pH controllers monitor potential of hydrogen (pH) levels and initiate controller functions. They provide rapid on/off control of dosing and are used in applications such as food processing, hydroponics, water purification, and waste control. pH controllers differ in terms of control type and provide limit control, linear control, or non-linear control. Several linear control styles are available. Feed-forward or compensation devices provide direct control from a reference signal and are used in either open-loop configurations or with proportional integral derivative control (PID). Automatic reset, bias, or offset control provides proportional, integral control and is used to eliminate steady-state errors. Proportional derivative control increases the speed of response, but is noisy and may seem unstable. Proportional control produces a control signal that is proportional to the error between the reference and feedback signals. Some pH controllers include minimum and maximum limits points. Others allow adjustments to deadband or hysteresis around limit points.
pH controllers differ in terms of performance specifications, features, and discrete input/ output (I/O) options. Performance specifications include the number of inputs, number of control outputs, number of control/feedback loops, update rate or bandwidth, and accuracy. Multi-function pH controllers have multiple modes which may or may not use different inputs and outputs. Multiple linked loops are designed to improve control performance and/or stability. Analog voltages, current loops, and switch outputs are common control signal outputs. Voltages and current loops are used as sensor excitation supplies. Some pH controllers include ambient temperature compensation, self-tuning functions, programmable set points, built-in alarms, or visual indicators. Others are waterproof or washdown capable. pH controllers that include signal computation functions or filters are also available. In terms of I/O options, important considerations include number of digital inputs, number of digital outputs, total digital channels, transistor-transistor logic (TTL) capability, and high power switching.
Selecting pH controllers requires an analysis of user interface options. There are three basic types of displays. Analog meters or gauges use needles or simple visual indicators. Digital readouts are numerical or application-specific displays. Video display terminals (VDT) include cathode ray tubes (CRT) and flat panel displays (FPD). User controls for analog front panels consist of potentiometers, dials, and switches for either setup or operational control. Digital front panels allow users to setup or program pH controllers with digital keypads or menus. Devices that interface to personal computers (PCs) via serial or parallel connections are also available. Some pH controllers include an integral chart recorder for data tracking. Strip charts, circular charts, and paperless chart recorders are commonly available.
There are many network protocols for pH controllers. Examples include ARCNET, AS-i, Beckhoff I/O, controller area network bus (CANbus), DeviceNet, Ethernet, general-purpose interface bus (GPIB), FOUNDATION Fieldbus, IEEE P1451, INTERBUS-S®, Sensoplex®, Seriplex, PROFIBUS®, and smart distributed system (SDS). INTERBUS-S is a registered trademark of Phoenix Contact GmbH & Co. Sensoplex is a registered trademark of Hans Turck GmbH & Co. PROFIBUS is a registered trademark of PROFIBUS International.
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