Schmidt SS 20.651 User manual

SCHMIDT®Flow Sensor
SS 20.651
Instructions for Use

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 2
SCHMIDT®Flow Sensor SS 20.651
Table of contents
1Important information.......................................................................3
2Application range.............................................................................3
3Mounting..........................................................................................4
4Electrical connection......................................................................10
5Commissioning..............................................................................18
6Service information........................................................................19
7Dimensions....................................................................................23
8Technical table ..............................................................................24
9Declarations of conformity.............................................................26
Imprint:
Copyright 2021 SCHMIDT Technology GmbH
All rights reserved
Version: 547608.02D
Subject to modifications

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 3
1 Important information
These instructions for use must be read completely and observed care-
fully, before putting the unit into operation.
Any claims under the manufacturer's liability for damage resulting from
non-observance or non-compliance with these instructions will become
void.
Tampering with the device in any way whatsoever - with the exception
of the designated use and the operations described in these instruc-
tions for use - will forfeit any warranty and exclude any liability.
The device is designed exclusively for the use described below (see
chapter 2). In particular, it is not designed for direct or indirect protec-
tion of personal or machinery.
SCHMIDT Technology cannot give any warranty as to its suitability for
a certain purpose and cannot be held liable for errors contained in
these instructions for use or for accidental or sequential damage in
connection with the delivery, performance or use of this device.
The following symbol has to be observed:
Danger warnings and safety instructions. Read carefully!
Non-observance of these instructions may lead to injury of per-
sonal or malfunction of the device.
2 Application range
The SCHMIDT®Flow sensor SS 20.651 is designed for the stationary
measurement of flow velocity as well as temperature of air. The sensor
measures standard velocity wN(unit: m/s) based on standard conditions
of 1013.25 hPa and 20 °C. The output signal is linear and independent of
pressure and temperature of the measured medium.
The basic version (without coating) is suitable only for clean air. Especially
the occurrence of aggressive components (e.g. sulfur, chlorine, phosphor,
etc.) can be done only on the customer's own responsibility.
Due to the high operating temperatures even low concentrations
of aggressive components can lead to a significant reduction of
the sensor’s lifetime.
With optional coating (Parylene) the sensor exhibits a higher tolerance
concerning pollution and an increased media resistance. The respective
suitability has to be considered in each case due to the different environ-
mental conditions.
When using the sensor outdoors, it must be protected against
direct exposure to the weather.
!
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 4
3 Mounting
Determination of installation site
Correct measurements require a flow low in turbulence. This can be
achieved by providing sufficiently long and straight distances without dis-
turbances in front of and behind the sensor.
The minimum run-in and run-out distances depend on the degree of dis-
turbance of the flow obstacle upstream (in front) of the measuring distance
and the inner pipe diameter
1
D (see Figure 1 and Table 1).
Figure 1
Flow obstacle upstream of measuring distance
Minimum distance length of
Run-in (L1)
Run-out (L2)
Light bend (< 90°)
10 x D
5 x D
Reduction,
expansion,
90° bend or
T-junction
15 x D
5 x D
Two 90° bends in
one plane
(2-dimensional)
20 x D
5 x D
Two 90° bends with
3-dimensional
change in direction
35 x D
5 x D
Shut-off valve
45 x D
5 x D
Table 1 Minimum run-in and run-out distances depending on flow obstacles
1
Minimum inner pipe diameter: 25 mm

