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 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 benchtop or floor-standing units with a full casing or cabinet and an integral interface. 

Humidity controllers use several different control techniques. Limit control (off-on control, 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 correspondingly 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 is a type of control in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Advanced or nonlinear controls include algorithms such as adaptive gain and neural networking. 

Specifications for humidity controllers include number of inputs, number of outputs, input types, output types, and 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 which 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 Electronics Equipment (WEEE) directives from the European Union (EU).


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