Vaisala DMP248 OPERATING MANUAL

DMP248 Dewpoint

Transmitter

OPERATING

MANUAL

M210492EN-A

JULY 2003

PUBLISHED BY
Vaisala Oyj Phone (int.): ([phone]

P.O. Box 26 Fax: [phone]
FIN-00421 Helsinki
Finland

Visit our Internet pages at http://www.vaisala.com/

© Vaisala 2003
No part of this manual may be reproduced in any form or by any

means, electronic or mechanical (including photocopying), nor may its
contents be communicated to a third party without prior written
permission of the copyright holder.

The contents are subject to change without prior notice.

_________________________________________________________________________CONTENTS

Table of contents

CHAPTER 1 GENERAL INFORMATION.......................................................................................................1

SAFETY................................................................................................................................................................1

WARRANTY .........................................................................................................................................................1

CHAPTER 2 PRODUCT DESCRIPTION.........................................................................................................3

GENERAL CHARACTERISTICS...............................................................................................................................3

THE OPERATING PRINCIPLE OF THE DMP248.......................................................................................................4

Use in high pressure.......................................................................................................................................6

CHAPTER 3 INSTALLATION...........................................................................................................................7

SELECTING THE PLACE OF INSTALLATION............................................................................................................7

MOUNTING THE TRANSMITTER ............................................................................................................................7

Mounting; overview.......................................................................................................................................9

Mounting the probe directly to the process..................................................................................................10

Installing the probe through the ball valve assembly...................................................................................11

Mounting the probe with a quick connect....................................................................................................17

Sample cell...................................................................................................................................................18

Grounding....................................................................................................................................................20

Connections..................................................................................................................................................22

Connection to an AC supply........................................................................................................................23

CHAPTER 4 COMMISSIONING.....................................................................................................................25

SECURITY LOCK JUMPER....................................................................................................................................25

SELECTING THE ANALOGUE OUTPUTS................................................................................................................26

CONNECTING THE RS 232C SERIAL BUS............................................................................................................28

CHAPTER 5 COMMANDS...............................................................................................................................33

COMMANDS AND SECURITY LOCK JUMPERS.......................................................................................................33

LED COMMANDS...............................................................................................................................................34

DISPLAY/KEYPAD COMMANDS...........................................................................................................................35

CHAPTER 6 CALIBRATION AND ADJUSTMENT.....................................................................................55

HUMIDITY CALIBRATION AND ADJUSTMENT......................................................................................................55

Two-point calibration and adjustment adjustment procedure ......................................................................56

Using serial commands.............................................................................................................................................56

Using display/keypad commands..............................................................................................................................57

Using LED commands..............................................................................................................................................58

Humidity calibration table............................................................................................................................59

TEMPERATURE CALIBRATION ............................................................................................................................59

One point offset correction...........................................................................................................................59

Using serial commands.............................................................................................................................................59

Using display/keypad commands..............................................................................................................................60

Using LED commands..............................................................................................................................................60

Two-point temperature calibration and adjustement....................................................................................61

Using serial commands.............................................................................................................................................61

Using display/keypad commands..............................................................................................................................62

Using LED commands..............................................................................................................................................62

CALIBRATION OF THE ANALOGUE OUTPUTS.......................................................................................................63

Using serial commands................................................................................................................................63

Using display/keypad commands.................................................................................................................63

VAISALA __________________________________________________________________________ I

OPERATING MANUAL_______________________________________________________________

Using LED commands .................................................................................................................................64

CHAPTER 7 MAINTENANCE.........................................................................................................................67

REFERENCE MEASUREMENTS.............................................................................................................................67

SELF-DIAGNOSTICS............................................................................................................................................67

TEMPERATURE CHANNEL ADJUSTMENT WITH PT 100 SIMULATORS ...................................................................68

MEASUREMENT OF OUTPUT CURRENTS USING TEST POINTS ...............................................................................70

ADJUSTING THE CONTRAST OF THE DISPLAY......................................................................................................71

