Model 420
Multi Channel Gas Monitoring System

SMS Model 420 Multi Channel Gas Monitoring System The Model 420 is a micro processor based controller designed to accept one to four gas detector heads


  • Zener Barrier protection from controller power to the sensor will provide intrinsic safety to remote mounted sensors.
  • True One Man Calibration allowing the monitor to be calibrated at the controller with two easy buttons, or at the sensor, non intrusively with a magnetic pen.
  • Non Volatile Microprocessor designed circuitry, means no backup battery needed to retain calibration data.
  • Dip Switch configurable data entry for easy change of concentration scales and sensor types.
  • Preset, factory adjustable, microprocessor controlled alarm levels to preventing alarm drift and provide higher accuracy.
  • Isolated low impedance 4-20 mA provides stable data transfer from sensor to controller.
  • Independent power supply isolation, from controller to sensor.
  • Independent alarms with optical isolation from controller to alarm relays, provides greater intrinsic safety to remote connected alarms.
  • Three independent optically isolated, solid state 2 amp alarm outputs and 10 amp dry contact relays provides larger alarm capacity.
  • Single real time multi channel display allows viewing of all channels, concentrations, alarm signal status, calibration data without having to change or scroll menus.

Model 420 Controller Specifications

  • Sensor: Electrochem and Metal Oxide
  • Humidity: 0-99%
  • Operation Pressure: Ambient +/- 10%
  • Maximum Circuit Noise: 1//2 PPM (peak to peak)
  • Interference Gases:
  • H2S When exposed to 100 PPM of Interference gas, the amount below will be the expressed reading by the electrochemical cell.
  • Hydrogen < 5 PPM
  • Sulfur Dioxide < 1 PPM
  • Carbon Monoxide < 1 PPM
  • Nitric Oxide 4 PPM
  • Methyl Mercaptan 100 PPM
  • Response: 10 PPM less than 10 seconds with 50 PPM applied


Power Requirements:
120/220 VAC or 12/24 VDC Isolated DC or batter backup voltage can be used for battery backup.

Gas Configuration by entry of binary code on dip switch:
Sulfur Dioxide 0-20 PPM, Phosgene 0-10 PPM, Ozone 0-5 PPM, Oxygen 0-30%, Nitrous Oxide 0-100 PPM, Nitric Dioxide 0-200 PPM, LEL 0-100%, Carbon Monoxide 0-200 PPM, Chlorine Dioxide 0-5 PPM, Chlorine 0-10 PPM, Arsine 0-5 PPM

Sensor Range:
0-5000 ft 4 - 20 ma transmitted digitally corrected.

Display Type:
Alpha - Numeric constant reading all channels, Back lighted. Contrast adjustable.

Latching / Non Latching Alarms:
2 solid state alarms, low and hi 2 amps max. 3 - 10 amp dry contact relays for remote use, 1 - 10 amp fail relay (dry contact).

Non Metallic NEMA 4 fiberglass impregnated Polycarbonate viewable window.

Operating Temp:

-30 degrees C to + 95 degrees C.

Performed by one person.



Gas Refineries

Underground Mines

Petro-Chemical Plants

Drilling Rigs

Tank Batteries

Sulfur Recovery

Well Maintenance Rigs

Commercial Laboratories

Blasting Black Powder

Sewage Plants

Paper Mill Refineries

Pulp Mills



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