Air-O-Swiss 7135 Microbial Ultrasonic Humidifier Brief Description This efficient air humidification technology uses high-frequency vibration to generate a micro-fine mist. A fan blows the mist into the room where it immediately evaporates. The 7135 Ultrasonic humidifier requires little power, has a built-in hygrostat and an output regulator button. A replaceable demineralization cartridge ensures a hygienic, pure water mist, without limescale or mineral residues. When operated with the pre-heating function, the water is heated to 176 F / 80 C before it enters the nebulizer chamber. The mist exits the appliance at a comfortable temperature of 104 F / 40 C. This has the advantage that the room temperature will not fall, and additionally the water can be kept cleaner. The Air-O-Swiss 7135 ultrasonic warm mist humidifier also features Automatic and Sleep modes. Automatic mode maintains ideal humidity you select in relation to room temperature. When the Sleep mode is set, the unit shuts off after eight hours. Ultrasonic humidifiers require little power. A replaceable demineralization cartridge ensures a hygienic, pure water mist, without limescale or mineral residues. Ultrasonic humidifiers have a built-in hygrostat and an output regulator button.
Customer Review: Air-O-Swiss 7135, Good but not good enough…
I bought the Air-O-Swiss believing that for the kind of money I was spending I would be getting the ultimate in humidification technology. Well it turns out that’s true, however there’s one caveat. As others have mentioned here the hygrometer is not very accurate and it reads terribly low. When I compare the hygrometer reading of the 7135 against my other thermo hygrometers it (7135) reads about 15~20% too low. This wouldn’t have been a problem had Air-O-Swiss provided the ability for users to set the humidity setpoint from 0~100%. Instead they’ve provided an absolute lower limit of 40% RH which means in reality you’ll end up driving the room well over 50% when the unit is set for 40%.

I contacted Air-O-Swiss about this and the gentleman on the phone told me that it’s probably the #1 complaint they get about this model and the others which are similar to it. I believe I have figured out the source of the problem and have shared that information with Air-O-Swiss. If you feel like a boring engineering diatribe read on, otherwise I’ve said everything that’s useful for John & Jane Q Consumer.

In my opinion the problem with the unit is as follows. Relative humidity, the parameter controlled by this unit, is as its name implies, relative. The value indicated on the face of the unit is supposed to indicate how close to saturation (i.e. 100% RH) the air is at a given temperature. If the temperature of the air mass goes up while you keep the total amount of water vapor in the air constant the RH% will fall as the temperature climbs. Likewise if you keep the total amount of water vapor in the air constant while lowering the temperature of the air mass the RH% will go up. Stated simply, warm air can hold a lot more water than hot air.

This unit calculates the RH using two electronic sensors which are inside the main housing. One is a thermistor (negative temperature coefficient resistor) whose resistance varies as a function of temperature and which is read by the microprocessor inside of the unit. The other is a hygrometer, probably a capacitive type whose capacitance varies as a function of humidity. Readings are taken from the hygrometer and using the temperature reading from the thermistor a figure is calculated for relative humidity. This works really well so long as the sensors are accurate. We’ll assume the hygrometer is perfectly accurate. However the temperature sensor (thermistor) is not. Unfortunately it has been placed inside of the box next to various electronics and not too far from the heater which warms the water. The result is that the unit corrupts its own sensor readings by heating up the thermistor. You can prove this to yourself by doing the following. Plug the unit in from dead cold and compare the readings against a good thermo/hygrometer. Mine is within a few percent…allow the unit for run for a while and you’ll notice as time goes on it drifts further and further off reading RH% that are lower and lower until they level off at about 15~20% error (on the low side).

The fix for this would have been very simple, remove the thermistor from the internal circuit board and place it on a cord that dangles from the unit. Unfortunately this is not what they’ve done. I’m rather tempted to modify my unit as the over humidification is intolerable. The solution given to me by Air-O-Swiss was unacceptable. They told me to turn it off and on as needed!?!?! The whole point of having these fancy electronics is so I don’t have to…

In all other regards the unit is very good, low noise, warm mist etc…the automatic aspects of it however are terribly flawed as I’ve explained above.

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