Vaisala GMD20 User Manual
CO2 transmitter GMD20 and GMD20D M210292en-A
INTRODUCTION
Vaisala’s GMD20 uses silicon based CARBOCAP sensor technology providing
excellent stability and reliability. The GMD20 transmitters require almost no
maintenance: the recommended calibration interval is five years in HVAC and related
benign applications. Transmitters are available without a display (GMD20 version) and
with a display (GMD20D version).
MOUNTING
The GMD20 transmitters are truly duct mounted. The distance between the probe and the
duct wall is easily adjusted by using the mounting plate. The mounting plate is fastened
with four screws ST.4.2x16-C-Z/A4m DIN 7981. Note that four ∅3.2 holes on the duct
surface are needed, as well as one ∅22...25 mm hole in the middle (see figure below).
Insert the transmitter through the mounting plate and adjust the distance.
64
min 80 max 140
Ø15
42
22
42
Figure 1 Dimensions of the GMD20 and the mounting plate (in mm)
ELECTRICAL CONNECTIONS
Open the transmitter cover and remove the display board if there is one installed. The
cable is inserted through a rubber plug included in the package. Pierce the rubber plug
with a screwdriver or a similar instrument in order to open the hole for the cable. Then
insert the plug in the appropriate hole in the transmitter cover. Strip a few millimetres of
the cable insulation and insert the cable carefully through the rubber plug so that you can
connect the wires to the terminal block on the righthand side of the component board.
1
CO2 transmitter GMD20 and GMD20D M210292en-A
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 reinstalling 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.
2
Loading...