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.
Selecting the Right Flow Sensor Sensor: Advantages Disadvantages Thermoanemometer very small air flows can also be measured (e.g.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (-).
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.
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.
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.
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.
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.
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.
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.
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.
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.