VAISALA SERVICE CENTERS ..............................................................................................................................71

CHAPTER 8 TECHNICAL DATA ...................................................................................................................72

MEASURED VARIABLES......................................................................................................................................72

OUTPUTS............................................................................................................................................................73

GENERAL...........................................................................................................................................................74

ELECTRONICS ....................................................................................................................................................75

SERIAL INTERFACE MODULES.............................................................................................................................75

ELECTROMAGNETIC COMPATIBILITY .................................................................................................................76

CHAPTER 9 OPTIONS......................................................................................................................................76

CHAPTER 10 SPARE PARTS AND ACCESSORIES ....................................................................................77

APPENDIX 1 SERIAL COMMANDS...............................................................................................................79

APPENDIX 2 POWER SUPPLY MODULE ..................................................................................................101

APPENDIX 3 INSTALLING AND USING THE RS 485/422 SERIAL PORT MODULE.........................105

APPENDIX 4 INSTALLING AND USING THE CURRENT LOOP MODULE........................................115

APPENDIX 5 ERROR MESSAGES................................................................................................................125

APPENDIX 6 WIRING DIAGRAM................................................................................................................129

APPENDIX 7 ALARM OUTPUT UNIT .........................................................................................................131

APPENDIX 8 PRESSURE CONVERSION CHART.....................................................................................137

PRESSURE CONVERSION CHART........................................................................................................................137

Multiplication factors .............................................................................................................................................137

II______________________________________________________________________M210492EN-A

CHAPTER 1_______________________________________________________ GENERAL INFORMATION

CHAPTER 1 GENERAL INFORMATION

Safety

Throughout the manual important instructions regarding the safety
considerations are focused as follows.

WARNING

CAUTION

NOTE

Warranty

Warning denotes a hazard. It calls attention to a procedure, practice,
condition or the like, which, if not correctly performed or adhered to,
could result in injury to or death of personnel.

Caution denotes a hazard. It calls attention to a procedure, practice,
condition or the like, which, if not correctly performed or adhered to,
could result in damage to or destruction of part or all of the product.

Note highlights important information. It calls attention to an essential
procedure, practice, condition or the like.

Vaisala issues a guarantee for the material and workmanship of this
product under normal operating conditions for one (1) year from the
date of delivery. Exceptional operating conditions, damage due to
careless handling and misapplication will void the guarantee.

VAISALA __________________________________________________________________________1

CHAPTER 2_______________________________________________________ PRODUCT DESCRIPTION

CHAPTER 2 PRODUCT DESCRIPTION

General characteristics

The DMP248 transmitter is a microprocessor-based instrument for the
measurement of dewpoint temperature in low humidities. The
transmitter measures other quantities as well: relative humidity,
temperature and ppm concentration (dry). When the dewpoint
temperature is below 0 °C, the transmitter calculates the frostpoint
instead of the dewpoint. The dewpoint output can be scaled freely, for
example, dewpoint -40...+20 °C can be set to correspond to 0...1 V.
The DMP248 transmitter has two analogue outputs and can be
connected to a serial bus via the RS 232C interface or optionally
through an RS 485/422 serial module or a current loop module.

The transmitter can be configured in many ways. It can have either a
blank cover or a cover with a local display and keypad with which the
user can operate the transmitter. The power supply voltage can be se­lected from three alternatives (24 VDC/VAC, 115 VAC, 230 VAC).
Two analogue output signals are selected from the measured
quantities; the signals can be scaled. The transmitter can be supplied
with two, five or ten metre sensor head cable. The alarm output option
enables two separate alarms that can be freely set by user.

