Duct Mounted CO2 Transmitter GM20 Duct Mounted CO2 Transmitter GM20
Duct Mounted CO2 Transmitter GM20

Vaisala GMD20 Duct-Mounted CO₂ Transmitter – Discontinued

Vaisala GMD20 Duct-Mounted CO₂ Transmitter – Discontinued

The Vaisala GMD20 Series was designed for carbon-dioxide measurement inside ventilation ducts and demand-controlled ventilation systems.

  • Discontinued legacy product; not recommended for new projects
  • Nominal CO₂ measurement range from 0 to 2000 ppm
  • GMD20 without display and GMD20D with local display
  • 0–20 mA, 4–20 mA and 0–10 V analogue outputs
  • Optional relay or LonWorks® interface on applicable configurations
  • IP65 electronics enclosure for duct installations
  • Use Vaisala GMD110 for new installations

Legacy Duct Carbon-Dioxide Transmitter

The Vaisala GMD20 Series was developed for measuring carbon dioxide directly inside air-handling systems and ventilation ducts. Its compact sensing head projects into the duct, avoiding external sampling tubes and the measurement errors that can result from leaks or incorrect sample-flow direction.

The series used Vaisala CARBOCAP® non-dispersive infrared sensing technology and was commonly installed for demand-controlled ventilation in commercial and institutional buildings. The GMD20 was supplied without a display, while the GMD20D added a local numerical display.

Discontinued model: Do not specify GMD20 or GMD20D for a new installation. For a replacement or new duct CO₂ measurement point, review the Vaisala GMD110 Duct CO₂ Transmitter. Existing wiring, output scaling, duct opening and controller configuration must be checked before replacement.

GMD20 Series Models and Options

Legacy model or optionFunctionImportant note
GMD20 Duct-mounted CO₂ transmitter without display Could be combined with selected optional output modules
GMD20D Duct-mounted CO₂ transmitter with local display Display version weighed approximately 170 g
GMR20 Relay-output option Not used together with the GMD20D display option
GML20 LonWorks® interface with CO₂ signal Legacy building-automation interface; not used with the display option
GMI21 External display and relay option Legacy accessory; verify availability before planning service work

Legacy GMD20 Specifications

SpecificationGMD20 Series
Nominal CO₂ range 0 to 2000 ppm
Other calibrated ranges 0 to 5000, 10,000 or 20,000 ppm
Accuracy ±(2% of range + 2% of reading), including repeatability, non-linearity and calibration uncertainty
Long-term stability Less than ±5% of range over five years
Response time Approximately one minute for 63% response
Warm-up time Approximately one minute; 15 minutes for full specification
Operating temperature −5 to +45 °C
Operating humidity 0 to 85 %RH, non-condensing
Duct air velocity 0 to 10 m/s
Analogue outputs 0–20 mA or 4–20 mA and 0–10 V
Supply voltage Nominal 24 V AC/DC; 18 to 30 V DC
Power consumption Less than 2.5 W
Housing protection IP65 electronics housing
Weight Approximately 140 g for GMD20 and 170 g for GMD20D

Vaisala GMD20 and GMD110 Comparison

Selection pointGMD20 SeriesGMD110
Lifecycle Discontinued legacy transmitter Recommended choice for new Vaisala duct CO₂ installations
Installation Duct-mounted sensing head with IP65 electronics housing Duct-mounted probe with IP65 enclosure
CO₂ range Nominally 0 to 2000 ppm; other ranges required recalibration Measures 0 to 10,000 ppm, with selectable analogue scaling
Accuracy ±(2% of range + 2% of reading) ±40 ppm from 0 to 3000 ppm; ±2% of reading above 3000 ppm
Outputs 0–20 mA, 4–20 mA and 0–10 V; optional relay or LonWorks Configurable analogue output and Modbus RTU
Local display GMD20D display version No integrated display; configuration and diagnostics through supported service tools
Calibration Legacy field-check and service arrangements Traceable calibration certificate included
Configuration Legacy serial service interface and hardware options Vaisala Insight PC software through the service connection
Portable service tool Legacy GM70/GMA70 arrangement Compatible with Indigo80 or MI70 using the correct cable
Product information This page documents installed GMD20 units Explore Vaisala GMD110

Before Replacing an Installed GMD20

GMD110 is the closest Vaisala choice for a new duct-mounted CO₂ measurement point, but it should not be treated as a blind terminal-for-terminal replacement. Review the complete installation before ordering.

  1. Record the full GMD20 or GMD20D model and any installed relay, display or LonWorks option.
  2. Confirm the calibrated CO₂ range and the scaling expected by the controller.
  3. Identify whether the existing signal is voltage, current, relay or LonWorks.
  4. Check the available AC or DC supply voltage and controller input type.
  5. Measure the existing duct opening, insertion arrangement and available service clearance.
  6. Review whether the old display or relay function must be recreated elsewhere.
  7. Confirm whether the replacement will use analogue output or Modbus RTU.
  8. Update the controller scaling, alarm limits and control sequence where required.
  9. Commission the replacement against a suitable reference and verify the ventilation response.

Typical GMD20 Applications

  • Demand-controlled ventilation in commercial buildings
  • Supply-air and return-air CO₂ measurement
  • Air-handling units and ventilation ducts
  • Offices, classrooms, meeting rooms and public buildings
  • Building energy-management systems using analogue signals

Choosing Another CO₂ Measurement Arrangement

Measurement pointSuggested product familyWhy it fits
Ventilation duct Vaisala GMD110 Purpose-built duct CO₂ transmitter with analogue and Modbus options
Occupied room Vaisala GMW80 Series Cost-effective wall-mounted room CO₂ measurement with several output options
Demanding room HVAC Vaisala GMW90 Series Higher-performance room measurement with temperature and optional humidity
High-concentration process Vaisala GMP251 Percentage-level CO₂ probe for incubators, cold storage and demanding processes
Demanding ppm-level process Vaisala GMP252 ppm-level CO₂ probe for demanding HVAC, refrigeration, agriculture and harsh environments

Vaisala GMD20 Replacement Support in Malaysia

Seacom Process Instruments supports users of installed Vaisala GMD20 and GMD20D duct CO₂ transmitters in Malaysia and can assist with migration to the Vaisala GMD110 for ventilation ducts and air-handling systems.

Our team can review the old transmitter model, calibrated range, analogue signal, optional relay or LonWorks module, supply voltage, controller input and physical duct installation. We can then recommend a suitable GMD110 configuration and identify changes needed for wiring, scaling and commissioning.

Replacing a GMD20? Send us a clear photo of the model label and wiring terminals, together with the CO₂ range, output signal, supply voltage, duct dimensions and controller details.

Download

  GM20 Catalogue   (158.16k)


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