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GM20 catalogue
Duct Mounted CO2 Transmitter GM20
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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 option | Function | Important 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
| Specification | GMD20 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 point | GMD20 Series | GMD110 |
|---|---|---|
| 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.
- Record the full GMD20 or GMD20D model and any installed relay, display or LonWorks option.
- Confirm the calibrated CO₂ range and the scaling expected by the controller.
- Identify whether the existing signal is voltage, current, relay or LonWorks.
- Check the available AC or DC supply voltage and controller input type.
- Measure the existing duct opening, insertion arrangement and available service clearance.
- Review whether the old display or relay function must be recreated elsewhere.
- Confirm whether the replacement will use analogue output or Modbus RTU.
- Update the controller scaling, alarm limits and control sequence where required.
- 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 point | Suggested product family | Why 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.
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The Vaisala GMD110 measures carbon dioxide directly inside ventilation ducts for precise HVAC control and demand-controlled ventilation.
- CO₂ measurement range from 0 to 10,000 ppm
- Accuracy of ±40 ppm from 0 to 3000 ppm
- 4–20 mA, 0–5 V, 0–10 V or Modbus RTU output options
- Analogue scaling of 0–2000, 0–5000 or 0–10,000 ppm
- Fast response with less than two minutes to full specification
- Heated CARBOCAP® sensor head helps prevent condensation
- IP65 enclosure for demanding duct installations
- Traceable calibration certificate included
The Vaisala GMW90 Series provides stable wall-mounted carbon-dioxide and temperature measurement, with optional humidity, for demanding HVAC and building-automation applications.
- Carbon-dioxide measurement from 0 to 5000 ppm
- CO₂ accuracy up to ±(30 ppm + 2% of reading)
- Temperature measurement from −5 to +55 °C
- Optional HUMICAP® relative-humidity measurement from 0 to 95 %RH
- Voltage, current or Modbus RTU output models
- Display and CO₂ status-indicator options
- Field-exchangeable calibrated measurement modules
- Traceable three-point CO₂ calibration certificate included
15 other products in the same category:
The Vaisala Indigo200 Series is the standard transmitter platform for one interchangeable Indigo-compatible smart probe, providing local display and practical automation-system connectivity.
- Indigo201 with three analog outputs and two relays
- Indigo202 with Modbus RTU and two relays
- Colour display with numerical and graphical views
- IP65 enclosure for industrial measurement applications
The Vaisala MGP262 continuously measures methane slip and carbon dioxide in offgas from biomethane-upgrading processes. Its in-situ design provides direct, always-on measurement without a separate sampling system.
- Optimised for low-level methane measurement in CO2-rich offgas
- CH4 measurement range from 0–5 vol-%
- CO2 measurement range from 0–100 vol-%
- Heated optical measurement for demanding and humid process conditions
- Three isolated 4–20 mA outputs and RS-485 Modbus RTU
- Ex-certified construction for direct installation in hazardous process areas
The Vaisala RFL100 uses secure, long-range VaiNet wireless technology to monitor temperature, relative humidity and carbon dioxide with the AP10 Access Point and viewLinc Continuous Monitoring System.
- At least 50 m typical indoor wireless range
- Up to 32 RFL100 data loggers per AP10
- 30 days of local measurement memory
- Approximately 18 months of battery operation for temperature and humidity monitoring
The Vaisala Indigo300 is a rugged industrial transmitter for one Vaisala Indigo-compatible smart probe, providing local measurement display and up to three configurable analog outputs.
Its corrosion-resistant IP66 metal enclosure, flexible probe mounting and interchangeable probe design make Indigo300 suitable for humidity, temperature, dew point, CO2, moisture in oil and other demanding industrial measurements.
The Vaisala MGP261 measures methane, carbon dioxide and humidity directly in raw biogas. Its in-situ design avoids a separate sampling system and helps operators monitor gas quality while protecting downstream equipment.
- Measures CH4, CO2 and water vapour in one probe
- 0–100 vol-% measurement range for methane and carbon dioxide
- Humidity output as 0–25 vol-% H2O or −10 to +60 °C dew point
- Heated optics help prevent condensation in wet biogas
- Three isolated 4–20 mA outputs and RS-485 Modbus RTU
- Ex-certified construction for direct installation in hazardous process areas
The Vaisala GMP231 is an OEM carbon-dioxide probe designed for accurate measurement and permanent wall installation in life-science incubators.
- CO₂ measurement range from 0 to 20%
- Measurement performance optimized around 5% CO₂
- Sensor head withstands heat sterilisation at +180 °C
- At least 120 specified +180 °C sterilisation cycles
- Internal temperature and pressure measurement for compensation
- Heated sensor head helps prevent condensation
- I²C, Vaisala RS-485 protocol and scalable 0–20 mA options
- Four-point traceable CO₂ calibration certificate included
The Vaisala GMP252 is an intelligent standalone probe for stable ppm-level carbon-dioxide measurement in demanding HVAC, refrigeration, agriculture and industrial applications.
