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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

JC-30D Smoke Opacity Monitor

The JC-30D Smoke Opacity Monitoring System assures an accurate measurement of smoke emissions from boilers, incinerators, kilns, and similar sources of imtermittent, visible emissions as well as an alarm contact that energizes when opacity exceeds a predetermined setpoint. The information provided encourages increased emphasis on clean, efficient combustion and effective air pollution control. Optical Sensing Assembly included.


The Preferred Instruments JC-30D Smoke Opacity Monitoring System assures an accurate measurement of the smoke emissions from boilers, incinerators, kilns, and similar sources of intermittent, visible emissions. Typical applications include commercial buildings, schools, hospitals, apartment complexes, and industrial plants. Theinformation provided encourages increased emphasis on clean, efficient combustion and effective air pollution control.

Ordering Information: JC-30D Monitor, 190711 Light Source, 190712 Light Sensor, and two Light Sensor Shields 190713 are required for a complete system.


The JC-30D Smoke Opacity Monitor is a microprocessor-based indicating instrument with a smoke duct mounted optical sensing assembly. The system provides a continuous opacity readout, alarm indication and shutdown capability. The smoke opacity percent is continuously displayed using a highly visible backlit LCD display. An intuitive bar graph display and alarm message provides rapid recognition of combustion conditions. All adjustments can be made directly from the faceplate of the instruments by scrolling through user-friendly, English language menus.   

The Optical Sensing Assembly consists of a Light Source and Light Sensor. These units are mounted on opposite sides of the smoke duct with the light beam focused from the source side to the sensor side. Smoke passing through the light beam reduces the intensity being received by the sensor. A signal is transmitted to the Indicating Instrument which continuously displays the percent smoke opacity and provides the alarm and control functions. This reliable single pass design requires no moving parts or complicated optical devices.   

The Light Source uses a low voltage, pre-focused, sealed beam lamp at a projection angle of 5° to reduce scattered light inaccuracies. The Light Sensor includes a solid state, photopically filtered photoelectric cell. The unit’s built-in optical “bulls eye” and variable blink rate LED assist in alignment verification. Lenses are specially designed for easy cleaning without dismantling the installed assembly.   

Operation: The indicating instrument continuously displays the percent smoke opacity and provides the following alarm and control sequences. Typically the JC-30D alarm relay activates an external alarm bell or horn and the shutdown relay is used to shut down the burner. 

Automatic Calibration: The JC-30D includes a 120 Vac input that monitors the burner fan starter coil and initiates an Automatic Calibration cycle whenever the burner fan stops running (ensuring a clear stack condition). This prevents nuisance burner trips due to dust build-up. “WARNING: CLEAN LENSES NOW” is displayed when the dust level is high. At the completion of the Calibration Cycle, the JC-30D turns off the light source until the fan restarts, extending the life of the bulb. A Calibration Cycle can also be manually initiated from the JC-30D Menu (after a lens cleaning, re-alignment, etc.) 

“Standard” Operating Sequence:

  • Smoke Opacity Exceeds Setpoint: Bar graph blinks and “ALARM” message appears.
  • After 20 Seconds “smoke-puff” Delay: Alarm relay (K1) contact activates, the remote alarm bell or horn is activated. Pressing the Alarm Silence button de-activates (K1) relayand the external bell or horn.
  • Smoke Opacity Continues For 120 Seconds (adjustable): The shutdown relay (K2) activates in manual reset mode, “SHUTDOWN” message appears, and the (K1) relayreactivates. When the smoke opacity condition clears, the JC-30D RESET pushbutton must be pressed to reset the shutdown relay in order for the burner to re-start.
  • Smoke Opacity Returns to Normal: “ALARM” message disappears.

“NYC BAR” Sequence: This sequence is similar to the Standard Operating Sequence (above), but conforms to the New York City D.E.P. Bureau of Air Resources requirements. The (K1) relay activates instantly and the (K2) relay delay is fixed at 120 seconds (not adjustable). The 190712 Light Sensor Alarm Test filter complies with NYC BAR requirements. 

“Wood / Coal” Overfire Air Fan Sequence: This sequence is used with stoker fired boilers. See the Standard Operating Sequence (above) for (K1) relay and Alarm Silence button operation. The (K2) relay is used to reduce opacity by activating an overfire air fan or damper. When opacity has been above setpoint for 15 seconds (adjustable) the overfire air relay (K2) activates and a “FAN ON” message appears. When the smoke clears and drops below setpoint, the (K2) relay remains activated for an additional 180 seconds (adjustable) to help prevent a reoccurrence of excessive smoke.