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 5
Mounting method
The sensor SS 20.651 is mounted by means of a compression fitting (con-
tent of delivery) which clamps the sensor probe by friction. Due to the dif-
ferent operating conditions (temperature and pressure range), there are
different types (see Table 2):
Max. temperature
Max. pressure
Fitting
Seal
Spare part no.
200 / 350 °C
atmospheric2
brass
None
549311
200 °C
16 bar
1.4571
FKM
535092
350 °C
16 bar
1.4571
clamping ring
549312
Table 2 Types of compression fittings
Systems with overpressure
The SS 20.651 is designed for atmospheric conditions (standard version),
optionally for a working (over) pressure up to 16 bar. As long as the me-
dium is operated with overpressure, make sure that:
There is no overpressure in the system during mounting.
Mounting and dismounting of the sensor can be carried out
only as long as the system is in a depressurized state.
Only suitable pressure-tight mounting accessories are used.
Appropriate safety devices are installed to avoid unintended discarding
of the sensor due to overpressure.
For measurements in media with overpressure, appropriate
safety measures must be taken to prevent unintended dis-
carding of the sensor.
If other accessories than the delivered pressure protection kit or alterna-
tive mounting solutions are used, the customer must ensure the corre-
sponding safety measures.
Pressure-tight mounting, fastening of the screw pipe connection
and discarding protection must be checked before pressure is
applied. These tightness checks must be repeated at reasona-
ble intervals.
All components of the pressure protection kit (bolt, chain and
bracket) have to be checked regularly for integrity.
2
p = 700 … 1,300 mbar
!
!
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 6
Thermal boundary conditions
With medium temperatures exceeding the permitted ambient temperature
of the electronics (main enclosure), a free cooling section of the probe of
at least 50 mm must be provided (see Figure 2) to prevent crosstalking of
the hot medium temperature into the electronics located in the enclosure.
Figure 2
Measures must be taken by the customer to prevent an over-
heating of the electronics due to crosstalk of the medium tem-
perature.
On the housing side, the sensor tube should project out of the
measuring tube in free air (without insulation) by at least 50 mm
(at sufficiently low ambient temperature).
Alignment of the sensor
The sensor head must be placed in the middle of the pipe (see Figure 1)
and adjusted correctly relative to the flow direction. A sensor mounted in
the wrong direction rotated by 180° leads to wrong (too high) measuring
values. As installation aid, a flow arrow is applied to the enclosure cover,
which must correspond to the flow direction.
Tilting of the measurement direction relative to the flow must not exceed
±3°, otherwise it can lead to major measurement deviations
3
.
The sensor measures unidirectionally and must be adjusted
correctly relative to the flow direction.
The axial tilting of the sensor head relative to the flow direction
should not exceed 3°.
General note:
Do not use the alignment surface of the housing for mechanical
adjustment, e.g. for locking.
There is a risk of damage to the sensor.
3
Deviations > 1 % of the measured value
!
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 7
Calculation of volume flow
A quasi-parabolic speed profile is formed over the pipe’s cross-section
under laminar
4
conditions. Whereas the flow velocity at the pipe walls re-
mains almost zero, in the middle of the pipe it reaches the optimum meas-
uring point, its maximum wN. This measurand can be converted into an
average flow velocity
N
w
that is constant over the pipe cross-section with
the aid of a correction factor the so-called profile factor PF.
The profile factor depends on the pipe diameter
5
and is shown in Table 3.
PF
Pipe Ø
Volume flow [m3/h]
Inner
Outer
Min. @
@ Sensor measuring range
[mm]
[mm]
0.2 m/s
2.5 m/s
10 m/s
20 m/s
40 m/s
60 m/s
0.796
26.0
31.2
0.304
3.804
15.21
30.43
60.86
91.29
0.748
39.3
44.5
0.653
8.166
32.66
65.33
130.7
196.0
0.772
51.2
57.0
1.144
14.31
57.22
114.4
228.9
343.3
0.786
70.3
76.1
2.197
27.46
109.8
219.7
439.3
659.0
0.797
82.5
88.9
3.068
38.34
153.4
306.8
613.5
920.3
0.804
100.8
108.0
4.620
57.74
231.0
462.0
923.9
1,386
0.812
125.0
133.0
7.175
89.68
358.7
717.5
1,435
2,152
0.817
150.0
159.0
10.40
129.9
519.8
1,040
2,079
3,119
0.829
206.5
219.1
19.99
249.9
999.5
1,999
3,998
5,997
0.835
260.4
273.0
32.02
400.2
1,601
3,202
6,404
9,605
0.84
309.7
323.9
45.56
569.5
2,278
4,556
9,112
13,668
0.841
339.6
345.6
54.85
685.6
2,742
5,485
10,969
16,454
0.845
388.8
406.4
72.23
902.9
3,612
7,223
14,446
21,670
0.847
437.0
457.0
91.47
1,143
4,573
9,147
18,294
27,440
0.85
486.0
508.0
113.5
1,419
5,677
11,353
22,706
34,059
0.852
534.0
559.0
137.4
1,717
6,869
13,739
27,477
41,216
0.854
585.0
610.0
165.3
2,066
8,263
16,527
33,054
49,581
0.86
800.0
311.2
3,891
15,562
31,124
62,249
93,373
0.864
1,000
488.6
6,107
24,429
48,858
97,716
146,574
0.872
1,500
1,109
13,869
55,474
110,948
221,897
332,845
0.877
2,000
1,984
24,797
99,186
198,373
396,745
595,118
Table 3 Profile factors and volume flows
4
The term “laminar” means here an air flow low in turbulence (not according to its physical
definition saying that the Reynolds number is < 2300).
5
Both inner air friction and sensor locking are responsible.

