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Preferred Instruments

Preferred Instruments manufactures microprocessor based controllers, instruments, and electric actuators for combustion and process applications. Preferred Instruments also manufactures oil tank gauging and leak detection equipment. A major component of the Instrument group sales is from control systems integration and personal computer based Supervisory Control and Data Acquisition systems. Preferred Utilities' industrial factory is located in Danbury, CT

PCC-III Digital Loop Controller

The PCC-III Multiple Loop Controller is a state of the art boiler or multiple loop process controller. The large I/O quantity, integral oxygen sensor, highly visible front panel, intuitive “Blockware”, redundant memories, 4-20 mADC input and outputs, 120 VAC discrete contact inputs, isolated relay outputs, 24 VDC transmitter power supply, the ability to control electric positioners directly and built-in industry standard communications allows the PCC-III to be integrated into complex systems with a minimum number of external components. The PCC-III is a complete control solution for individual boilers or multiple control loops.


The PCC-III digital multiple loop controller is a powerful, expandable process controller that can be function block programmed for a variety of applications. Preferred offers the PCC-III pre-configured for the following applications:

  • Plant master controller
  • Plant master controller with 3 boiler lead/lag control
  • Boiler master (jackshaft control)
  • Boiler master (jackshaft control plus oxygen trim)
  • Boiler master (parallel positioning)
  • Boiler master (parallel positioning plus oxygen trim)
  • Boiler master (full metering combustion control)
  • Boiler master (full metering with oxygen trim)
  • Feedwater control (single, two, or three element)
  • Stand-alone oxygen anaylyzer

Custom configurations are available as well. With PCC3_EDIT you can program the PCC-III to suit your applications.


The PCC-III integrates a flexible, highly visible HMI, up to 31 I/O points (including isolated discrete and analog inputs, relay outputs and Triac and 4-20 mAdc outputs), 24Vdc power supplies and network communications into a single panel mounted NEMA 13 controller.

The PCC-III comes with large numeric and English language displays, pushbuttons and intuitive bar graphs which provide clear process information.  A flexible display allows the operator to view up to 10 different process variables, each with up to a 16 character English language description message. The “first in – first out” annunciator allows the operator to view the order of alarms and acknowledge each separately. Configurable “lights and buttons” allow incorporation of auxiliary functions into the controller without the addition of external hardware.

The PCC-III uses an intuitive “Blockware” configuration language. Functions (AIN, PID, LOALM, F(x), etc) are simply copied into a configuration, and then the control signals are “wired” from block to block.  Preferred’s innovative PC3_Draw™ for MS Windows® uses a graphical, “drag and drop” interface. Blockware can also be edited directly from the PCC-III front panel or the spreadsheet style PC3_Edit™. The PCC-III includes automated setup tools to help reduce commissioning time and improve control system tuning.

Safe Boiler Operation: The controller, when married with proper fuel and combustion air flow regulating elements, assures accurate and repeatable combustion control. Fuel and air flows are matched in order to maintain proper fuel to air ratio at all times. Too little air causes unburned fuel losses and too much air causes excessive stack losses. Improper fuel air ratio can be DANGEROUS. Any combustion project requires the assessment of existing piping, burner and boiler conditions, process measurements, analyzers, linkages, actuators, control valves, control dampers and operational requirements. Preferred’s focus on combustion applications, with thousands of boiler control system installations, brings the required experience to the evaluation, development and commissioning of each project. With its redundant non-volatile memories and “Service Manual” mode, the PCC-III Controller is capable of “self-recovery” in the event of a severe 120 Vac electrical power disturbance or some other occurrence that alters data in its memory. Moreover, an integral optically isolated RS485 Data Highway helps prevent a single controller failure from disrupting an entire

Lower Installed Cost: When considering any controller, one should consider the total “installed” cost. The PCC-III offers the lowest installed cost in the industry. Other controllers routinely require external hardware such as interfacing relays, communication isolators, 24V power supplies, laptops or configurators and possibly extra controllers to manage a specific application. The PCC-III overcomes many of these issues by incorporating these features right into the basic controller. The PCC-III “Z” Option Card, as an example, eliminates the need for an external flue gas excess oxygen transmitter, while standard 120 Vac inputs and 8 Amp relay outputs eliminate the need for interposing relays. While most other controllers are well over 15″ long, the compact PCC-III fits easily into less expensive 8″ deep cabinets or enclosures.

Multiple Loops: Due to its ability to handle several control loops, application of a single PCC-III may allow a reduction in the total number of controllers necessary for an installation. The large 160 function block configuration memory allows any function to be assigned to each block, providing unequalled flexibility and enough power to handle just about any boiler control strategy.