Options
Alarm output 2 relays 8A/230V SPCO relays
Power supply 24 VDC (VAC) (standard), 115/230 VAC
Serial interface RS 232C (standard), RS 485/422, current loop
Display cover cover with or without local display & keypad
Cable length 0.56, 2, 5 or 10 metres

VAISALA __________________________________________________________________________3

OPERATING MANUAL_______________________________________________________________

Pw

The operating principle of the DMP248

The DMP248 transmitter incorporates the DRYCAP® sensor which is
optimized to be used in low humidities but has also an excellent
tolerance against condensation. The DRYCAP® sensor uses an
operating principle based on changes in capacitance as its thin
polymer film absorbs water molecules together with a combined
temperature measurement with a Pt 100 resistive temperature sensor.
The capacitance of the thin polymer film has a direct response
proportional to RH, but combined with the temperature signal the
response of the DRYCAP® sensor is rather proportional to Pw (water

vapour pressure) or to the dewpoint.
While frostpoints (dewpoints below 0 °C) in principle can be

determined by using traditional RH transmitters, it is very difficult
considering the required accuracy at the dry end calibration. As
relative humidity levels approach zero the accuracy rapidly decreases
and the offset soon becomes the largest source of errors when
monitoring frostpoints. Therefore, the focus is to minimize the offset
(error at 0% RH) when monitoring the process gas.

For example to monitor a process with a frostpoint of -40 C and a
temperature of +20 C translates to a relative humidity of 0.55%. An
offset error of -0.2 %RH, which is well within specifications for a
normal RH-transmitter, brings the measured RH down to 0.35%. This
would bring the calculated frostpoint down to -44 C. Thus seemingly
minor offset errors caused by drift or bad calibration translate into
unacceptable frostpoint errors when the RH is low.

To solve the accuracy problem the DMP248 transmitter utilizes a
patented method that automatically adjusts the dry end measurement
in frostpoints. The offset calibration algorithm incorporated into the
DMP248 transmitter uses the fact that the capacitance of a thin film
polymer sensor is proportional to RH as seen in formula 1.

RH RH Gain

= +

out

0

*

Pws T

( )

(2-1)

where:
RH0=output in completely dry state

Pw=water vapor pressure
Pws(T)=temperature dependent water vapor saturation pressure

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CHAPTER 2_______________________________________________________ PRODUCT DESCRIPTION

0.4

The function Pws(T) is well known from literature. Thus, it is
possible to determine the offset (RH0) if measurements are made at

two or (preferably) more temperatures assuming a constant Pw
during the process.

0.3

20°C

0.2

0.1

0

RHout(%)

-0.1

-0.2

-0.3
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

FIGURE 2-1 Device output during hypothetical offset calibration

30°C

1/Pws(T)

cycle.

For example, for a transmitter with a -0.2%RH offset error is made 11
measurements at temperatures 20...30 °C. The frostpoint is assumed to
stay constant at -40 °C and that the 'Gain' is nominal 100%.

As the temperature increases the RH drops. Since the response is
linear, a straight line is fitted through the data points. The line

intercepts the y-axis at RH0 as shown in FIGURE 2-1. Now the RH0 is
known and it can be subtracted from the indicated value 0.35% RH to
get the correct value of 0.55% RH.

FIGURE 2-2 shows the DRYCAP



sensor as mounted on a DMP248
probehead. Through a combination of the polymer sensor and Pt 100
sensor, the DRYCAP will accurately measure the water vapor
pressure used in determining low dewpoints. During auto-calibration
the Pt 100 element is used to first heat and then measure the
temperature of the sensor while cooling back to ambient temperature.
A complete cycle of auto-calibration takes 60...70 seconds. When the
auto-calibration cycle is active, the transmitter locks the output values
to those measured prior to auto-calibration.

VAISALA __________________________________________________________________________5

OPERATING MANUAL_______________________________________________________________

FIGURE 2-2 The DRYCAP

Note that the auto-calibration takes place only if the DMP248 is used
in ambient humidities below 10 %RH (dewpoint below -12 °C at 20 °
C) and at ambient temperature 0...+80 °C.

Use in high pressure

If the process pressure differs from the normal ambient pressure, the
value has to be entered in the transmitter memory to ensure the best
possible measurement accuracy. The pressure setting is used for
pressure compensation of the DMP248 transmitter. Note that although
dewpoint is a pressure dependent parameter, this setting cannot be
used for calculating dewpoints in different pressures. The probe
should be installed to a place with pressure equal to that of the process
in order to ensure the most reliable measurement. For converting
pressure units, see Appendix 8.

