DDCM
Differential pressure switches
FEMA differential pressure monitors are suitable for
monitoring and controlling differential pressures,
flow monitoring and automatic control of filter
systems. A double chamber system with stainless
steel bellows or Perbunan diaphragm detects the
difference between the two applied pressures.
The desired switching pressure is continuously
adjustable within the ranges mentioned
in the product summary.
All differential pressure monitors can also
be used in the vacuum range.
The switching differential is not adjustable.
DDCM252
SIL 2 according IEC 61508-2
Technical data
Pressure connection
Internal thread G 1/4
Switching device
Robust housing (200) made of seawater
resistant die cast aluminium GD Al Si 12.
Protection class
IP 54, in vertical position.
Pressure sensor materials
DDCM014–16:
Pressure bellows of 1.4571
Sensor housing of 1.4305.
DDCM252–6002:
Perbunan diaphragm.
Aluminium sensor housing.
Mounting position
vertically upright.
Ambient temperature at switching device
–25…+70 °C
Max. medium temperature
The maximum medium temperature at
the pressure sensor must not exceed the
permitted ambient temperature at the
switching device. Temperatures may reach
85°C for short periods). Higher medium
temperatures are possible provided the above
limit values for the switching device are
ensured by suitable measures (e.g. siphon).
Mounting
Directly on the pressure line or on a flat
surface with two 4 mm Ø screws.
Note the connection of pressurized lines:
P (+) = high pressure
S (–) = low pressure
Switching pressure
Adjustable from outside with screw driver.
Type
Setting range
(differential
pressure
Switching
Max.**
Materials in Dimen-
differential
permissible contact with sioned
(mean values) pressure
medium
drawing
page 25 + 26
mbar
mbar
mbar
mbar
bar
bar
bar
bar
bar
0,5
1,5
3
3
15
15
25
15
25
bar
bar
bar
bar
bar
bar
bar
bar
bar
Aluminium
+ Perbunan
1 + 20
Stainless steel
1.4305 +
1.4571
1 + 21
* without graduation (only ± scale) set according to pressure gauge
** also loadable on one side
For more differential pressure monitors, see the HCD and DPS series, page 71 and 72.
+
For accessories, see VKD… and MAU8…, on pages 152 and 153.
Calibration
The
DDCM
series are calibrated for falling pressure.
This means that the adjustable pressure on the scale
corresponds to the switching point at falling pressure.
The reset point is higher by the amount of the switching
differential. (See also page 23, 1. Callibration at lower
switching point).
Switching
capacity
Normal
The differential pressure switch (e.g. DDCM1)
monitors differential pressure through the pump.
250 VAC 250 VDC 24 VDC
(ohm) (ind) (ohm) (ohm)
The system shuts down if values fall below an
adjustable switching threshold. Pump monitoring
8A 5A
0.3 A
8A
does not depend on the static pressure in the
system.
DDCM
see page 67
s
Protection Class:
IP 54/65
Accessories
Solenoid valves
Switching differential
Not adjustable. For values see Product
Summary.
Pump monitoring application example
Flow monitors
Temperature sensors
Switching differential not adjustable
DDCM252*
4…25 mbar
2
DDCM662*
10…60 mbar
15
DDCM1602*
20…160 mbar
20
DDCM6002*
100…600 mbar
35
DDCM014
–0.1…0.4 bar
0.15
DDCM1
0.2…1.6 bar
0.13
DDCM4*
1…4 bar
0.20
DDCM6
0.5…6 bar
0.20
DDCM16
3…16 bar
0.60
Thermostats
Pressure transmitters
Pressure switches
Mechanical pressure switches
Liquids and gases
43
Mechanical pressure switches
Technical features / Advantages
Diecast aluminium housing
IP 54 or IP 65
version also available
Wall mounting
or directly on the pressure line
Switching element (microswitch)
Lead sealable setpoint adjustment
Setting spindle locking element
Stainless steel sensor housing
Stainless steel bellows
with internal stop
Pressure connection
G 1/2" external
G 1/4" internal
Centring pin
Accessories
Solenoid valves
Flow monitors
Terminal connection
or plug connection to
DIN EN175301 Form A
Temperature sensors
Thermostats
Pressure transmitters
Pressure switches
Mechanical pressure switches
Technical features / Advantages
19
20
Mechanical pressure switches
Definitions
Definitions
Pressure data
Overpressure
Pressure
over
the relevant atmospheric pressure. The reference point is
atmospheric pressure.