Expandable I/O: The wide variety of PCC-III option boards allows one to expand the I/O capabilities of a given controller to fit any application.  Refer to the specifications page for expansion card possibilities.  Up to 15 analog inputs can be used to support complex control strategies or satisfy data acquisition needs. The PCC-III can be equipped with up to 6 relay outputs. These outputs are rated for up to 8A inductive loads and can be used on most applications without external “helper” relays. Two ZP Oxygen Sensors can be connected to a PCC-III without any external Oxygen transmitter needed. Additionally, the PCC-III offers Triac outputs to drive reversible AC electric actuators, solenoids, horns and other devices. The “G” Triac Output option card includes a “position feedback” potentiometer or 4-20 mAdc inputs for closed loop servo positioning.

Multiple Function Generator F(x) Blocks: The F(x) block is used to “characterize” or “curve fit” an analog signal by allowing each value of the input to be assigned an independent output value. The PCC-IIIs F(x) block allows 11 breakpoints to define the input vs. output profile. The blocks are used to set up feedforward and remote setpoint signals as well as linearize relationships. For example, in combustion air flow control applications, F(x) blocks are used to “characterize” the Air Flow Demand signal to create Air Flow Controller and Oxygen Trim Controller setpoints. The example shown to the right uses separate F(x) blocks to “characterize” different setpoints for gas and oil firing. The PCC-III has the capacity to use as many F(x) functions as are required by the application.

“Learn Mode”: The PCC-III “Learn Mode” feature allows multiple F(x) block input vs. output profiles to be entered simultaneously with a push of a button! During commissioning, the control technician adjusts air and fuel flows manually to establish optimal firing conditions for a particular load point. With the conditions set, the technician simply presses the “Store” button for the PCC-III to store (learn) the current firing rate, oxygen value and burner air flow rate into the appropriate F(x) blocks simultaneously. This results in considerable savings in commissioning time, as well as an optimally “characterized” process.

Boiler Efficiency Function Block: The boiler efficiency function allows real time calculation of the boiler’s efficiency with results displayed on the controller’s front panel, recorder and/or data acquisition system. Boiler efficiency is calculated using the ASME “by losses” method. This function requires inputs of flue gas temperature, combustion (ambient) air temperature, percent oxygen in the flue gas, and percent firing rate to evaluate the efficiency. Not only does this function help establish “on-line” fired equipment efficiency, but it also helps one detect changes in efficiency, thereby alerting operating personnel of potential maintenance concerns before they become a problem. The results of efficiency calculations can also be used in advanced control applications.


  • 160 Block Memory, 18 I/O Points, 3 Additional I/O Card Slots For Available Expansion Up To 31 I/O
  • Highly Visible LED Displays
  • Intuitive “Blockware” Configuration Language
  • Dual Redundant Memory Modules
  • Optically Isolated RS485 Modbus Data Highway
  • 120 VAC Discrete Input, Relay and Triac Output
  • Flush Mounted NEMA 13 Front Panel


Case Size: 7.38″ H X 3.00″ W X 7.75″ D

Enclosure Type: Flush Panel Mounted

Front Panel Size: 8.00″ H X 3.75″ W

Panel Cutout: 7.50″ H X 3.13″ W (+/- .062)

Weight: 6 lbs. (Excluding Option Boards)



Operating Temperature: 32° to 122° F (0° to 50° C)

Storage Temperature: -20° to 150° F (-28° to 65° C)

Humidity Limits: 15 to 95% (Non-Condensing)

Front Panel: NEMA 13/IP65


Accuracy: 0.025% Analog Inputs and Outputs, 70° F

Resolution: 16 Bit Input/16 Bit Output

Microprocessor: 32 Bit, 256k RAM

Execution Cycle: Ten Per Second Non-Volatile

Memory Life: 10.8 – 30 years, Blockware Dependent

Operator Control Panel

Displays: Alphanumeric – 8 Character LED (0.2″) Numeric – 4.5 Digit LED (0.43″)

Bar Graphs: PV & SP – 51 Segment LED (5.1″) Output – 21 Segment LED (2.1″)

Pushbuttons: Membrane, Tactile Feedback

Faceplate: Mylar, Splash Proof

Alarm Annunciator: 10 Point, First Out

Status Indicators: 6 LED, Configurable

User Defined Pushbuttons: 4 Configurable


Language: Function Block Style, 60 functions, 160 Blocks

Security: 4 Password Levels Redundant Memories

User Interface: Faceplate – Fully Front Face Configurable 4 Dedicated EDIT Keys Located Under Hinged Cover

Laptop (Optional): PC3_Edit™ Spread Sheet Based Editor or PC3_Draw™ Graphical, Object-Oriented Editor


Network: Protocol – Modbus (ASCII or RTU mode), Speed – 1200 to 38,400 Baud, Type – RS485, Optically Isolated

Configuration: Speed – 1200 to 38,400 Baud, Type – RS232 With Telephone Modular Handset Connector


Input Power: 120 VAC (+/- 15%), Built In Surge Suppressors

Internal Power Supplies: 24 VDC @ 215 mADC (For 2 Wire Transmitters) 5 Vdc @ 50 mADC (For Potentiometers)