  • RS485 Modbus Communication
  • 4-20 mADC Retransmission
  • 4 Digit Display, 200 Segment Bar Graph
  • Large LCD Graphic Display
  • Single Pass Design, No Moving Parts
  • Field Adjustable Using English Language Menus
  • Automatic Calibration When Burner Cycles Off
  • Early Warning Message and Relay Contact For Deteriorating Combustion Conditions
  • Burner Safety Shutdown Message and Relay Contact
  • Three Alarm Sequences: Standard, NYC BAR, Wood / Coal Overfire Air Fan


Power Supply: 120 VAC 50/60 Hz, 15 VA

Case Size: 8″H x 3.5″W x 7.5″D

Enclosure Type: NEMA 4 Faceplate, Flush Mounting

Ambient Temperature: +32° to +122° F Ambient

Digital: High Contrast LCD Display 4″ High, 0.5% Resolution Bar Graph


Alarm Setpoints: Adjustable 5-50%

Light Source and Light Sensor

Calibration: Automatic, Off-Line

Spectral Response: PhotopicPeak and mean within 500-600nm

Angle of Projection: <5°

Operational Error: <5%

Span Drift: <2%

Zero Drift: <2%

Calibration Error: <1% (Linearity)

Response Time: <10 Seconds for 95% Change in Opacity

Sighting Distance: 1 to 8 Foot Smoke Duct, 3 to 10 Foot Optical Path Length

Wiring Distance: 500 FT. Max (16 Ga. Wire), 250 ft. Max (20 Ga. Wire)

Sensor Housing: ¾” Pipe Mount


Relay Outputs: Two SPDT Relays, 10 A Resistive, 8 FLA, ½ HP, 120 VAC

Retransmission: 4-20 mADC, 650 Ohm Load Max

Network Output: 1200-38400 Baud, RS485 Modbus, ASCII or RTU


Provide a Smoke Opacity Monitor for each stack. The smoke monitor shall consist of a light source, solid-state optical detector, and microprocessor based alarm and indicating instrument. The instrument shall provide a smoke opacity percent display in engineering units, early deteriorating combustion conditions warning indication, burner Safety Shutdown indication and relay contacts. The instrument shall continuous indicate smoke density on a highly visible backlit LCD display. Provide an integral or separate 4″, 0.5% resolution (minimum) bar graph display in engineering units with visual alarm setpoint indication.  Provide an “Alarm Silence” and “Manual Reset” pushbutton and two 10 ampere relays. The housing shall be panel mountable, fully gasketed with NEMA 4 front face. All adjustments shall be made from the front panel display in engineering units. No external “configurator” or laptop shall be required. The “Standard” Operating Sequence shall be as follows: When smoke density exceeds setpoint, the bar graph shall flash, and an “Alarm” message shall appear. After a 20 second “smoke-puff” delay the “Alarm” relay contact closes the circuit for the remote alarm device. Pressing the “Alarm Silence” pushbutton resets the alarm relay to silence the alarm device. If the smoke density reduces below the setpoint within 120 seconds (adjustable), the bar graph stops flashing and the “Alarm” message disappears. If the smoke density exceeds the setpoint for 120 seconds or more, the “Shutdown” message shall appear, the “Alarm” relay re-closes and manually reset “Shutdown” relay contact open to shut down the violating burner.  Pressing the “Alarm Silence” pushbutton resets the alarm relay to silence the alarm device. When the smoke density then falls to below the setpoint, the “Shutdown” message will remain on and the “Shutdown” relay will remain in the manual reset mode. With smoke opacity cleared, pressing the “Reset” pushbutton will reset the system, the “Shutdown” message will disappear, and the “Shutdown” relay will reset to permit normal burner operation. In order to avoid nuisance burner trips due to dust build-up on the light source and sensor, the instrument must include an automatic, unattended re-calibration cycle whenever the burner fan stops running (ensuring a clear stack condition). The optical sensing unit lenses shall be designed to accommodate regular cleaning without dismantling the installed assembly. Light Source and Detector will sight 100% of the effluent path length. Light Source will utilize a pre-focused sealed beam lamp with a maximum 5 degree projection angle. Photo Detector shall include a photopically filtered, solid state photo cell and alignment verification bull’s eye. The Instrument shall include a RS485 Modbus network interface and a 4-20 mAdc retransmit output to communicate to a future Data Acquisition System (DAS) or Building Automation System (BAS). The Instrument shall be manufactured and labeled in accordance with UL508A requirements (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). The draft control system shall be a Preferred Instruments, Danbury, CT, Model JC-30D