NOTE

The probe can be installed in the process through the ball valve
assembly provided that the process pressure is less than 10 bars. This
way, the process does not have to be shut down when installing or
removing the probe. However, if the probe is not removed from the
process as such (e.g. the process is shut down first), the process
pressure can be max. 20 bars.



sensor mounted on a DMP248 probe.

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CHAPTER 3_______________________________________________________________INSTALLATION

CHAPTER 3 INSTALLATION

Selecting the place of installation

Select a place which gives a true picture of the environment or
process; also select a place that is as clean as possible. Air should
circulate freely around the sensor.

It is recommended that the sensor head is installed directly in the
process through the ball valve assembly. When the ball valve
assembly is used, the chamber or the duct does not have to be emptied
or shut down for installation or removal of the probe. Install the sensor
head transversely against the direction of the process flow.

If the probe head has to be installed aside of the process gas flow or
the process is very hot or particularly dirty, the probe can be installed
in a “leak-through” position. In this installation, the probe is mounted
behind the ball valve assembly and if necessary, a cooling coil and/or
a filter can be mounted in between. The flow passes through the
sensor head and leaks out through a vent hole in the fitting body
enabling a reasonable response time. In hot and dirty processes, a
sample system can also be used.

Mounting the transmitter

In FIGURE 3-1 and FIGURE 3-2, you can see the dimensions of the
DMP248 transmitter:

VAISALA __________________________________________________________________________7

OPERATING MANUAL _______________________________________________________________

PROBE UP

PROBE
PUSHED
DOWN

cable length
2, 5 or 10 m

ø5.5

178

31

ø13.5

adjustment

range120 mm

29

R1/2 ISO 7/1

non leaking screw (A)
(factory setting)
or leak screw (B)
(included in the package)

149

clasp nut

fitting body

NOTE

104

CL

ENT

120

ø6.5

65

133

145

FIGURE 3-1 Dimensions of the DMP248 electronics housing (in

mm).

1. Always mount the transmitter housing with the cable bushings

pointing downwards to ensure IP65 (NEMA4) rating

2. Make sure that the connection cable has the right thickness

(∅7...10 mm) and that the cable bushing is carefully tightened.

3. Pay always special attention to closing the transmitter cover

carefully and remember to tighten all four screws.

FIGURE 3-2 Probe dimensions (in mm).

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CHAPTER 3_______________________________________________________________INSTALLATION

NOTE

Take care not to damage the pipe of the probe. If the pipe is damaged,
the probe head is less tight and it will not go through the clasp nut.

Mounting; overview

sealing with:

1. LOCTITE® No 542 + activ. No 7649 (t=-55...+150 °C)

2. MEGA-PIPE EXTRA No 7188 (t=-55...+170 °C)

3. PTFE tape (t=-60...+210 °C) NOTE: the tape does not lock
the parts together. Therefore, use two fork spanners (hex 24 and
27 mm) for tightening and opening the clasp nut of the probe

FIGURE 3-3 Sealing and thread cutting for the fitting body.

The fitting body can be installed e.g. on standard pipe fittings (G 1/2
ISO 228/1) or on a thread in the process wall. If the wall thickness is
less than 10.5 mm, it is recommended to use a welded sleeve (see

FIGURE 3-3). Note that the minimum recommended distance of the

fitting body and probe head is 40 mm (see FIGURE 3-3).

fitting body
hex = 24mm
tapered thread
R1/2 ISO 7/1

parallel thread
G1/2 ISO 228/1
(BS 2779, JIS B0202)

ø19mm drilling

>10.5mm

>40mm

Process or pipe wall

Adjust the probe to a suitable distance according to the type of
installation, and tighten the clasp nut first manually. Then, mark the
fitting body and the clasp nut and tighten the nut a further 50...60°
with a fork spanner (see FIGURE 3-4).

probe

a pen

fitting body

FIGURE 3-4 Tightening the clasp nut.

clasp nut

60°

max.

