What are the specs for the Aprilaire 600 humidity sensor?

Question

The manufacturer replied to me that the humidity sensor used in their devices is "proprietary". They didn't even answer my first question: "is the device based on capacitance or resistance?"

In this other stackexchange question I asked about how I might alter the circuit to deliver more humidity than the device does when turned to the maximum setting; the device reads 45% and turns off while my house only gets to 35%. That question presumes a mechanism based on capacitance based sensor, and includes a circuit for how to "fool" the device into seeing a lower humidity than the sensor "sees".

In order to execute on this modification, though, the information I'm asking for in this question would be very helpful. Regardless if the humidity sensor turns out to be resistive or capacitive technology, knowing the specification would allow me to design a circuit modification.

Has anyone been able to find the specifications or do your own test on the humidity sensor component used in Aprilaire humidity controllers?

The sensor on my controller looks like this:

Answer 1

The idea that the sensor was based on capacitance is true, but the idea to "design a circuit modification" will not work, because the output of the sensor is digital.

The sensor is from Sensirion and is known as a SHT11:

The SHT1x digital humidity sensor ... As with every other Sensirion sensor type from the SHTxx humidity sensor family, they are fully calibrated and provide a digital output.

The SHT1x humidity sensor series (including SHT10, SHT11, and SHT15) is Sensirion’s family of surface-mountable relative humidity sensors. The humidity sensors integrate sensor elements plus signal processing on a tiny footprint and provide a fully calibrated digital output. A unique capacitive sensor element is used for measuring relative humidity, while temperature is measured by a band-gap sensor. Its CMOSens® technology guarantees excellent reliability and long-term stability. The humidity sensors are seamlessly coupled to a 14-bit-analog-to-digital converter and a serial interface circuit. This results in superior signal quality, a fast response time, and insensitivity to external disturbances (EMC).

It appears that although the chip is calibrated during manufacturing, there is no possibility of changing the sensor's calibration because it's done in one time programmable memory.

The calibration coefficients are programmed into an OTP memory on the chip. These coefficients are used to internally calibrate the signals from the sensors. The two-wire serial interface and internal voltage regulation allows for easy and fast system integration.

There are published projects that use sensors like this one, and it would probably be possible to connect an additional processor, such as an Arduino, then send a serial signal to the humidity controller that followed the protocol of the chip, but really was coming from a program which altered the digital message. This seems like an awful lot of effort, though.

I suspect that the specs listed above are enough, I haven't covered the serial protocol, but generally, the specifications for the sensor is two wire digital serial communication.

Reference: Sensirion SHT1X Reference Page

Answer 2

I wouldn't hold my breath waiting for the manufacturer to reveal this information. That is not how they do business. Besides the manufacturer does not want your house much over 30% for liability reasons. All kinds of bad things can grow in the condensation left-over from too much humidity. If you want more humidity you can install a common snap switch to over-ride the humidistat control. You will have to control it by hand and if you forget in in the on position, bad things will happen. Be informed that hardwood floors will buckle and mildew grows in an overly humid climate. You could install an off-delay timer in conjunction with the manual control for some protection.