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 8
Thus, it is possible to calculate the standard volume flow of the medium
using the measured standard flow velocity in a pipe with known inner di-
ameter:
AwV
wPFw
DA
NN
NN




2
4

D
Inner diameter of pipe [m]
A
Cross-section area of pipe [m2]
N
w
Flow velocity in the middle of the pipe [m/s]
N
w
Average flow velocity in the pipe [m/s]
PF
Profile factor (for pipes with a circular cross-section)
N
V

Standard volume flow [m3/s]
SCHMIDT Technology provides a "flow calculator" on its homepage for
the calculation of flow velocity or volume flow in (circular) pipes or (rectan-
gular) shafts for the different sensor types:
www.schmidt-sensors.com or www.schmidttechnology.de
Installation in systems with square cross-section
For most applications, two borderline cases can be distinguished with re-
gard to flow conditions:
Quasi-uniform flow field
The lateral dimensions of the flow-guiding system are approximately
as large as its length in the flow direction and the flow velocity is small
so that a stable trapezoidal
6
speed profile of the flow is formed. The
width of the flow gradient zone at the wall is negligible in relation to the
chamber width so that a constant flow velocity can be expected over
the whole chamber cross-section (the profile factor is in this case 1).
The sensor must be mounted here in such a way that the sensor head
is far away enough from the wall and measures in the area with the
constant flow field.
Typical applications are:
oExhaust ventilation shafts for drying processes
oChimneys
6
A uniform flow field prevails in the largest part of the space cross-section.

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 9
Quasi-parabolic flow profile
The system length is large compared to the cross-section surface and
the flow velocity is so high that the ratios correspond to that of the cir-
cular pipe. This means that the same requirements apply here to the
installation conditions.
Since the situation is similar to that in a pipe
7
, the volume flow in a
square chamber can be calculated by equating the hydraulic diameter
of both cross-section forms. The result for a rectangle according to Fig-
ure 3 is a hydraulic “pipe diameter” DR:
bK: Width of rectangular channel
hK: Height of rectangular channel
DR: Equivalent pipe diameter
kk
kk
Rhb hb
D

2
Figure 3
According to this, the volume flow in a shaft is calculated as:
N
KK
KK
RNN
NN
KK
KK
KK
KK
RR
w
hb hb
PFAwV
wPFw
hb hb hb hb
DA



































2
22
22
44




bK/ hK
Width / height of square chamber [m]
DR
Hydraulic inner diameter of square chamber [m]
AR
Cross-section area of the equivalent pipe [m2]
N
w
Maximum flow velocity in the middle of the pipe [m/s]
N
w
Average flow velocity in the pipe [m/s]
PF
Pipe profile factor
N
V

Standard volume flow [m3/s]
Typical applications are:
oVentilation shaft
oExhaust air duct
7
The profile factors are equal for both cross-section forms.

