Instruction Manual

The nominal 24 V supply must be connected between terminals + and -. The analogue

output is available at remaining terminals. The common wire is connected to terminal 0

and the other wire either to terminal V (voltage output) or to terminal mA (current

output). The current output type is chosen with the jumper 0/4mA (see Figure 2):

4...20mA is chosen by connecting the jumper (default) and the 0...20 mA is chosen by

disconnecting the jumper. If the relay output of the display board (in GMD20D ) is used,

connect the wires to the terminal block at the back of the display board before re-

installing it. The default relay trigger point has been set to 1000 ppm and it can be

changed with the optional software kit 19222GM. For further details, contact your local

Vaisala representative. If an optional LonWorks interface or a relay output option is

installed in the GMD20, please, refer to the corresponding manuals for details.

Note that during normal operation the green led (V10) at the lower righthand corner is

blinking. In case the self-diagnostics procedure comes across some abnormality, the red

led (V7) lights up (see Figure 2).

Figure 2 Electrical connections and leds V7, V10

SERVICE AND MAINTENANCE

The GMD20 transmitters have an excellent stability and require almost no maintenance.

In benign environments the recommended calibration interval is five years. If the reading

differs too much from the reference value during checking, the transmitter should be

removed and preferably sent to Vaisala or Vaisala distributor for re-calibration. In case

the user prefers to do the re-calibration himself, Vaisala's software Kit 19222GM and

accurate calibration gases are needed.

If full accuracy is required, the transmitter has to be calibrated against accurate and

traceable calibration gases in stable environmental conditions (temperature, pressure).

Accurate calibrations are usually performed in laboratories; in these calibrations,

temperature and pressure corrections have to be made. A full after sales calibration and

service facility is naturally provided by Vaisala and its distributors. For further details,

consult your local Vaisala representative.

transmitter GMD20 and GMD20D M210292en-A