In Average annual power use, I mentioned
I will be buying a dehumidifier for our house, to reduce the condensation on the walls. Current Energy Start rated dehumidifiers remove about 1.85 liters of water per kWh used, and I don’t think we need to remove 2775 liters of water a year (7.6 l/day) from our house, so the dehumidifier will add nothing to our electricity bill. Based on reviews (in Consumer Reports and on Amazon), we’re looking at the 30-pint Whynter RPD-321EW Energy Star Portable Dehumidifier, as it has good performance in cool rooms (our house gets quite cool in winter, especially when we’re both at work) and is relatively easy to empty (we don’t have a convenient way to rig up a drain hose).
We bought the dehumidifier and it seems to work ok. The timer is a bit crude, as it does not have a time-of-day clock, but you can set up a delay of up to 24 hours for turning on and turning off. This means that we’ll have to reset the timer daily. That’s not as much of an imposition as one might think, because we have to empty the water daily any way. People running a dehumidifier with a drain hookup might find the need to reset the timer more of a nuisance.
Our biggest disappointment with the dehumidifier is that it is loud. They claim that it is only 53dB, but it seems louder than that to me—unfortunately, I don’t have a sound pressure meter to measure it with. The compressor is fairly quiet, but the fan is loud, which suggests a poorly designed air flow. We can run the dehumidifier in the living room, but not in the bedroom, because of the noise it makes.
I can’t measure the loudness of the dehumidifier in calibrated units, but I could record chunks of sound from it using an electret mic, an amplifier, and the BitScope USB oscilloscope. I then wrote a program using SciPy to take the FFT of each recorded trace, and averaged the absolute values of FFTs for a hundred or so traces:
The noise is “pink” noise, with more low-frequency components than high frequency ones. The 5kHz cutoff is not an artifact of my amplifier, which has fairly flat gain out to about 100kHz, nor of the microphone, which is fairly flat to at least 20 kHz.
We can use a lower sampling rate to zoom in on the lower frequency end of the spectrum:
The 60Hz peak is sharper here, and 120Hz is clearly missing, but the next several harmonics of 60Hz are present. I don’t have an explanation for the dip between 400Hz and 480Hz.
I don’t know whether the spike around 1067Hz (in both plots) is an artifact of my test setup or is present in the sound. I suspect it is present in the sound.
The dehumidifier takes 340 watts to run (about 40W for the fan and 300W for the compressor), and it extracts about 5 pints in 5 hours (somewhat slower than its claimed 30 pints/24 hours, but that is probably based on warmer, wetter air, which is easier to extract water from). We only run for 5 hours a night, because we don’t want to be woken up by the loud beeping that occurs when the 7-pint bucket fills up. (There is no way mentioned in the manual to disable the alarm—my son and I might disassemble the dehumidifier to disconnect the irritating beep.)
We plan to run the dehumidifier only during off-peak hours (at night or weekend days), for about 45 hours a week, which will consume about 800kWh a year. At PG&E off-peak rates, that is around $120 a year, but we had $98 of unused Net-Electric Metering (NEM) credits, and our minimum bill is around $111, so the extra electricity use will make no change in our bill. We will probably want to pay for carbon offsets, though, since we are increasing our carbon footprint by about 0.23MT (so about $2 in carbon offsets).
Filed under: Data acquisition Tagged: BitScope, dehumidifier, solar power 