Pressure
under
the relevant atmospheric pressure. The reference point is
atmospheric pressure.
Overpressure relative to absolute vacuum.
Vacuum
Absolute pressure
Differential pressure
Difference in pressure between 2 pressure measuring points.
Relative pressure
Overpressure or vacuum relative to atmospheric pressure.
Pressure data in all FEMA documents refers to relative pressure.
That is to say, it concerns pressure differentials relative to atmospheric pressure.
Overpressures have a positive sign, vacuums a negative sign.
Permissible working pressure
(maximum permissible pressure)
The maximum working pressure is defined as the upper limit at which the operation, switching reliability
and water tightness are in no way impaired (for values see Product summary).
Bursting pressure (test pressure)
Type-tested products undergo a pressure test certified by TÜV affirming that the bursting pressure
reaches at least the values mentioned in the Product summary. During the pressure tests the measuring
bellows are permanently deformed, but the pressurized parts do not leak or burst. The bursting pressure
is usually a multiple of the permissible working pressure.
Setting range
Pressure range in which the cutoff pressure can be set with the setting spindle.
Pressure units
Unit
bar
mbar
Pa
kPa
MPa
(psi) Ib/m
2
1
1
1
1
1
bar
mbar
Pa
kPa
MPa
1
0.001
10
-5
0,01
10
1000
1
0.01
10
10
4
10
5
100
1
1000
10
6
100
0.1
0.001
1
1000
0.1
10
-4
10
-6
0.001
1
14.5
0.0145
1.45 · 10
-4
0,145
145
In FEMA documents pressures are stated in
bar
or
mbar.
Pressure data for a pressure
switchbased on the example
of DWR625:
Setting range: 0.5-6 bar
Perm. working pressure: 20 bar
Bursting pressure: >100 bar
Important:
All pressure data refers to overpressures or vacuums relative to atmospheric pressure.
Overpressures have a positive sign, vacuums a negative sign.
Definitions
Maximum pressure monitoring
RSP = SP – xd
Switching differential
Pressure transmitters
Accessories
Solenoid valves
Flow monitors
Temperature sensors
Thermostats
The switching differential (hysteresis) is the difference in pressure between the
switching point (SP)
and the
reset point (RSP)
of a pressure switch. Switching differential tolerances occur due to
tolerances in the microswitches, springs and pressure bellows. Therefore the data in the product
summaries always refers to average values. In the case of limiter functions the switching differential
has no significance, as one is only interested in the switching point at which cutoff occurs, not the
reset point. For a
controller function,
i. e. in the case of pressure switches used to switch a burner,
pump etc.
on and off,
a pressure switch with an
adjustable switching differential
should be
chosen. The switching frequency of the burner or pump can be varied by changing the switching
differential.
Adjustable switching differential/ calibration
In the case of pressure switches with adjustable switching differential, the hysteresis can be set within
the specified limits. The switching point (SP)
and
reset point (RSP) are precisely definable. When
setting the pressure switch, the switching differential situation and the type of factory calibration must
be taken into account. Some pressure switches (e.g. minimum pressure monitors of the DCM series)
are calibrated under "falling" pressure, i.e. switching under falling pressure takes place at the scale
value with the switching differential being above it. The device switches back at scale value +
switching differential. If the pressure switch is calibrated under rising pressure, switching takes place
at the scale value and the device switches back at scale value - switching differential (see direction of
action). The calibration method is indicated in the data sheets.
Minimum pressure monitoring
RSP = SP + xd
Direction of action
In principle, any pressure switch can be used for both maximum pressure and minimum pressure
monitoring. This excludes pressure limiters, whose direction of action (maximum or minimum) is
predefined. The only thing to remember is that the scale reading may deviate by the amount of the
switching differential. See example at bottom left: The scale value is 2.8 bar.
Maximum pressure monitoring
With rising pressure,
switching takes place once the preset switching pressure is reached (SP).
