Last revised: November 4, 2024
Reviewed by: Scott Orlosky, consulting engineer

Humidity controllers monitor and maintain proper humidity levels in environmental test applications, food storage areas, and electronic equipment rooms. They often include rate indication features and totalizer, data logger, and chart recorder capabilities. Humidity controllers that provide heating and cooling control are also available. Multi-functional products can be used to control thermoelectric, thermistor, or thermocouple heaters; or to control resistance temperature detectors (RTD), resistive heaters, or heating elements.

Typically, suppliers indicate whether humidity controllers are designed for use with liquids, solids, powder, gases, air, or steam. Some humidity controllers have a printed circuit board (PCB) form factor. Others are designed for mounting in a rack, on a DIN rail, or on a wall, chassis, enclosure, or cabinet. Stand-alone process controllers are bench-top or floor-standing units with a full casing or cabinet and an integral interface. 

Features

Humidity controllers use several different control techniques.

• Limit control (off-on, or bang-bang control)
  • Establishes set points or limits that, when reached, send a signal to stop or start a process variable.
• Linear control
  • Matches a variable input signal with a corresponding variable control signal. Signal conditioning, filtering, and amplification are then used to produce the proper output control signal.
• Proportional, integral, and derivative (PID) control
  • Requires real-time system feedback.
• Feedforward control
  • Provides direct-control compensation from the reference signal. This type of control technique can be open-loop or used in conjunction with more advanced PID control.
• Fuzzy logic
  • This is a type of control in which variables can have imprecise values rather than a binary status (completely true or completely false).
• Advanced or nonlinear controls
  • Includes algorithms such as adaptive gain and neural networking. 

Specifications

Specifications for humidity controllers include:

• number of inputs
• number of outputs
• input types
• output types
• number of zones (if applicable)

The number of inputs is the total number of signals sent to the humidity controller. The number of outputs is the sum of all outputs used to control, compensate, or correct the process. Input types for humidity controllers include direct current (DC) voltage, current loops, analog signals from resistors or potentiometers, frequency inputs, and switch or relay inputs. Output types include analog voltage, current loops, switch or relay outputs, and pulses or frequencies.

Some humidity controllers can also send inputs or receive outputs in serial, parallel, Ethernet, or other digital formats that indicate a process variable. Others can send inputs and receive outputs from information converted to an industrial fieldbus protocol such as CANbus, PROFIBUS^®, or SERCOS. PROFIBUS is a registered trademark of PROFIBUS International. 

Humidity controllers differ in terms of user interface features and regulatory compliance. Many products feature a digital front panel or analog components such as knobs, switches, and meters. Computer-programmable, web-enabled, and Ethernet or network-ready humidity controllers are also available. In terms of compliance, devices that are destined for sale in the European marketplace should meet the requirements of the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives from the European Union (EU).

Humidity Controllers FAQs

How do temperature changes affect humidity control?

Temperature changes can significantly affect humidity control, particularly in environments where maintaining specific humidity levels is crucial. Here are some insights into how temperature changes impact humidity control:

Relationship Between Temperature and Relative Humidity (RH)

As air temperature increases, the relative humidity (RH) typically decreases. This is because warmer air can hold more moisture, so the same amount of water vapor results in a lower RH percentage. Conversely, when the temperature decreases, RH increases because cooler air holds less moisture.

Challenges in Maintaining Desired Humidity Levels

A rise in temperature can lead to a significant drop in RH, which poses challenges in achieving and maintaining the desired humidity levels. For instance, a 5ºC increase in temperature can reduce RH from 95% to approximately 67%.

Influence of Environmental Factors

Other factors, such as the construction of the environment and the nature of the load within the controlled space, can also influence humidity control. Materials with high water retention capacities, like bare concrete floors can impede the rapid achievement of desired humidity levels.

What is the relationship between temperature and relative humidity?

Temperature Changes

Humidity levels fluctuate relative to temperature changes. As air temperature increases, relative humidity (RH) typically decreases because warmer air can hold more moisture. Conversely, when the temperature decreases, RH increases since cooler air holds less moisture. This relationship poses challenges in maintaining desired humidity levels, especially when there are significant temperature fluctuations.

Nature of the Load

The nature of the load within the controlled space also influences humidity control. For example, large and dense devices affect how quickly desired humidity levels are achieved. These objects can retain moisture, making it more challenging to adjust humidity levels rapidly.

Additional Measures

To counteract the effects of environmental factors, additional measures may be necessary. These can include introducing humidity downstream of the blower or directly into the controlled environment to maintain the desired RH levels.

How do humidity controllers work in conjunction with HVAC systems?

Humidity controllers continuously monitor the relative humidity (RH) and temperature of the environment. When a change in temperature is detected, the controller adjusts the humidity settings to compensate for the expected change in RH. As noted earlier, as air temperature increases, RH typically decreases, and vice versa.

Humidity controllers are often integrated with HVAC systems to provide a comprehensive climate control solution. The HVAC system regulates temperature, while the humidity controller ensures that the RH is maintained at the desired level. This integration allows for coordinated adjustments to both temperature and humidity, optimizing the overall environmental conditions.

How do factors like construction materials affect HVAC and humidity control systems?

Construction materials with high water retention capacities, such as bare concrete floors, can slow the achievement of desired humidity levels. This is due to the fact that those materials can absorb or release moisture, affecting the overall humidity in the space.

To counteract the effects of high water retention materials, additional measures may be necessary. These can include introducing humidity downstream of the blower or directly into the controlled environment.

These factors highlight the importance of considering the construction materials and the nature of the load within the controlled space when designing and operating HVAC and humidity control systems.

Humidity Controllers Media Gallery

References

GlobalSpec—Air Innovations temperature and humidity control can enhance clean room operations

GlobalSpec—Semiconductor temperature control systems and humidity regulation

Image credit:

OMEGA Engineering, Inc.