- CO₂ measurement range from 0 to 10,000 ppm
- Measurement up to 30,000 ppm with reduced accuracy
- Accuracy of ±40 ppm from 0 to 3000 ppm
- Compensation for temperature, pressure, humidity and oxygen
- Heated sensor head helps prevent condensation
- 0–20 mA, 4–20 mA, 0–5 V, 0–10 V and RS-485 output options
- Standalone Modbus RTU operation or connection to an Indigo transmitter
- Compatible with Vaisala Indigo80 and Insight PC software
The Vaisala GMP251 is an intelligent probe for stable percentage-level carbon-dioxide measurement in incubators, controlled-atmosphere storage and demanding industrial applications.
- CO₂ measurement range from 0 to 20% CO₂
- Accuracy of ±0.1% CO₂ at 5% CO₂
- Compensation for temperature, pressure, humidity and oxygen
- Heated sensor head helps prevent condensation
- 0–20 mA, 4–20 mA, 0–5 V, 0–10 V and RS-485 output options
- Standalone Modbus RTU operation or connection to an Indigo transmitter
- Compatible with Vaisala Indigo80 and Insight PC software
- IP65 probe body with traceable calibration certificate
The Vaisala GMW80 Series provides dependable, cost-efficient room measurement for demand-controlled ventilation and indoor air-quality applications.
- CO₂ measurement ranges up to 2000 or 5000 ppm, depending on model
- Temperature measurement from 0 to +50 °C on selected models
- Optional relative-humidity measurement from 0 to 95 %RH
- Voltage, current, relay, Pt1000 and Modbus RTU options
- Display or colour CO₂ indicator available on selected models
- Stable Vaisala CARBOCAP® technology for continuously occupied buildings
- Quick installation with correct CO₂ readings immediately after start-up
The Vaisala GMP343 is a rugged, high-accuracy carbon-dioxide probe for outdoor monitoring, ecological research, soil respiration, plant-growth chambers and demanding OEM applications.
- Selectable CO₂ ranges from 0–1000 ppm to 0–2%
- Diffusion-aspirated or flow-through sampling models
- Accuracy up to ±(3 ppm + 1% of reading)
- Temperature, pressure, humidity and oxygen compensation
- RS-232, RS-485, 4–20 mA, 0–2.5 V or 0–5 V outputs
- Raw and numerically filtered measurement data
- IP67 probe housing with the cable connected
- Compatible with MI70 for portable display and logging
The Vaisala VDL200 is a secure Power-over-Ethernet data logger for monitoring temperature, relative humidity, carbon dioxide and analog or Boolean inputs in controlled and GxP-regulated environments.
Designed for the Vaisala viewLinc Continuous Monitoring System, the VDL200 combines interchangeable smart probes, onboard memory and battery backup to provide reliable, gap-free environmental monitoring.
The Vaisala GM70 was a portable carbon-dioxide meter for diffusion measurements, pump sampling, data logging and field checking fixed CO₂ instruments.
- Discontinued product; not recommended for new purchases
- GMP221 probes for percentage-level CO₂ up to 20%
- GMP222 probes for ppm-level CO₂ up to 10,000 ppm
- GMH70 diffusion handle or GM70PUMP sampling system
- MI70 indicator with numerical, graphical and logging functions
- Used for field checking compatible fixed Vaisala CO₂ instruments
- Use Vaisala Indigo80 for new portable measurement systems
The Vaisala GMW90 Series provides stable wall-mounted carbon-dioxide and temperature measurement, with optional humidity, for demanding HVAC and building-automation applications.
- Carbon-dioxide measurement from 0 to 5000 ppm
- CO₂ accuracy up to ±(30 ppm + 2% of reading)
- Temperature measurement from −5 to +55 °C
- Optional HUMICAP® relative-humidity measurement from 0 to 95 %RH
- Voltage, current or Modbus RTU output models
- Display and CO₂ status-indicator options
- Field-exchangeable calibrated measurement modules
- Traceable three-point CO₂ calibration certificate included
The Vaisala GMD110 measures carbon dioxide directly inside ventilation ducts for precise HVAC control and demand-controlled ventilation.
- CO₂ measurement range from 0 to 10,000 ppm
- Accuracy of ±40 ppm from 0 to 3000 ppm
- 4–20 mA, 0–5 V, 0–10 V or Modbus RTU output options
- Analogue scaling of 0–2000, 0–5000 or 0–10,000 ppm
- Fast response with less than two minutes to full specification
- Heated CARBOCAP® sensor head helps prevent condensation
- IP65 enclosure for demanding duct installations
- Traceable calibration certificate included
The Vaisala Indigo500 Series provides flagship industrial measurement integration: choose Indigo510 for a simplified single-probe system or Indigo520 for full dual-probe capability.
- Indigo510: one probe and two isolated analog outputs
- Indigo520: two probes, four isolated outputs and two relays
- Ethernet web interface and Modbus TCP communication
- Rugged IP66 and NEMA 4 metal enclosure