Vaisala GMP252 Carbon Dioxide Probe X2 Integration Guide

The Vaisala GMP252 Carbon Dioxide (CO2) Probe measures atmospheric COconcentrations (ppm). The probe is compatible with the X2 data logger using the Modbus-RTU communication protocol and the RS-485 sensor interface. Below is information on the settings required for integration with an X2 data logger.

Vaisala GMP252 Carbon Dioxide Probe product page

Figure 1: Vaisala GMP252 Carbon Dioxide Probe


Wiring for Direct Communication

For direct communication with the sensor, follow the wiring information below and skip to the Configure Communication section to set the appropriate settings for communication with an X2 data logger.

Vaisala M12 Cable Wire ColorSignal
WhiteRS-485 B
BlackRS-485 A


NexSens UW8 Connector Wiring & Pin Information

In nearly all applications involving NexSens integration, a connectorized UW8 plug will be added to the M12 male connector that comes with purchasing the sensor. In the event of connecting a NexSens UW8 connector into an external device that is not an X2 data logger, below is the pinout of the NexSens UW8 connector, along with the signal associated with the communication, ground and power pins from the sensor.

Figure 2: NexSens UW8 plug pinout.


NexSens UW8 Pin NumberSignal
7RS-485 B
8RS-485 A


Configure Communication

The GMP252 sensor can be configured in a terminal emulator software such as Tera Term. Changes to the communication output and frame must be made before the sensor can communicate with the X2 data logger.

  1. Set the terminal emulator serial port communication to the following:
    • Baud Rate: 19200
    • Data: 8 Data Bits
    • Parity: None
    • Stop: 1 Bit
    • Flow Control: None
    • Enable Local Echo to visualize the entered commands.
  2. While connecting the GMP252 sensor to the Vaisala USB cable or an external power source, simultaneously press and hold the Enter key.
    1. If a successful communication is established, the sensor model and version will appear in the terminal.
      • Ex: GMP252 1.3.4
  3. Once connection is established, the following commands must be configured for communication with an X2 data logger:
    • smode Modbus
    • addr 1
    • seri 19200 N 8 1
  4. Send the ? command to confirm the changes to the device settings.
  5. Send the seri command to confirm the changes to the serial communication.


X2 Script Information

The X2 data logger includes a pre-loaded Vaisala GMP251/252 script. Use the CONNECT software to enable the script for sensor detection.

Configure Sensor Scripts

X2 Script NumberCommunication ProtocolBaud RatePower TypeWarmup Period (sec)Frame


Script Parameters

The Vaisala GMP252 CO2 probe only outputs a single CO(ppm) measurement. The sensor measures atmospheric CObased on the known absorption of infrared light by CO2 at a characteristic wavelength. As CO2 passes through the open channel at the top of the sensor, an infrared light source travels through the gas and reflects off a mirror to a thermopile detector. The detector has a band-pass filter that is channeled to measure the characteristic wavelength in which infrared light is absorbed by CO2. By measuring the ratio of absorbed infrared light with a reference signal, the sensor is able to determine the concentration of CO2 (ppm) in the measured sample.


For more information, download the Vaisala GMP252 User Guide.