Here you can find the sources for the firmware, software utilities, and hardware of OpenHumidistat, a free and open-source humidistat for laboratory-scale humidity control that is affordable and easy to build.

Using OpenHumidistat, you can realise humidity control for your experimental setups for about €500 (and some of your time to build it). OpenHumidistat is designed to provide standalone control of the humidity in a small measurement chamber (~ 10 - 100 mL). It features:

• Fast and precise dynamic response
  • Settling time of ~ 10 s
• Broad attainable humidity range of approximately 10-90% (depending on humidifier efficacy and feed gas conditions)
• Flowrate up to 2 L/min
• High accuracy and good disturbance rejection through closed-loop control
• Easy and intuitive operation
• Excellent versatility and portability: can be applied to a wide range of experimental setups
• Optional monitoring/logging of controller state using Python utility on a PC connected over USB

The design is based around mixing a humid and dry air flow in varying proportions, using proportional solenoid valves and flow sensors to control flow rates. The mixed flow is led into a measurement chamber, which contains a humidity sensor to provide feedback to the controller, to achieve closed-loop humidity control.

There are two versions of the humidistat, the development of which are described in the two papers listed below.

v1	v2 (OpenHumidistat)

The second version features upgraded components including a much more powerful microcontroller and better humidity sensor, performance and stability improvements to the controller itself (cascade PID control using flow sensors), a much fancier interface, and on-the-fly configurability.

If you are interested in building OpenHumidistat, it is strongly recommended to consider the second version. Besides upgrades to the hardware, the article describing it includes comprehensive step-by-step build instructions. PCB designs for the custom circuits and 3D models for the enclosure are also provided for your convenience.

It was found that the heat from the solenoid valves could affect the temperature in the measurement chamber. To alleviate this issue, a (60 mm) fan can be installed in the OpenHumidistat enclosure. Updated CAD files, as well as an updated version of the solenoid driver PCB with a fan header, are available to this end.

The Teensy LC microcontroller board is getting less and less available due to supply issues with its microcontroller chip. Instead, the Teensy 4.0 can be used as a drop-in replacement. It is pin-compatible and an upgrade in terms of specifications compared to the Teensy LC. The firmware has been prepared for compatibility with the Teensy 4.0.

First version, using Arduino Uno, single-loop PID controller, DHT22/AM2302 humidity sensor, and 16x2 character display:

Veldscholte, L.B., Horst, R.J. & de Beer, S.
Design, construction, and testing of an accurate low-cost humidistat for laboratory-scale applications.
Eur. Phys. J. E 44, 48 (2021).
https://doi.org/10.1140/epje/s10189-021-00062-5

Second, improved version, using Teensy LC, cascade PID controller, SHT85 humidity sensor, and 128x64 graphical display:

Veldscholte, L. B., de Beer, S.
OpenHumidistat: Humidity-controlled experiments for everyone.
HardwareX 11, e00288 (2022).
https://doi.org/10.1016/j.ohx.2022.e00288

If you are using OpenHumidistat in your research, it is much appreciated if you cite the relevant paper(s).