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 10
4 Electrical connection
Make sure that no supply voltage is active during electrical instal-
lation and that the supply voltage cannot be switched on inad-
vertently.
The sensor is equipped with a plug-in connector which is firmly integrated
in the housing (pin assignment see Table 4).
Number of connection pins: 8 (plus shield connection at the metallic housing)
Type: Male
Fixation of connecting cable: M12 thread (spigot nut at the cable)
Type of protection: IP67 (with screwed cable)
Model: Binder, series 763
Pin numbering:
View on plug-in connector of sensor
Figure 4
Pin
Designation
Function
Wire color
1
Pulse 1
Output signal: Flow / volume (digital: Impulse)
White
2
US
Supply voltage: 24 VDC ± 20 %
Brown
3
Analogue TM
Output signal: Temperature of medium (U / I)
Green
4
Analogue wN
Output signal: Flow velocity (U / I)
Yellow
5
AGND
Reference potential for analogue outputs
Gray
6
Pulse 2
Output signal: Flow / volume (digital: Relay)
Pink
7
GND
Supply voltage: Ground
Blue
8
Pulse 2
Output signal: Flow / volume (digital: Relay)
Red
Shield
Electromechanical shielding
Meshwork
Table 4
The specified wire colors are valid using a connecting cable delivered by
SCHMIDT®Technology GmbH.
The analogue signals have an own AGND reference potential.
The metal sensor housing is indirectly coupled to GND (with a varistor
8
,
parallel to 100 nF) and should be connected to a protective potential, e.g.
PE (depending on the shielding concept).
The appropriate protection class III (SELV) respective PELV
(EN 50178) has to be considered.
8
Votage-dependant resistor (VDR); breakthrough voltage 27 V @ 1 mA
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 11
Operating voltage
The flow sensor SS 20.651 is protected against reverse polarity of the
operating voltage. For its intended operation, it requires a DC voltage of
24 VDC with a tolerance of ±20 %.
Deviating values can lead to measurement errors or even defects and,
therefore, should be avoided.
Operate the sensor only within the defined voltage range
(24 VDC ± 20 %).
Undervoltage may result in malfunction, overvoltage may lead
to irreversible damage.
The specifications for the operating voltage apply to the connection to the
sensor. Voltage drops generated due to line resistances must be taken
into account by the customer.
The operating current of the sensor (analogue signal currents included,
without any of the impulse outputs) is normally approx. 50 mA. With pulse
output, the required current value increases to max. 250 mA
9
.
Wiring of analogue outputs
Both analogue outputs, for flow and temperature, are designed as high-
side drivers with "Auto-U/I" characteristic which are short circuit protected
against both rails of the operating voltage.
The load resistance RLmust be connected between the corresponding
signal output and the electronic reference potential AGND or GND of the
sensor.
Depending on the value of resistance RL, the signal electronics switches
automatically between its operation as voltage interface (mode: U) or cur-
rent interface (mode: I), hence the designation "Auto-U/I". The switching
threshold is in range between 500 and 550 (for details, refer to the next
subchapter Signalling of analogue outputs).
However, a low load resistance value in voltage mode may cause signifi-
cant voltage losses via line resistances (especially in the GND line), which
can lead to measuring errors.
For voltage mode, a load resistance of at least 10 kis rec-
ommended.
The maximum load capacity CLis 10 nF.
9
Both signal outputs with 22 mA (maximum measurement values), both impulse outputs
with maximum signal current, supply voltage minimal
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 12
The following points must be also taken into account:
Use of only one analogue output
It is recommended to terminate both analogue outputs with the same
resistance value, even if only one of the outputs is used.
Unused analogue outputs
In this case, both outputs can remain disconnected or should be termi-
nated with high impedance against (A)GND (with the same resistance
value).
Short circuit mode
In case of a short circuit against the positive rail of the supply voltage
(+US), the signal output is switched off.
In case of a short circuit against the negative rail (A/GND) of the supply
voltage, the output switches to current mode (RLis calculated to
0 ) and provides the required signal current.
If the signal output is connected to +UBvia a resistance, the value RL
is calculated incorrectly and false signal values are caused.
Signalling of analogue outputs
Switching characteristic “Auto-U/I”
Range of load value RL
Signalling mode
Signalling range
≤ 500 (550) 
Current (I)
4 ... 20 mA
> 500 (550) 
Voltage (U)
0 ... 10 V
Table 5 Switching characteristic “Auto-U/I”
A hysteresis of approx. 50 ensures a stable transition behavior,
which is shown in Figure 5 below.
Figure 5
Depending on the set output signal, accuracy of the switching point
detection can be reduced. Therefore, it is recommended to select the
load resistance RLin such a way that a secure detection can be main-
tained (≤300 for current mode / ≥10 kfor voltage mode).
R [ ]
L
0500 550
Modus
I
U
Mode

