Ahlborn ALMEMO 2390-5 Technical data

Selecting the Right Flow Sensor
3.5 Flow Sensors
3.5.1 Selecting the Right Flow Sensor
For measurement of flow velocities the ALMEMO® sensor range provides thermoanemometer probes, pitot tubes and rotating vanes.
The selection criteria are the measuring range and the operating temperature
as follows:
Sensor
Flow Velocities
Oper. Temperatures
Thermoanemometer
0.1 to 50 m/s
to 60°C
Rotating vanes
0.2 to 40 m/s
to 140°C
Pitot tubes
7 to 100 m/s
to 600°C
3
Measuring Range and Operating Temperature of Different Probes
Thermoanemometer
up to 60°C
Rotating vanes
up to 140°C
Pilot tubes
up to 600°C
ALMEMO® Manual
3-5-1
Selecting the Right Flow Sensor
Sensor:
Advantages
Disadvantages
Thermoanemometer
very small air flows can also
be measured
(e.g. draft
measurements), non-directional measurement is possible
sensitive sensor electronics,
sensitive with regard to mechanical stress and contamination, sensitive with regard to
turbulent flow, high current
consumption, limited ambient
temperature
Rotating vanes
high accuracy at medium flow sensitive sensor electronics,
velocities and medium ambi- sensitive
to
mechanical
ent temperatures, insensitive stress, depending on direction
to turbulent flow
Pitot tubes
copes with high flow velocities
and rough operating conditions, high operating temperatures possible, easy to clean
highly depending on direction,
low flow velocities cannot be
measured, depending on temperature,limited
accuracy,
sensitive to turbulent flow
Positioning The Air Velocity Probe
The correct position of the sensing probe is the main condition for reliable and
accurate measurement of the air velocity.
Turbulence appears after fans as well as after turns, junctions or section
changes in the duct. Reliable measurements are only possible if the probe is
placed far enough from such places. The minimum distance is a function of
the duct’s diameter.
The equivalent diameter of a rectangular duct a x b is:
3-5-2
ALMEMO® Sensors
D=
2ab
a+b
Selecting the Right Flow Sensor
The following pictures are guidelines for correct installation of air velocity transmitters.
Reliable measurements can be made
by positioning the
transmitter after filters (clean rooms),
air heaters or air
coolers, where the
turbulence is very
low.
The probe shall be
installed
in
the
middle of the duct.
Preferred
location
after filters,
rectifiers, coolers
(no turbulances)
The probe shall be
placed in front of diffusers or confusers.
Filters and coolers
calm down the air
flow.
ALMEMO® Manual
3-5-3
3
Thermoanemometer
3.5.2 Thermoanemometer
Thermistors and hot-wire anemometers are highly sensitive sensors that can
also measure very small air velocities. They are suitable for use in all fields of
air conditioning, ventilating, indoor applications and measurements at working
places (draft). The ALMEMO® sensor range provides thermoanemometer
probes for different measuring ranges and measuring accuracies:
• Thermoanemometer FV A645-THx with plug-on module housing
and fixed measuring range.
• Thermoenemometer MT 84x5 with separate housing for electronics,
ALMEMO® connecting cable and selectable measuring range.
The measuring variables, temperature (FVA935-TH) and air velocity are programmed within the ALMEMO® sensor connector on two measuring channels.
They can be activated and displayed with the correct scaling and dimension by
every ALMEMO® measuring instrument. For volume flow rate measurements
the cross section or the diameter of the ventilating shaft can be easily entered
into ALMEMO® hand-held devices.
Measuring Principle
A temperature-dependent semiconductor
(NTC) is integrated in the measuring
probe and is heated by a current.
The heated semiconductor cools down
as soon as it is exposed to an air flow.
The amount of heat loss is a measure for
the air velocity. A control circuit keeps the
temperature of the element, which has
cooled down by the air flow, on a constant value. The control current is proportional to the flow velocity.
3-5-4
ALMEMO® Sensors
Thermoanemometer
Thermoanemometer FVA935-THx
The air velocity sensor is a
hot film anemometer. An electrical current is increasing the
temperature of a resistor on
the substrate. The flowing air
causes a reduction of this
temperature. The cooling effect is directly proportional to
the mass flow and consequently to the air velocity
and inversely proportional to
the air temperature. At equilibrium, the temperature of the
sensor’s surface is the measure for mass flow.