VAISALA __________________________________________________________________________9

OPERATING MANUAL_______________________________________________________________

NOTE

Be careful not to tighten the clasp nut more than 60° as this may
result in difficulties when trying to open it.

The probe is delivered with non-leaking screw A mounted. For by­pass measurements, this screw is removed and replaced with leaking
screw B (included) and an O-ring is placed on the groove of the
sintered filter prior to installation. Make sure to tighten the screw
carefully.

Screw B has a small (0.08 mm) laser-made hole in the middle; the gas
or air to be measured passes through the sintered filter and by the
sensor, and leaks out through the screw.

Mounting the probe directly to the process

Select a point, which gives a true picture of the process. The
transmitter can be installed directly in the process wall, especially if
the pressure of the process is 1 bar (atmospheric processes).

process wall

recommended adjustment range

25 mm

FIGURE 3-5 Installing the probe in an atmospheric process.

25...135 mm

Leave at least 25 mm (1 ")
of probe head free to enable
a faster response time in
unpressurized processes

welded sleeve (G1/2, Ø 40 mm)

non leaking screw
(screw A, factory setting)

If the probe is installed in process pipes where the water is likely to
collect at the measurement point, take care to install the sensor head
so that it will not be immersed in water.

When the probe is installed directly on the process wall or pipe, note
that a closing valve may be needed on both sides of the installed probe
so that the sensor head can be removed from the process for
calibration and maintenance.

10_____________________________________________________________________M210492EN-A

CHAPTER 3_______________________________________________________________INSTALLATION

If the sensor head is installed in a pressurized chamber, always make
sure that the pressure of the chamber is equalized with the ambient
pressure prior to removing the probe.

capped nut
DIN 917-M22x1.5

when the probe is pulled
out for maintenance, cap
the hole with a capped nut;
this way, the process can be
open although the probe is
not in place

sealing

welded sleeve

(G1/2)

process pipe

FIGURE 3-6 Installing the sensor head directly on the process pipe.

Non leaking screw
(screw A)

closing valve
(ball valve)

Installing the probe through the ball valve assembly

The best way to install the sensor head is through the ball valve
assembly. Use a 1/2” ball valve assembly with a ball hole of ∅14 mm
or more. In this kind of installation, it is not necessary to empty or
shut down the process for installing or removing the sensor head. If
the sensor head is installed in a process pipe, please note that the
nominal size of the pipe must be at least 1 inch. See FIGURE 3-7 -

FIGURE 3-8 for detailed instructions.

VAISALA _________________________________________________________________________11

OPERATING MANUAL_______________________________________________________________

probe

non-leaking
screw A

handle

>30 mm

ball of the ball
valve
(hole diameter
at least 14 mm)

NOTE

process pipe / chamber

FIGURE 3-7 Installing the sensor head through the DMP248BVS

ball valve assembly.

The probe can be installed in the process through the ball valve
assembly provided that the process pressure is less than 10 bars. This
way, the process does not have to be shut down when installing or
removing the probe. However, if the process is shut down before
removing the probe, the process pressure can be max. 20 bars.

See FIGURE 3-8- FIGURE 3-11 for detailed description of installation
through the ball valve assembly. This installation is possible
provided that the process pressure is less than 10 bars. Note also
that if the sensor head is installed in a process pipe, the nominal size
of the pipe must be at least 1 inch.

12_____________________________________________________________________M210492EN-A

CHAPTER 3_______________________________________________________________INSTALLATION

• STEP 1: mount the probe with the ball valve assembly closed;

tighten the clasp nut manually.

bushing R1/2 cone/G1/2(40 bar)
e.g. Camozzi 2520-1/2-1/2

(the bushing serves for
moving the probe (sinter)
to such a distance from the
ball valve that the valve
can be closed)

clasp nut

fitting body
R1/2 cone, sealed

ball valve 1/2" (40 bar)
e.g. Atlas Copco:BAL-1A 15 (G1/2)

FIGURE 3-8 Installing the probe through the ball valve assembly;

>30 mm

bushing
R1/2 cone
sealed

nipple
R1/2 cone
sealed

step 1.