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 13
For measuring of RLin an actual zero signal (voltage mode), the elec-
tronics generates test pulses that correspond to an effective value of
approx. 1 mV. However, the latest measuring devices may trigger in
response to such a pulse in the DC voltage measuring mode and dis-
play short-term measuring values of up to 20 mV. In this case, it is
recommended to install an RC filter before the measuring input with a
time constant of 20 … 100 ms.
Severe interferences on the connection cable may shift the switching
threshold out of specification. In this case the use of isolated amplifiers
for the measuring signals are recommended.
Error signalling
In current mode
10
, the interface output is 2 mA.
In voltage mode, the output switches to 0 V.
Representation of measuring range
The measuring range of the corresponding measurand is mapped lin-
earlyto the signalling range of its associated analogue output, depend-
ing on the signal type.
For flow velocity measurement, it ranges from zero flow to the end of
the measuring range wN,max (see Table 6).
Voltage mode (U)
Current mode (I)

  

  󰇛 󰇜
Table 6 Analogue signals for flow measurement
10
In accordance with the Namur specification
110 wN [%]
020 40 8060 100
IOut
[mA]
20
4
21,6
12
8
16
U
Out
[V]
0
110
w
N
[%]
10
0
20
11
40
80
60
100
4
2
6
8

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 14
The measuring range of the medium temperature TMstarts at 0 °C and
extends up to TM,max = +200 / +350 °C (see Table 7).
Voltage mode (U)
Current mode (I)

  

  󰇛 󰇜
Table 7 Analogue signals for measurement of medium temperature
Exceeding measuring range of flow velocity wN
Measuring values above wN,max are still output linearly up to 110 % of
the signalling range (this corresponds to maximum 11 V or 21.6 mA,
see images in Table 6). With even higher values of wN, the output sig-
nal remains constant.
Error signalling does not take place.
Medium temperature TMout of specification range
Operation outside the specified limits can lead to damage to the sensor
and, therefore, is considered as a critical error. Depending on the tem-
perature limit
11
, this leads to the following reaction (see also images in
Table 7):
oMedium temperature below TM,min = 0 °C:
The analogue output for TMswitches to error (0 V or 2 mA).
The measuring function for flow velocity is switched off; its analogue
output also signals an error (0 V resp. 2 mA).
oMedium temperature above TM,max = +200 / +350 °C:
TMis output in a linear way up to TM,max + 10 %.
Above this critical limit flow measurement is switched off and its an-
alogue output switches to error (0 V or 2 mA). The signal output for
TMswitches, contrary to standard error signalling, directly to the
maximum values of 11 V resp. 22 mA.
11
The switching hysteresis for decision threshold is approx. 5 K.