The integrated temperature
sensor is used for the purposes of automatic temperature compensation.
FVA935-TH4 / TH5
FVA935-TH4Kx / TH5Kx
Technical data
Flow
FVA935TH4/TH4Kx
FVA935TH5/TH5Kx
Measuring range:
0 ... 2 m/s
0 ... 20 m/s
Resolution:
0.001 m/s
0.01 m/s
Response time:
Accuracy
< 1,5 s
±(0.04 m/s + 1% of meas. val.) ±(0.2 m/s + 2% of meas. val.)
Temperature compensation:
0 ...+50 °C
Direction facing the flow:
bidirectional
Angle dependence:
<3% of meas. val. with deflection < 15°
Temperature
Measuring range:
-20 ...+70°C
Resolution:
0.1°C
Accuracy:
±0.7 °C
Nominal conditions
ALMEMO® Manual
3-5-5
3
Thermoanemometer
Temperature:
22 °C ± 2 K
Atmospheric humidity:
45 ±10 % r.H. (non-condensing)
Atmospheric pressure:
1013 mbar
Power supply:
6 ... 13V / 40 mA
Dimensions
Probe diameter
6 mm
FVA935TH4/TH5 Probe with handle, probe lengths : 210 mm
(plus handle) ALMEMO® cable, 1.5 meters
FVA935TH4Kx/TH5Kx Probe with remote electronics in the cable housing
Probe lengths THxK1, 80 mm / THxK2, 300 mm
Probe cable 5 m to electronics, ALMEMO® cable, 1.5 m
Maintenance
Due to the absence of moving parts, the E+E air velocity transmitters are very
reliable. Their innovative hot film anemometer principle makes them highly insensitive to dust and dirt. Under normal environmental conditions no maintenance is required.
For operation in polluted environment we recommend to clean the sensor periodically by washing it in isopropylalcohol and let it dry. Do not touch or rub.
3-5-6
ALMEMO® Sensors
Thermoanemometer
Thermoanemometer FV A605-TA
For measuring the air velocity the ALMEMO® range of sensors includes rotating vanes, Pitot tubes and laser-calibrated thermoanemometers with unidirectional or omnidirectional measuring sensitivity. By storing the sensor data in the
ALMEMO® connector, the measured values are indicated in m/s and with the
correct scaling. It is also possible to measure the volume flow by entering a
factor or the cross sectional area.
Measuring Principle:
The sensor contains a NTC resistance, which, with respect to the ambient temperature, is heated up to a constant overtemperature. The flow rate is determined by measuring the required heating power. As this measurement strongly
depends on the ambient temperature a further precision NTC resistance is
used to measure and automatically compensate the ambient temperature.
Thermoanemometers are particularly suitable for low air velocities, e.g. draught
measurements.
Types:
FVA605-TAx
FVA605-TAxO
The thermoanemometer consists of a sensor tube that contains the NTC temperature sensor, the heated miniature thermistor, and the sensor converter
module including the detector electronics for the transducer. The thermoanemometer has been adjusted together with the sensor to be used with it. The converter module and the sensor can, therefore, not be interchanged! The module
is marked with the same fabrication number as its sensor (see type plate).
ALMEMO® Manual
3-5-7
3
Thermoanemometer
Two mechanical types are available with two different measuring ranges:
Unidirectional (sensitive in one direction) with a protected measuring tip:
FV A605-TA1:
air velocity
0.010 .... 1.000 m/s
FV A605-TA5:
air velocity
0.15 ... 5.00 m/s
Omnidirectional (direction-sensitive spherical tip) with protecting cage
FV A605-TA1O: air velocity
0.010 .... 1.000 m/s
FV A605-TA5O: air velocity
0.15 ... 5.00 m/s
Programming:
Range:
Output signal:
Range:
Dimension:
Factor:
Exponent:
Base:
FV A605 TA1/1O
0.010 .... 1.000 m/s
0 .... 1V
d2600
m/s
0.1
+1
-
FV A605 TA5/5O
0.15 ... 5.00 m/s
0 .... 1V
d2600
m/s
0.05
+2
-
Mounting and Handling:
1. Connect the sensor to the blue sensor cable.
2. Use the black ALMEMO® cable to connect the detector module to the
measuring instrument.
