range, it means that if the reading on an RH meter says 45%RH, the actual RH level would be somewhere between 42 and 48% RH.
2. Drift over time
As mentioned, RH sensors tend to drift over time, much like how an analog watch may lose its accuracy over time. The potential loss of accuracy every year is called a drift, something that RH meters, like watches, suffer from.
When an RH meter leaves the factory, it will be set up to read accurately within its specification. However over time, it will drift away from this, usually at a rate of 2 to 5% RH per year.
Manufacturers will expect their users to either calibrate their humidity yearly, or replace them every few years. But let’s be honest… domestic users will usually not bother to do that, or even know of the importance of calibration.
So with the accuracy range and the RH meter’s drift over time, it’s easy to see how measuring RH with a meter can be highly inaccurate. RH meters are even less accurate if you’re using a cheap one, such as digital weather stations and alarm clocks that can be easily purchased online or in-store.
Note that the term “Relative Humidity” is used because they are relative to temperature.
So, if you are measuring RH in a room that is at 22°C and 50% RH without the influence of any dehumidifier, humidifier, or human, if the temperature drops to 21°C, the RH will rise to 55% RH on its own.
If we were at extremes of RH at the top or bottom of the scale, then the RH would move by 5 or 7%RH with every movement of 1°C.
This movement is always inverse – as the temperature increases, the RH will fall, and vice versa.
The environment factor is why it’s important for you to not put your humidity meter or dehumidifier in front of a heat source like a radiator or a window. And, if you have your humidity meter on one side of the room and your dehumidifier on the other side of the room, then they could be reading different humidity levels just because of their positions in the room.
Other factors include drafts, someone being close by (we give off heat and moisture; try blowing on your humidity meter to see what happens!), moisture from sinks, the kitchen, bathrooms, drying laundry, etc., and you can see there are a lot of factors that can influence a RH reading.
Putting the humidity meter above the dehumidifier is a bad idea, too. Air coming out of the dehumidifier is warm and dry, especially from desiccant dehumidifiers, and this would give your meter a false reading. The air above our Ionmax desiccant dehumidifiers, for example, would be about 2°C warmer than room temperature and range at about 40% RH (depending on the setting), so anything in its air flow will read the RH level of the air it is emitting, not of the whole room.
5. Speed of reaction
Speed of reaction is another problem. A dehumidifier would have its own fan that draw air across its sensor to help it measure the latest room reading and spot fluctuations, whereas the humidity meter does not benefit from this and might be suffering from a lack of airflow over its sensor.
The humidity sensor might also be quite slow to react or only measure in steps of 5%RH. To test out your RH meter, try blowing over the sensor – how quickly does it react and how long does it take it to settle back down? If you run it through this cycle a few times, does it settle back down to the original reading?
So another important question comes to mind: how does a dehumidifier actually measure the room conditions?
Dehumidifiers have small electronic sensors behind the filter where the air comes into the dehumidifier from the room.
This is the best and most sensible spot for them to sample the air.
Dehumidifiers also are deliberately designed to not be more accurate than +/- 5%RH because you wouldn’t want them to be turning themselves on or off too frequently if the RH is hovering around the target RH level.
Avoiding this sort of on/off/on/off control helps to make the dehumidifier last longer, too.
So there you have it! That’s why your humidity meter reading and the dehumidifier performance does not always match each other.
Neither one is the more accurate one – but because so many factors come into play, it is not possible to base the dehumidifier’s performance on the humidity meter’s reading.
The best thing to do is to let the dehumidifier do its job based on its own built-in humidistat that is designed to help it operate.
We hope this post has helped you better understand your dehumidifier and how it works.
If you’re concerned about the performance of your Ionmax dehumidifier, just give us a call or email us to talk to us!
Disclaimer: The information provided in this article is for general reference only. Please seek advice from professionals according to your needs.