VAISALA _________________________________________________________________________13

OPERATING MANUAL_______________________________________________________________

• STEP 2: open the ball valve assembly.

manual
press
tool

120mm

probe pipe

148 mm

ø5.5

handle

ball of the
ball valve

adjustment range

61

> 14

=

ø14

marking groove

leak screw (B)
(hex. 1.5 mm)

(40)

29

15

R1/2 ISO 7/1

ø13.5

fitting
ferrule

clasp nut
(hex 27 mm)

fitting body
(hex. 24 mm)

O-ring

DRYCAP® sensor

filter

FIGURE 3-9 Installing the probe through the ball valve assembly;

step 2 (measures in mm).

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CHAPTER 3_______________________________________________________________INSTALLATION

STEP 3: push the probe head through the ball valve assembly into the
process. If the pressure is high, use a manual press tool. Note that the
sensor head must be pushed so deep that the filter is completely inside
the process flow.

MANUAL
PRESS TOOL

VALVE OPEN

VALVE CLOSED

FILTER

FIGURE 3-10 Installing the probe through the ball valve assembly;

step 3.

For by-pass measurements, the probe is mounted behind the ball valve
assembly and non-leaking screw A on the fitting body is replaced with
leaking screw B and O-ring is placed on the groove of the sintered
filter. Screw B has a small (0.08 mm) laser-made hole in the middle;
the gas or air to be measured passes through the sintered filter and by
the sensor, and leaks out through the screw. The process pressure
reduces in the hole of the screw B. This installation is recommended if
the process flow rate is >20 m/s and there is over-pressure in the
process.

VAISALA _________________________________________________________________________15

OPERATING MANUAL _______________________________________________________________

gas escape channel:
use this position for
by-pass measurements

leak screw (B)

sintered
filter

O-ring

DRYCAP® sensor

process pipe
or chamber

NOTE
keep the marking
groove in sight when
using leak screw (B)

FIGURE 3-11 Installing the sensor head for by-pass measurements.

When pushing the probe head through the ball valve assembly, be
careful not to break the sintered filter. Open and close the ball valve
assembly with the marking groove always in sight. In by-pass
measurements, the clasp nut is tightened manually prior to pressing
the probe through the valve. When the probe has been pressed through
and the valve is open, the nut is tightened 50...60° with a fork spanner
(hexagon 27 mm).

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CHAPTER 3_______________________________________________________________INSTALLATION

Mounting the probe with a quick connect

The probe can also be installed with a quick-connect, which acts as a
closing valve between the process and the probe. The installation can
be easily done even in small ducts with standard parts, and the probe
is easily removed when necessary. It is necessary for the probe to be
installed in the leak-through position for a reasonable response time.

FIGURE 3-12 illustrates an example of using a quick connect with the

DMP248 probe. The chamber can be made of stainless steel AISI 316.

keep the marking
groove in sight
when using
leak screw

leak screw

leak screw

15 mm

30 mm

chamber made of
hexagonal bar
(hex=27 mm)

sealing

G1/2

50 mm

G1/8
or G1/4

bayonet
socket

fitting part

of the quick-

connect

process pipe

FIGURE 3-12 Installing the probe with a quick-connect.

VAISALA _________________________________________________________________________17

OPERATING MANUAL_______________________________________________________________

Sample cell

It may be necessary to use the sample cell if the process (e.g. a pipe) is
too small for the DMP248 sensor head. Furthermore, if the process is
very hot (>80 °C) or particularly dirty, the probe is installed in a
sample cell behind a cooling coil and/or filter. In this case, the
ambient temperature must be at least 10 °C warmer than the process
dewpoint in order to avoid condensation in the sample tubing.