3. Switch on the measuring instrument.
As shown in the illustration the sensor can be pushed into the detector module.
As a result, one compact and easy to handle unit is available. Alternatively, it is
also possible to use the sensor separately from the detector module:
1. Disconnect the sensor cable from the sensor tube.
2. Pull the sensor tube out of the module.
3. Re-connect the sensor cable to the sensor.
3-5-8
ALMEMO® Sensors
Thermoanemometer
Fühlerschutz:
FV A605 TAxO
1.When in meas. position, hold sensor
tip at the knurled handle band (a).
2.Turn the blue sensor handle (b)
clockwise.
3.The handle snaps back (c) and the
sensor tip disappears in the protected position.
4.Follow the steps 3 to 1 in reverse to
bring the sensor tip back into the
measuring position.
FV A605 TAx
1.To open the sensor tip push the
black protecting cap (a) back in the
direction of the handle.
2.With the sensor tube held in a fixedposition, it can be loosened or locked
by slightly twisting the blue sensor
handle (b).
3.Bring the sensor tube in the requiredposition and secure by twisting.
Measurement:
After connecting the probe and switching on the ALMEMO® device the measured values will be correctly scaled including their dimension and can be immediately read out in m/s.
When performing measurements in an air duct the safety distances to turbulent points must be observed. To obtain undisturbed measured values the instrument ALMEMO® 2390-5 or 2690-8 is particularly suitable because it continuously averages the measured values by the function TIME CONSTANT.
Further measuring functions for averaging and for volume flow measurements
are described in the ALMEMO® Manual (chapt. 3.5.5) or Description on Averaging.
ALMEMO® Manual
3-5-9
3
Thermoanemometer
Zero Point Correction:
Generally, a re-adjustment of the zero point is not required. However, during
long term operation or when exposed to heavy shocks during transportation
the zero point can slightly shift.
If the output signal is not equalling 0m/s when the sensor is covered, a zero
point correction should be performed.
1. The sensor must be closed. Wait at least 3min after closing.
2. For approximately 3 to 4 seconds short out pin 1 and pin 5 located
at
the socket "LEMO (large)" and then disconnect them again.
3. The detector module is now in the adjustment mode and automatically performs a zero point correction. After a waiting period of approximately 2
minutes the sensor is operational again.

It must be ensured that the ambient temperature remains stable
during the adjustment.
Fuse:
If no output signal is available despite the supply voltage being applied, the
power supply must be interrupted for at least 30 seconds. This allows the (possibly) triggered PTC fuse to regenerate again.
Cleaning:
Generally, the sensors are maintenance-free. However, the sedimentation of
contamination at the high temperature NTC can lead to errors of measurement.
For cleaning purposes the head part can be thoroughly rinsed in a non-aggressive cleansing solution. Afterwards rinse in distilled water and allow to
completely dry.

3-5-10
Attention!
Do NOT touch the tip of the sensor! NEVER dry the head part
using a hot-air blower or compressed air!
The sensitive measuring cells could be damaged.
ALMEMO® Sensors
Thermoanemometer
Technical Data:
Electronics Box with Sensor
Measuring range:
FV A605 TA1(O):
FV A605 TA5(O):
Resolution:
FV A605 TA1(O):
FV A605 TA5(O):
Accuracy:
FV A605 TA1(O):
FV A605 TA5(O):
Nominal conditions:
22°C, 960hPa
Autom. Temp.-compensation:
Temp. influence:
±0.5% of final value/°C
Sensor
Head size:
Shaft:
Operative range:
Angle of attack:
Inlet opening:
Sensor length:
Cable length:
Storage temperature:
Ø 8 mm
Ø 15 mm
0 to 40°C
FV A605 TA1/TA5:
FV A605 TA1O/TA5O:
FV A605 TAx
FV A605TAxO:
FV A605 TAx:
FV A605 TAxO:
1.5m
-30 to +90°C
0.01 to 1m/s
0.15 to 5m/s
0.001m/s
0.01m/s
±1.0% of final val., ±1.5% of meas. v.
