Vaisala DMT242 User Manual

Vaisala DRYCAP

®

Dewpoint Transmitter DMT242

USER'S GUIDE June 2008 | M010036EN-D

INTRODUCTION

DMT242 is a dewpoint transmitter for wide range of OEM applications. DMT242 measures dewpoint
with excellent long term stability which is maintained automatically with the patented auto-calibration
procedure. The Vaisala DRYCAP® polymer sensor technology used in DMT242 is also durable against
dew in case condenced water exists in the process during system malfunction. The product mechanics
have been designed for harsh environments requiring protection against dust, dirt and splashed water.

The disconnection and reconnection of the transmitter is easy with the connector where the output signal
and supply voltage wires are connected. The unit also has a serial line for rescaling the analog output.

PRESSURE SETTING PROCEDURE FOR PRESSURIZED PROCESSES

For achieving the most accurate measurements
in pressurized processes, set the process
pressure to DMT242 according to Figure 1 by
using the pressure switches (see Figure 3, item

8). As shipped from factory, the pressure
switch setting is 1 bar, as in switch number 4 in
the ON position.

MOUNTING

Switches Pressure (absolute)

2341

(bar)
0 … 2
2 … 4

4 … 6
6 … 8
8 … 10

10 … 12
12 … 14
14 … 16
16 … 18
18 … 20

THE SWITCH MARKED WITH
A BLACK SQUARE IS IN

ON POSITION

THE

Figure 1 Pressure Setting Table

1. Insert the sealing washer (see Figure 1) on

the probe and set the probe through the
fitting of the process pipe. The probe has
G½" ISO 228/1 parallel thread.

2. Fasten the transmitter to the fitting of the

process pipe by tightening from the nut of
the probe (24 mm).

1 = Cover
2 = Connector
3 = Flat gasket
4 = Counter connector
5 = Transmitter body
6 = Nut
7 = Sealing washer

WIRING

8 = Probe

1. Remove the cover.

2. Take out the connector.

3. Take out the screw terminal from the

connector by pushing it out, for example,
with the fixing screw.

4. Use a three-wire cable. Suitable 2 m or

10 m optional cable is available from
Vaisala (items: 221475 for 2 m cable and
221476 for 10 m cable). Connect the wires
to the connector terminals as follows:

Terminal nr 1 = V supply + (VAC line)
Terminal nr 2 = V supply - (VAC neutral)/signal ­Terminal nr 3 = Signal +
Leave the ground terminal free

Figure 2 DMT242

­+

Wire colours in cables
221475 and 221476:

1 = brown
2 = blue
3 = black

Vaisala DRYCAP

®

Dewpoint Transmitter DMT242

USER'S GUIDE June 2008 | M010036EN-D

5. Insert the wired screw terminal back into the connector excatly in the position indicated in Figure 3.

Push the fixing screw through the connector. Fasten the cable clamp. Insert the wired connector into
the counter connector.

NOTE

The connection is incompatible if the positioning of the screw terminal is NOT as
indicated in Figure 3. Fasten the fixing screw.

6. Install the back cover allowing the cable to run through the hole in the cover. The transmitter is ready

for use.

NOTE

When the power is switched on, wait about 7 minutes before taking measurements.
Start-up self-diagnostics freeze the output during the first operation minutes.

SCREW TERMINAL MUST BE INSTALLED TO THE
CONNECTOR IN THE POSITION OF THE FIGURE

(TERMINALS 2 AND 3 AS UPPER EDGE)

1 = Connector
2 = Screw terminal of the connector
3 = Connector
4 = Cable clamp
5 = RS-232 serial port
6 = Terminals 2 and 3
7 = Three-wire cable
8 = Pressure setting switches
9 = Fixing screw

Figure 3 Wiring and Installation of Connector

CALIBRATION AND MAINTENANCE

Replacing the sintered filter

If the sintered filter is dirty it can prolong the response time of the measurement. If the filter needs to be
changed unscrew the filter and replace the old filter with a new one.

Calibration

It is recommended to perform a calibration check every other year. A field check can be carried out by
using a calibrated reference probe and comparing the readings measured with the transmitter and the
reference probe. The Vaisala DRYCAP® Hand-Held Dewpoint Meter DM70 is ideal for confirming the
performance of the transmitter in the field. By using a connection cable the readings of DMT242 and
DM70 can be viewed simultaneously on the display of DM70. If there is need for adjustement contact
Vaisala Instruments Service Centers or local Vaisala representative.