6

40

28

80

G1/4

ISO

228/1

PUSH PROBE

S

INTER AGAINST

THIS EDGE,

TIGHTEN THE

G1/4

6

S

Sample gas inlet

use connector

R1/4 ISO 7/1

68

SAMPLE CELL

CLASP NUT.

DMP248SC

IN

G1/2

ISO228/1

OUT

view

A - A

Sample gas outlet

use connector R1/4 ISO 7/1

FIGURE 3-13 Dimensions (in mm) of the DMP248SC sample cell.

25

22

80

25

20

for DMP248
probe

A

A

fixing screws

(e.g. M6x60 or 1/4" x 11/4")

TOP VIEW SIDE VIEW

40

FIGURE 3-14 Fastening to a metal plate (top view) and on a concrete

frame (side view)

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CHAPTER 3_______________________________________________________________INSTALLATION

process
pipe

28

6

6

S

G1/4

ISO

228/1

SAMPLE CELL

DMP248SC

CLASP NUT.

THIS EDGE,

PUSH PROBE

TIGHTEN THE

S

INTER AGAINST

G1/4

screw

FRONT VIEW TOP VIEW

metal
hose
clamp

thread M6 or
1/4"-20 UNC

metal

plate

thickness e.g.

3mm (1/8")

AISI 316

40

Dimensions of the metal plate

68

80

FIGURE 3-15 Fastening to a process pipe with the help of a metal

plate

An overpressure in the process is necessary to create a flow through
the sample cell. Note that the pressure of the sample cell must not
differ from that of the process because dewpoint temperature changes
with pressure. In dirty processes, it may be necessary to use a filter
between the cooling coil and the sample cell. One more simple way of
using the sample cell with user provided accessories is shown in

FIGURE 3-16. The flow through the sample cell is controlled with the

needle valve and the pressure is kept equal to that of the process.

FIGURE 3-16 Installing the probe in high temperatures (an example).

VAISALA _________________________________________________________________________19

OPERATING MANUAL_______________________________________________________________

Grounding

A single electrical cable with a screen and three to ten wires is
recommended for power and analogue output/serial bus connections.
The cable diameter should be 7...10 mm.

The screen of the electrical cable must be grounded properly to
achieve best possible EMC performance. Recommended cable shield
is done in the cable gland as shown.

• remove the brass disks, rubber ring and nut from the transmitter

housing

• strip 165 mm of the cable insulation, but leave 25 mm of the braid

visible

• slip the nut and rubber ring over the cable insulation

• slip the brass disk that has the bigger hole in it over the braid so

that it rests against the cable insulation

• slip the other brass disk over the wires to the middle of the braid

flexible wires 0.5 mm²

(AWG 20), stranded wires

recommended

3

140

165

braid

brass
disks

rubber
ring

nut

cable

25

D = Ø 7...10 mm

(If the cable diameter is less

than 7mm, use a shrinking

tube or an adhesive tape)

shielding tube

braid

brass disks

• push back the braid and press it between the two brass disks to

achieve a full 360° grounding; the fold between the disks should
have the same diameter as the brass disks

• secure the braid with a shielding tube

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CHAPTER 3_______________________________________________________________INSTALLATION

• insert the wires into the transmitter housing through the gland

• tighten the nut

• connect the wires into the screw terminals and fasten a cable tie

around the wires

cable tie

transmitter housing

gland

brass disks

rubber ring

nut

NOTE

When the cable is grounded as explained, the metallic parts of the
sensor head, the screen of its cable, the transmitter housing and the
screen of the signal cable to external system are all connected to
each other. After this, the whole system can be grounded from one
point only. If the grounding is made via several points (sensor head,
transmitter housing, signal cable), make sure that the different
groundings are made to the same grounding potential. Otherwise,
harmful grounding currents may be generated. If you do the
grounding via the transmitter housing, use one serrated lock washer
between a mounting screw and the housing; the lock washer breaks
the paint on the housing.