±0.5% of final val., ±1.5% of meas. v.
effective in range 0 to 40°C
3
±30°
±180°
Ø 9mm
Ø 110 mm (protective cage)
300m
310mm
General Technical Specifications
Meas. medium:
dry air or inert gases
Response time:
FVA605TAxD:
smoothened: 1t = 2s
FVA605TAxU:
not smoothened: 1t = 100ms
Power supply:
through ALMEMO® device (approx. 7 to 10V)
Current consumption: approx. 70mA
Output signal:
0 to 1V, linearised, minimum load resistance 10kW
Housing:
Dimensions:
100 x 60 x 35 mm (L x W x H)
Protection system:
IP 40 (aluminium housing)
Weight:
approx. 250g
Operat. temperature: 0 to 40°C
Storage temperature: -30 to 90°C
Air humidity:
0 to 90% r.h., non-condensing
Adjusting reference:
Laser-Doppler wind tunnel,
(certificate according to SN EN 45001)
adjustment at 22°C/approx. 960hPa
ALMEMO® Manual
3-5-11
Thermoanemometer
Thermoanemometer MT 84x5
The thermoanemometers MT 84x5 are laser-calibrated, high-precision probes
with adjustable measuring ranges and standard output signals.
MT 8455:
MT 8465:
MT 8475:
Multi-purpose sensor with protected measuring tip
Rod sensor with small measuring tip
Omni sensor with symmetrical ball tip
A special connector cable allows to connect the thermoanemometers to all ALMEMO® instruments. The instruments automatically recognise the sensor and
indicate the correct measured value, with the corresponding dimension, in the
display.
Measurement
The probe must be tightly fixed before it is used. The flow direction is marked
at the probe. A high time constant should be set or the averaging function of
the instruments should be used to obtain undisturbed measured values, which
enables an easier read-out of fluctuating values provided on the display.
Cleaning
Dust and dirt can sediment on the probe. If necessary, the probe can be
cleaned using a soft brush and a mild detergent solution, e.g. isopropyl alcohol.
3-5-12
ALMEMO® Sensors
Thermoanemometer
Technical Data MT 84x5:
Measuring ranges:
Accuracy:
Nominal temperature:
Nominal position:
Resolution:
Reproducibility:
Supply voltage:
Current consumption:
Time constant:
Operating range:
Dimensions:
MT 8455, MT 8465 adjustable from 0.125 m/s to
1.0 / 1.25 / 1.5 / 2.0 / 2.5 / 3.0 / 4.0 / 5.0 / 7.5 / 10.0
12.5 / 15.0 / 20.0 / 25.0 / 30.0 / 40.0 / 50.0 m/s
MT 8475 adjustable from 0.05 m/s to
0.5 / 0.75 / 1.0 / 1.25 / 1.5 / 2.0 / 2.5 m/s
MT 8455/8465:
± 2% of meas. value, ±0.5% of selected range
MT 8475:
± 3% of meas. value, ±1 % of selected range
MT 8455/8465: 18–28°C<,> °C +0.2% per °C
MT 8475: 20–26°C<,> °C +0.5% per °C
horizontal
0.07 % of selected range
<±1% of measured value
11 to 30 V DC
maximum 350 mA
selectable from 0.05 to 10 s
0 to 60 °C
probe 300 mm long, meas. tip 32 mm, cable 5 m
housing 126 x 80 mm, 60 mm high
ALMEMO® Manual
3
3-5-13
Pressure Modules for Dynamic Pressure
3.5.3 Pressure Modules for Dynamic Pressure
Basic Principles
static pressure
total pressure
The air velocity is determined by the dynamic pressure and the static pressure, which
develops when a Prandtl Pitot tube is placed
in an air flow.
The total pressure strikes the opening of the
Pitot tube and is available at the terminal (+)
of the pressure module.
The pure static pressure is measured via
side slots and is available at the terminal (-).