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 15
Wiring of pulse output (high-side driver)
The pulse output is current-limited, short-circuit protected and has the fol-
lowing technical characteristics:
Design: High-side driver, open collector
Minimum high level US,H,min: US–3 V (with maximum switching current)
Maximum low level US,L,max: 0 V (load RLto GND required)
Short circuit current limit: Approx. 100 mA
Maximum leakage current IOff,max: 10 µA
Minimum load resistance RL,min: Depending on supply voltage US(see below)
Maximum load capacitance CL: 10 nF
Maximum cable length: 100 m
Wiring:
Figure 6
The pulse output can be used as follows:
Direct driving of low-impedance loads (e.g. optocoupler, relays, etc.)
with a current consumption of maximum 100 mA.
This allows calculating the minimum permitted (static
12
) load resistance
RL,min depending on the operating voltage US:
   

Example:
In case of the maximum permissible supply voltage of US,max = 28.8 V
the minimal load is RL,min = 258 Ω.
Here the excessive heating power of the load has to be considered.
The pulse output is protected by various mechanisms:
Current limiting:
The analogue current is limited to approx. 100 mA.
If the load value is too low, the output switches to chopping (cycle du-
ration of 300 ms, with short interconnection phases of approx. 100 µs).
The maximum load capacitance CLis 10 nF. A higher capacitance re-
duces the limit of the current limiter.
12
Overcurrent peaks are absorbed by the short circuit limiter.

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 16
In case of a high capacitive load CL, the inrush current im-
pulse may trigger the quick-reacting short-circuit protection
(permanently) although the static current requirement is be-
low the maximum current IS,max. An additional resistor con-
nected in series to CLcan eliminate the problem.
Protection against overvoltage:
The pulse output is protected against short-term overvoltage peaks
(ESD, burst or surge) of both polarities by a TVS diode
13
.
Prolonged overvoltages destroy the electronics.
Overvoltages can destroy the pulse output.
Wiring of relay
The galvanically decoupled switching output is realized by a semiconduc-
tor relay with following technical characteristics:
Type: SSR (PhotoMOS relay)
Maximum leakage current IOff,max: 2 µA
Maximum ON-resistance RON: 16 Ω(typ. 8 )
Maximum switching current IS: 50 mA
Maximum switching voltage US: 30 VDC / 21 VAC,eff
Wiring:
Figure 7
The relay output is protected against short-term overvoltage peaks (ESD,
burst or surge) of both polarities by a TVS diode.
Prolonged overvoltages destroy the electronics.
Exceeding the specified electrical operating values lead to irre-
versible damage.
Protective measures for incorrect wiring or overload are not
taken for this output.
13
Transient Voltage Suppressor Diode, breakdown voltage approx. 30 V, peak pulse ca-
pacity 4 kW (8 / 20 µs)

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 17
Signalling of pulse outputs
Both pulse outputs represent the same information synchronously where-
as two measurands are selectable:
The actual flow velocity wN= 0 … wN,max is mapped proportionally to
the frequency
14
range f = 0 …fmax (see Figure 8):
- Standard version: fmax = 100 Hz
- Optional: Selectable maximum frequency fmax = 10 … 99 Hz

   
max,
max N
w
f
f
N
w
max,
max N
V
f
f
N
V 
N
V

: Standard volume flow
Figure 8 Example for fmax = 100 Hz
The volume flow and the pulse valence VN,Imp (= volume per pulse) can
be determined on base of the output frequency, the measuring range
of the sensor and the inner pipe diameter D:
2
4DPF
N
w
D
APF
N
w
N
V


;
max
max,
Imp, f
N
V
N
V


Another option supplies pulses with a fixed pulse valence of
1 m³/pulse.
For this purpose, the inner pipe diameter must be specified when or-
dering (minimum inner pipe diameter: Dmin = 25 mm).
Exceeding the measuring range of flow wNis still output up to 110 % of
the measuring range. The output of higher flow values is limited to 110 %
of the measuring range.
If an error occurs, 0 Hz respective no pulses will be output. The current
initial state remains unchanged.
Note:
The relay can be used as a S0-Interface according DIN EN 62053-31
15
.
14
Duty cycle is 1 : !
15
Formerly: DIN 43864
f
Out
[Hz]
0110 w
N
[%]
100
020
110
40 8060 100
40
20
60
80