Standard Inputs / Outputs

Analog Inputs: 5, Type: 4 – 20 mADC, 0 – 5 VDC

Analog Outputs: 2, Type: 4-20 mADC, 750 Ohm Load

Discrete Inputs: 5, Type: 120 VAC Opto-Isolated

Discrete Outputs: 6, Type: (2) SPDT relay, 8A, 120V (4) 24 VDC, (Sinking), 100 mA


Supply a microprocessor-based multiple loop controller suitable for flush mounting on the face of a control panel. The controller shall provide for balance less – bump less transfer. In normal operation, this unit receives analog inputs, calculates the output and positions the associated final control element. The operator can manually override this function by selecting the manual mode of operation and adjusting the final control element. Primary operating voltage shall be 120 volts, 60 Hz, single phase. Plug-in field wiring shall allow removal of the controller circuit boards without disconnecting any panel or field wiring from the terminal strips.  The controller shall include all necessary 24 VDC power to supply input and output circuits.

Enclosure: Controllers shall be flush mounted on the control cabinet and shall have NEMA 13 rated (oil/water spray) front panel displays, keyboard, cabinet to case and case to front panel gasketing. Controllers shall be able to operate indefinitely in 120° F locations with no cooling fans. Controller Tuning and configuration menus shall be concealed behind a panel front located door.

Operator Interface: Digital displays and keypad shall be provided on the controller front panel. The controller shall be capable of displaying up to 10 process variables with English language descriptions. Additionally, controllers shall have a ten point English language “first – out” annunciator. Controllers shall have two 50 segment red LED bar graphs to locally display the process variable and setpoint and one 20 segment red LED bar graph to display the output. All operator pushbuttons shall be of the membrane type and shall have tactile feedback. Status indications shall be long life LED’s.

Configuration: Control strategy shall be free form “Blockware” type language. The controller shall be supplied with an onboard library of 61 analog and discrete functions and have a minimum 160 block memory.  Any quantity or combination of function blocks (AIN, PID, LOALM, F(x), etc.) shall be easily copied into a control strategy and “wired” from block to block. The controller shall be capable of calculating real time boiler efficiency based on the ASME “by losses” method.  The calculation must utilize real time inputs of boiler firing rate, flue gas oxygen, flue gas temperature and fuel selected. Two sets of adjustable fuel chemistry data parameters shall be included, and firing rate scaled radiation losses shall be used for maximum accuracy.  Calculations that rely on fixed constants or manually inputted values for these conditions are not acceptable. Fuel/Air ratio curve and Oxygen trim setpoint curve adjustment shall be automated for rapid, error-free burner setup. Only a single operator action shall be required to store commissioning data into multiple characterizer curves for a particular load point. “Examine” and “Block Force” modes shall be provided to allow rapid system troubleshooting.  Each controller shall be completely field configurable from the front panel keypad without the use of external computers or handheld terminals. Configuration changes, tuning, and oxygen trim setpoint curve adjustment shall be accomplished via a keypad on the controller front panel. Each controller shall have several levels of access security to prevent unauthorized configuration. Provide laptop computer, software, and cables if all forms of controller configuration cannot be performed from the controller faceplate.

Input/Output Signal Types: The Controller shall utilize 120 VAC for discrete inputs and outputs to assure system compatibility. Analog inputs shall be 1 – 5V or 4-20 mADC. Analog input and output signals shall be isolated from earth ground for ground loop prevention. The controller shall include optional TRIAC, 24-120 VAC solid state switch output boards to directly interface with electric actuators. Controller I/O quantities shall be expandable with plug-in I/O option cards.

Reliability: Control signals that are shared by multiple boilers (e.g. Plant master demand, and RS485 communications links) shall have signal isolators in each boiler section. This is to prevent equipment or wiring failure in one boiler from shutting down any other boiler.  Each Transmitter and Sensor input channel shall have individual power supply short circuit protection. Configuration and calibration data shall be stored in a non-volatile EEPROM plug-in memory module. In addition, a redundant plug-in backup memory module shall be furnished that will automatically download into the primary memory in the event of primary memory data corruption.  As an alternate to redundant plug-in memory modules, provide all necessary portable computers, software and hardware to allow configuration downloading and archival.

Communication: Each controller shall be equipped with an isolated RS485 communications data highway that communicates in Modbus protocol.  Isolation is required to prevent damage to all controllers on the same RS485 cable should a 120 VAC short occur anywhere in the system. The RS485 protocol shall allow: Auto/Manual mode change, setpoint change, variation of the manual output, sensing and silencing of alarms, change of any configuration parameter (including PID tuning constants), change of timers, etc. Controllers shall not depend on a serial communications link to send sensor data required for control from controller to controller unless the link is fully redundant. Provide all equipment capabilities specified in this paragraph, even if a connecting SCADA system is not included in this project Quality Assurance: The controllers shall be manufactured and labeled in accordance with UL508A (CSA C22.2 #14 for use in Canada). Inspection and labeling shall be supervised by UL or other OSHA approved Nationally Recognized Test Lab (NRTL). Lack of an NRTL certified UL508A wiring methods inspection and labeling will be grounds for controller rejection.