The pressure difference, i.e. the dynamic
pressure is a measure for the velocity in the
flow. The dynamic pressure is calculated
and indicated.
The dynamic pressure relates to the air velocity as follows:
1.
Density of the air:
= 0
2.
with ρ0 =
T =
1.292 kg/m3 (density at 0°C)
air temperature in °C
Air velocity (valid up to approx. 40 m/s):
p=
dynamic pressure in Pa
k=
probe-related coefficient
Prandtl tube:
k=1
cylindrical probe:
k = 1.7
Air velocity with consideration of the compressibility of the air
(also valid at more than 40 m/s):
v=
3.
273
273T
v=


2p
kp
p
2
p /2 p /4 c
with
with c =
speed of sound in air
(331 + 0.6 x T m/s)
The formulae show the influence of the air temperature to the density of the air
and, consequently, to the result of the measurement of the dynamic pressure.
Furthermore, the deviation of the atmospheric pressure pa from the normal
pressure 1013mbar also has an effect on the result. The following factor can
be used for correcting the velocity:
K=
3-5-14

1013mbar
Pa
ALMEMO® Sensors
K ≈ 1 + (1013 - pa) · 0.0005 (in 1st approximation)
Pressure Modules for Dynamic Pressure
Air velocity for selected dynamic pressures (Prandtl Pitot tube, T = 22 °C)
Dynamic pressure [Pa]
Dyn. pressure [mmWS] Air velocity [m/s]
1
0.1
1.29
2
0.2
1,83
3
0.3
2,24
4
0.41
2.59
5
0.51
2.89
10
1.02
4.09
20
2.04
5.78
30
3.06
7.08
40
4.08
8.18
50
5.1
9.14
100
10.2
12.93
Correction factors for consideration of temperature and air pressure:
The true air velocity is depending on the air temperature and the barometric air
pressure. Therefore, the measured value must be corrected according to the
following table to obtain exact measurements of the air velocity.
Air
temperature
- 30 °C
- 20 °C
- 10 °C
0 °C
10 °C
20 °C
30 °C
40 °C
50 °C
60 °C
70 °C
80 °C
90 °C
100 °C
150 °C
200 °C
250 °C
300 °C
400 °C
500 °C
600 °C
700 °C
940
mbar
0.942
0.961
0.98
0.998
1.016
1.035
1.051
1.069
1.085
1.102
1.118
1.135
1.151
1.167
1.242
1.314
1.381
1.446
1.567
1.68
1.784
1.884
960
mbar
0.932
0.951
0.97
0.988
1.005
1.024
1.04
1.057
1.074
1.09
1.106
1.123
1.139
1.154
1.229
1.3
1.367
1.431
1.55
1.663
1.766
1.865
980
mbar
0.922
0.941
0.96
0.978
0.995
1.013
1.029
1.047
1.063
1.079
1.095
1.111
1.127
1.142
1.216
1.287
1.353
1.416
1.534
1.646
1.748
1.846
1000
mbar
0.913
0.932
0.95
0.968
0.985
1.003
1.019
1.036
1.052
1.068
1.084
1.1
1.116
1.131
1.204
1.274
1.339
1.402
1.519
1.629
1.73
1.827
1020
mbar
0.904
0.923
0.941
0.958
0.975
0.993
1.009
1.026
1.042
1.057
1.073
1.089
1.105
1.12
1.192
1.261
1.326
1.388
1.504
1.613
1.713
1.809
ALMEMO® Manual
1040
mbar
0.895
0.914
0.931
0.949
0.966
0.983
0.999
1.016
1.031
1.047
1.063
1.078
1.094
1.109
1.18
1.249
1.313
1.375
1.489
1.597
1.696
1.791
3-5-15
3
Pressure Modules for Dynamic Pressure
Example:
Air velocity 50 m/s, air temperature 80 °C, atmospheric pressure 960 mbar.
The measured value must be multiplied with the correction value 1.123. The air
velocity is, therefore, 56.1 m/s.
ALMEMO® Pressure Connector:
For flow measurements the ALMEMO® sensor range provides plug-on pressure connector FDA602-SxK and, as accessories, robust stainless steel or
nickel-plated brass Prandtl Pitot tubes. They are connected to the pressure
connector via hoses. Cylindrical probes can be used if the specific probe coefficient (1.7) is considered by programming the factor 1/  1,7 = 0.767.