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 18
5 Commissioning
The valid measuring ranges are specified on the rating plate.
After applying the supply voltage, the sensor signals the initialization of
the measuring operation by means of all four Duo-LEDs (sequence: red,
orange and green).
If the sensor detects a problem during initialization, it signals the problem
according to Table 8. An extensive overview of errors and their causes as
well as troubleshooting measures are listed in Table 9.
If the sensor is in the correct operational state, it switches to measuring
mode after initialization. Flow velocity signalling (both LEDs and signal
outputs) jumps briefly to maximum and settles down to the correct meas-
uring value after approx. 10 seconds, provided that the sensor probe was
already at medium temperature. Otherwise, the process will last longer
until the sensor has reached the medium temperature.
LED display
No.
State
LED 1
LED 2
LED 3
LED 4
1
Ready for operation & flow < 5 % 16
2
Flow > 5 %
3
Flow > 20 %
4
Flow > 50 %
5
Flow > 80 %
6
Flow > 100 % (= Overflow)
7
Sensory element defective
8
Supply voltage too low
9
Supply voltage too high
10
Temperature of electronics too low
11
Temperature of electronics too high
12
Temperature of medium too low
13
Temperature of medium too high
Legend
LED off
LED shines orange
LED shines green
LED flashes red (approx. 2 Hz)
Table 8 LED signals of sensory functions
16
„%“ of measuring range of flow velocity

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 19
6 Service information
Maintenance
Contaminations of the sensor head lead to distortion of the measured
value and can damage the sensor chip.
Therefore, the sensor head must be checked for contamination at regular
intervals. If contaminations are visible, the sensor must be cleaned as de-
scribed below and examined with respect to flow at a certain volume flow
(calibration). Ideally, the entire characteristic line of the sensor should be
calibrated by SCHMIDT Technology.
If the maintenance is performed not properly or not at the re-
quired intervals, the warranty will be rendered.
Cleaning of sensor head
If the sensor head is dusty or contaminated, it can be carefully cleaned by
means of compressed air.
The sensor head is a sensitive measuring system.
During manual cleaning proceed with great care.
In case of persistent deposits, the sensor chip as well as the interior of the
chamber head can be cleaned carefully with the aid of residue-free drying
alcohol (e.g. isopropyl alcohol) or soap water, using special cotton buds
(example see Figure 9) or pipe cleaners.
Carefully insert the cotton pad between chamber head wall and sensor
chip, then move it gently back and forth with controlled, minimal pressure
on the chip.
Do not try to apply great force to the sensor chip (e.g. using cot-
ton pads with a head that is too thick or making levering move-
ments with the pad stick).
Mechanical overload may lead to irreversible damage.
Move the cotton pad with great care back and forth parallel to the surface
of the chip to rub off the contamination. Use several cotton pads if re-
quired.
!
!
!

Instructions for Use –SCHMIDT®Flow Sensor SS 20.651 Page 20
Figure 9 Example of cotton pads with narrow cleaning pads
For washing off the sensor element, a short rinsing with liquid (preferably
using cleaning agents or alcohol, that dry out without leaving residues) is
allowed. Immersion of the sensor head into liquids is not permitted.
Immersion into liquids is not permitted and can irreversibly dam-
age the sensor head.
Before recommissioning, the sensor head must be completely dry. The
drying process can be accelerated by gently blowing it off.
If this procedure does not help, the sensor must be sent to SCHMIDT
Technology for cleaning or repair.
Eliminating malfunctions
The following Table 9 lists possible errors (error images). A description of
the way to detect errors is given. Furthermore, possible causes and
measures to be taken to eliminate errors are listed.
Causes of any error signalling have to be eliminated imme-
diately. Significant exceeding or falling below the permitted
operating parameters can result in permanent damage to the
sensor.
!
!