The reset point (RSP) is lower by the amount of the switching differential.
Minimum pressure monitoring
With falling pressure,
switching takes place once the preset switching pressure is reached (SP).
The reset point (RSP) is higher by the amount of the switching differential.
Direction of action in vacuum range
It is particularly important to define the direction of action in the vacuum range.
Rising does not mean a rising vacuum, but rising pressure (as viewed from absolute "0"). "Falling"
pressure means a rising vacuum.
For example: Vacuum switch set to -0.6 bar falling means: Switching (SP) takes place under falling
pressure (rising vacuum) at -0.6 bar. The reset point is higher by the amount of the switching
differential (e.g. at -0.55 bar).
Setting a pressure switch
To define the switching point of a pressure switch exactly, it is necessary to determine the direction of
action in addition to the pressure. "Rising" means that switching takes place at the set value when the
pressure rises.
The reset point is then lower by the amount of the switching differential. "Falling" means exactly the
opposite.
Please note when specifying the setting of a pressure switch:
In addition to the switching point it is also necessary to specify the direction of action (falling or rising).
Example for selection of a pressure switch:
A pump is to be turned on at 2.8 bar and off again at 4.2 bar.
Chosen type: DCMV6 according to data sheet DCM. Setting: Scale pointer to 2.8 bar (lower switching
point). Switching differential to 1.4 bar (set according to pressure gauge).
Cutoff point: 2.8 bar +1.4 bar = 4.2 bar.
Pressure switches
Mechanical pressure switches
Definitions
21
22
Mechanical pressure switches
General description
Operating mode
The pressure prevailing in the sensor housing (1) acts on the measuring bellows (2). Changes in
pressure lead to movements of the measuring bellows (2) which are transmitted via a thrust pin
(4) to the connecting bridge (5). The connecting bridge is frictionlessly mounted on hardened points
(6). When the pressure rises the connecting bridge (5) moves upwards and operates the microswitch
(7). A counter-force is provided by the spring (8), whose pre-tension can be modified by the adjusting
screw (9) (switching point adjustment). Turning the setting spindle (9) moves the running nut (10) and
modifies the pre-tension of the spring (8). The screw (11) is used to calibrate the microswitch in the
factory. The counter pressure spring (12) ensures stable switching behaviour, even at low setting values.
1
2
3
4
5
6
7
=
=
=
=
=
=
=
8
=
9
=
10
=
11
=
12
=
Pressure connection
Measuring bellows
Sensor housing
Thrust pin
Connecting bridge
Pivot points
Microswitch or other
switching elements
Setting spring
Setting spindle (switching
point adjustment)
Running nut (switching point
indicator)
Microswitch calibration
screw (factory calibration)
Counter pressure spring
Pressure sensors
Apart from a few exceptions in the low-pressure range, all pressure sensors have measuring bellows,
some made of copper alloy, but the majority of high-quality stainless steel. Measured on the basis of
permitted values, the measuring bellows are exposed to a minimal load and perform only a small lifting
movement. This results in a long service life with little switching point drift and high operating reliability.
Furthermore, the stroke of the bellows is limited by an internal stop so that the forces resulting from
the overpressure cannot be transmitted to the switching device. The parts of the sensor in contact with
the medium are welded together without filler metals. The sensors contain no seals. Copper bellows,
which are used only for low pressure ranges, are soldered to the sensor housing. The sensor housing
and all parts of the sensor in contact with the medium can also be made entirely from stainless steel
1.4571 (DNS series). Precise material data can be found in the individual data sheets.
Pressure connection
The pressure connection on all pressure switches is executed in accordance with DIN 16288 (pressure
gauge connection G 1/2A). If desired, the connection can also be made with a G 1/4 internal thread in
accordance with ISO 228 Part 1.
Maximum screw-in depth on the G 1/4 internal thread = 9 mm.
Centring pin
In the case of connection to the G 1/2 external thread with seal in the thread (i.e. without the usual
stationary seal on the pressure gauge connection), the accompanying centring pin is not needed.
Differential pressure switches have 2 pressure connections (max. and min.), each of which are to be
connected to a G 1/4 internal thread.