The measuring variables, dynamic pressure and air velocity are programmed
within the ALMEMO® sensor connector on two measuring channels and can be
activated and displayed with the correct scaling and dimension by every ALMEMO® measuring instrument.
Pressure Connector FDA602-SxK
Designation Channel
FDA602-S1K: 1st chan:
2nd chan:
FDA602-S6K: 1st chan:
2nd chan:
3-5-16
Meas. Range
0.5...40.0 m/s
± 1250.0 Pa
1.8...90.0 m/s
± 6800 Pa
ALMEMO® Sensors
Pitot tube FD9912
Dim
ms
Pa
ms
Pa
Range
L840
Volt
L890
Volt
Factor
0.4
Exp
3
4
Pressure Modules for Dynamic Pressure
Zero Point Correction of the Pressure Sensors
The zero point of the pressure sensors can shift due to positional changes and
temperature variations. Therefore, it is useful to perform a zero point correction
before each measurement. For the zero point correction, the pressure hoses or
the Pitot tube must be removed from the flow conduit. When the measured
value has stabilised, the zero point correction can be performed. This is described in the operating instructions supplied with each device under “zeropoint / sensor adjustment” For the necessary interface command please refer
to Section 6.4.2.
The zero point correction must be performed separately for each
active channel (m/s, Pa).
The zero point correction data is lost on switch off. Therefore, a new
correction must be performed for the next measurement.

Temperature Compensation
If the measuring temperature largely deviates from the reference temperature
of 25°C the temperature influence (range -50.0 to +700.0°C) should be compensated by performing a measurement preferably with a NiCr-Ni temperature
sensor. With ALMEMO® V5 and V6 devices every suitable temperature sensor
(resolution 0.1°C) can be used for compensation (see 6.3.4). By means of the
reference channel, the air temperature can, at the measuring instrument ALMEMO® 2295-6, for compensation purposes, be entered for one flow measurement channel if the switch position COMPENSATION is selected. If the operating conditions are relatively constant it is sufficient to enter a correction factor
according to the above table.
Handling the Pressure Modules

Ensure that the Pitot tube is properly connected. A confusion of the
pressure connectors will cause incorrect measurements.
Attention:
The pressure sensors contain very sensitive pressure cells.
Note the permissible maximum pressures - these must not be exceeded!
Caution when removing the hoses! Do not squeeze the hoses.
Harmful negative pressures can then be avoided. Avoid strong vibrations! Do not let aggressive gases penetrate the membrane or
pressure cells, as they could be destroyed!
ALMEMO® Manual
3-5-17
3
Pressure Modules for Dynamic Pressure
Technical Data
Pressure connector:
Overload capacity:
maximum triple measuring range
Max. common mode pressure: 700 mbar
Accuracy (zero-pt adjusted):
±0.5% of final value
in range 0 to positive fin. val.
Nominal temperature:
25°C
Temperature drift:
max. 1.5% (typ. 0.5%) of final value
Compensated temp. range:
0 … +70°C
Operating range:
–10 to +60°C, 10 to 90% r.H. non-condensing
Dimensions:
90 x 20 7.6 mm
Hose connection:
Ø 5mm, 12mm long
Sensor material:
aluminium, nylon, silicone, silica gel, brass
Pitot tubes:
Order No.