VAISALA _________________________________________________________________________21

OPERATING MANUAL_______________________________________________________________

+

Connections

CH1- and CH2- are connected

CH2

CH1+

CH1 -

CH2 -

together internally

+

+

X2

X1

OPENED COVER OF THE DMP248

CURRENT/VOLTAGE

-

V

mA

OUTPUTS

-

V

mA

POWER SUPPLY
(INTERNAL OR
EXTERNAL)

Do not use power supply
ground (-) as output signal
ground

24 V +

FIGURE 3-17 Electrical connections

Power supply 24 VDC

24 VAC (see Chapter Connection
to an AC supply)

Output signals 0...20 mA

4...20 mA

0...1 V

0...5 V

0...10 V

Power supply ground (-) is connected to the housing with parallel
connection of 15 nF capacitor and 300 kΩ resistor.

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CHAPTER 3_______________________________________________________________INSTALLATION

Connection to an AC supply

The DMP248 transmitter can also be connected to an AC supply
without an external rectifier. However, when more than one
transmitter is connected for example to one 24 VAC transformer, a
common loop is formed and there is an increased risk of a short­circuit. To avoid this, always use separate floating supply for each
transmitter (see FIGURE 3-18A). However, if several transmitters have
to share one transformer, the phase (∼) must always be connected to +
connector in each transmitter (see FIGURE 3-18 B).

A) NO COMMON LOOP FORMED - RECOMMENDED

DMP248 transmitter Controller

24 VAC

24 VAC

DMP248 transmitter

B) COMMON LOOP FORMED -

DMP248 transmitter

24 VAC

supply

voltage

supply

voltage

DMP248 transmitter

signal

supply

supply

output

voltage

signal

output

voltage

NOT RECOMMENDED!

Controller

signal

output

signal

output

shared
common

line

FIGURE 3-18 Connecting the transmitter to an AC supply.

VAISALA _________________________________________________________________________23

CHAPTER 4_____________________________________________________________COMMISSIONING

CHAPTER 4 COMMISSIONING

When the DMP248 transmitter leaves the factory, its measurement
ranges and output signals have already been selected. The user can
subsequently change the measurement units between metric and non­metric and select and scale the output signals with software functions,
see Chapter Selecting and scaling the analogue output quantities and
Appendix 1.

Security lock jumper

Before the settings can be changed, the user must first remove the
security lock jumper in connector X15 (see FIGURE 4-1). The security
lock jumper makes it impossible to change the transmitter settings by
mistake. The jumper should be removed only for changing the settings
and for calibration; the auto-calibration is active only with the jumper
connected. When the security lock jumper is connected, some
commands cannot be used (see Chapter Commands and security lock
jumpers).

CHANGE OF SETTINGS
DISABLED

X15

OPENED COVER OF THE DMP248

FIGURE 4-1 Location of the security lock jumper.

VAISALA _________________________________________________________________________25

OPERATING MANUAL_______________________________________________________________

OPENED COVER OF THE DMP248

Selecting the analogue outputs

The DMP248 transmitter can be ordered ready with the current or
voltage outputs required. If the outputs need to be changed, move the
jumpers in connector X15 into positions as shown in FIGURE 4-2.

CH1

CH2

C H 2

C H 1

CURRENT OUTPUTS
0 ... 20 / 4 ... 20 mA

CH2

CH1

C H 1

C H 2

VOLTAGE OUTPUTS
0 ... 5 V / 0 ... 10 V

CH2

CH1

C H 2

C H 1

VOLTAGE OUTPUTS
0 ... 1 V

CH2

CH1

C H 1

C H 2

CH1 0 ... 1 VOLTAGE OUTPUT
CH2 CURRENT OUTPUT

X15

FIGURE 4-2 Selecting the analogue outputs with jumpers.

The software also has to be informed which outputs are in use. This is
done either through the serial interface or the menus on local display
when in use. The serial command is AMODE and the display/keypad
command "Mode ð Analog outputs ð Mode" (see Chapter 5

commands). If the outputs need to be scaled, see serial

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