FD 9912-33MS
FD 9912-33VA
FD 9912-54MS
Head-Ø
3 mm
3 mm
5 mm
Shaft-Ø
6 mm
6 mm
8 mm
Length
300 mm
300 mm
400 mm
FD 9912-54VA
FD 9912-56MS
FD 9912-56VA
5 mm
5 mm
5 mm
8 mm
8 mm
8 mm
400 mm
600 mm
600 mm
500 °C
350 °C
500 °C
FD 9912-84MS
FD 9912-84VA
FD 9912-88MS
8 mm
8 mm
8 mm
8 mm
8 mm
8 mm
400 mm
400 mm
800 mm
350 °C
500 °C
350 °C
FD 9912-88MS
FD 9912-97VA
FD 9912-97VA
8 mm
10 mm
10 mm
8 mm
10 mm
10 mm
800 mm
1000 mm
1000 mm
600 °C
350 °C
600 °C
FD 9912-98MS
FD 9912-98VA
FD 9912-99MS
10 mm
10 mm
20 mm
20 mm
20 mm
20 mm
1500 mm
1500 mm
2000 mm
350 °C
600 °C
350 °C
FD 9912-99VA
20 mm
20 mm
2000 mm
600 °C
MS = brass nickel-plated, VA = chrome-nickel steel
3-5-18
ALMEMO® Sensors
Oper. up to
150 °C
300 °C
350 °C
Rotating Vanes
3.5.4 Rotating Vanes
For flow measurements the ALMEMO® sensor range provides the rotating
vanes FV A915-Sxxx with an exchangeable snap-on head or the type FV
A915-MA1 with a fixed measuring head. They are especially suitable for use in
air conditioning. The air velocity can be activated and displayed with the correct
scaling and dimension by every ALMEMO® measuring instrument.
Measuring Principle
The flow velocity is determined by means of a
frequency measurement. The flowing medium
initiates the movement of the rotating vane. By
an inductive pulse count, which is performed using a microcontroller and is integrated in the ALMEMO® connector, the revolutions of the rotating vane are measured and displayed as
velocity.
Measurement
To obtain correct measuring data with a rotating vane measuring probe the
shaft of the rotating vane must be aligned in parallel to the flow direction. If the
measuring probe is slightly moved in the air flow, the value indicated at the instrument will change. The measuring probe is correctly placed in the air flow,
when the indicated value is at maximum. When flow measurements are performed using rotating vanes there can be measuring situations with a very inhomogeneous flow profile:
-
-
Measurement at air outlets:To obtain correct measuring data the measurement must be performed some distance away from the grill-screen and
large rotating vanes must be used allowing the measured values to be integrated and averaged over the larger measuring head. If smaller rotating
vanes are used it is necessary to average either spatially over individual values or over a specific time.
Measurements at air intakes:To obtain defined flow conditions in a fixed
cross section the rotating vane is positioned in a measuring hopper.
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3
Rotating Vanes
Design
Our rotating vanes are sensitive transducers with diamond bearings that are
very precisely adjusted. This ensures high accuracy. The aluminium rotating
vanes have stream-lined measuring heads made from plastic material (polysulfone). The shafts are, as standard, guided in protected, oiled berylliumbrass bearing sleeves and supported in tips made from special casehardened
steel. This makes the transducers especially suitable for use in air conditioning.
The rotating vanes are partly equipped with snap-on measuring heads and are,
therefore, very easy to maintain.
Meas. Range
0.3 to 20 m/s
0.4 to 40 m/s
0.5 to 20 m/s
0.6 to 40 m/s
0.2 to 20 m/s
Head Ø
22 mm
22 mm
11 mm
11 mm
80 mm
Opening
from 35 mm
from 35 mm
from 15 mm
from 15 mm
from 108 mm
Technical Data:
Accuracy:
FV A915 S120-S140:
FV A915 S220-S240:
FV A915 SMA1:
Max. resolution:
Nominal temperature:
Operating range:
3-5-20
ALMEMO® Sensors
Shaft Ø
15 mm
15 mm
15 mm
15 mm
15 mm
Length
175 mm
175 mm
165 mm
165 mm
235 mm
±0.5% of f.sc.v. ±1.5% of m.v.
±1 % of f.sc.v. ±3 % of m.v.
±0.5% of f.sc.v. ±1.5% of m.v.
0.01 m/s
22°C ±2K
–20 to +140°C
Order No.
FV A915 S120
FV A915 S140
FV A915 S220
FV A915 S240
FV A915 SMA1
Volume Flow Measurement
3.5.5 Volume Flow Measurement
For determining the volume flow V in ventilating channels the medium flow velocity is multiplied with the cross section area F:
V = vM F 0.36
V
= volume flow in m3/h,
F
= cross section in cm2,
vM
= medium flow velocity in m/s
Air Volume Measurement with Mountable Hopper
For air volume measurement at air vents (e.g. disk valve) up to 200mm diameter the mountable hopper ZV 9915-LM is available for the macro rotating
vane FV A915-MA1 as an accessory. By scaling the air velocity with the factor
1.3762, exponent +1 and dimension mh the air volume is obtained in m3/h. A
correction factor for the forced circulation of the rotating vane is already considered. The measuring variable volume flow can also be programmed as 2nd
channel.
Chan.
1st ch.:
2nd ch.:
Function
air velocity
volume flow
Meas. Range
0.2...20.00 m/s
1.0...275.0 m3/h
Dim
ms
mh
Range
L420
L420
Factor
1,376
Exp
+1
Volume Flow Measurement with Center Probe
For a rough measurement of the volume flow it is sufficient to place a flow
probe in the center of the flow channel. The medium flow velocity is approximately 0.8 v (see below 'net measurement center method'). By scaling the velocity with the factor (0.8 F 0.36) the momentary volume flow can be continuously indicated in m3/h. It may be also necessary to program the exponent and
the dimension.
Determining a Volume Flow from Average Value & Cross Section
To obtain measured values as accurately as possible, the flow velocity must be
integrated and averaged over the whole area. The hand-held devices 2390-5,
2690-8, 2890-9 and system 5690-2 allow for entering the cross section area
directly via keyboard in function QF, as area F with 32000cm2 at maximum, or
in the function DN via the diameter with 2000mm at maximum. The volume
flow V can then be read out in function channel “Flow” directly in m3/h - the
product of average value and surface; (see device instructions, “Volume flow
measurement”).
It is also possible, by means of function channels “Flow” and “n(t)”, to output
and to save the volume flow and the number of measuring operations; (see
Section 6.3.3).
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3
Volume Flow Measurement
Determination of the Average Flow Velocity
The most important parameter for the measurement of the volume flow is the
average flow velocity vM. As the velocity has the highest value in the center of
a channel and because it is significantly smaller at the walls, it is necessary to
use one of the following methods for averaging it over the cross section.
Time-based Averaging:
With air volume measurements at grill-screens the mean flow velocity can be
determined by a time-based averaging:
1.
Set the averaging mode for the time-based averaging
2.
Place the rotating vane properly at one end and start averaging.
3.
Proceed uniformly over the whole cross section.
4.
When the other end has been reached, stop the averaging.
Area Measurements:
If flow measurements are performed for acceptance tests, according to the
guidelines VDI/VDE 2640, the medium flow velocity is determined in a net of individual measuring points within the measuring cross section, which is placed
vertically with respect to the flow direction. The averaging over single measurements should be used for these net measurements (see device manual). An
overview of the different methods can be found on the following page. Precise
measuring results can be obtained with a Pitot tube or a micro anemometer,
which has a minimal influence on the flow. Depending on the measuring method the average value of the single measurements vM must be corrected by the
correction factor k: v = k vM
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ALMEMO® Sensors
Volume Flow Measurement
3
ALMEMO® Manual
3-5-23
Volume Flow Measurement
Procedure:
If the measurements are performed according to the guidelines VDI/VDE 2640
the following notes must be observed:
- Depending on the type of ventilation system there can already be turbulences at low flow velocities.
- Perform the measurements in a quiet section of the ventilation system
where turbulences are minimal.
- Select the measuring point so that maximum safety distances are available
before and after the measuring point. The safety distance N is the distance
from a turbulence point to the measuring point. Turbulences can develop,
for example, at fans, pipe bends, reductions, control flaps, rectifiers, heating
units and filters etc.
- The following formulae can be used to calculate the safety distances N for
selecting the measuring point. L represents the free straight leg of a ventilating channel that is free from obstructions.
- Select the safety distance N1=L1/D before the measuring point so that it is
equal or larger than 6 and select the safety distance N2=L2/D after the measuring point so that it is equal or larger than 2.
- If short, straight elements of a piping system are only available, a shorter
safety distance N can be chosen when a larger number of measuring points
is used for the averaging. However, N1=L1/D must at least total 2.5.
3-5-24
ALMEMO® Sensors