Measurement Technologyfor Compressed AirIn the field of compressed air, measuring pressureprovides the data basis for rating the correct pres-sure head of pressure differences in the distributionsystem as well as for controlling and regulating thecompressors. Before a compressed air system isdimensioned or optimised, the volume flow shouldbe measured. If especially high compressed airquality is required, the relevant measurements pro-vide the basis to ensure compressed air quality aswell as to optimise compressed air processing.

Pressure measurement ordifferential-pressure measurement

Measuring pressure under flow conditions is mainlydone to control and regulate compressors or com-pressor systems as well as to assess compressedair systems.

Differential-pressure measurement is used in addi-tion to monitor the functional performance and eco-nomic efficiency of the air processing systems suchas filters.

Membrane pressure switchIn many of the compressors and compressor sys-tems used today, membrane pressure switches rec-ord the pressure and pass on the data measured inthe form of an electrical switch signal.

Please note:

• the ageing of the mechanical components impairsthe repeatability.

• Membrane pressure switches require a high dif-ferential gap before they react and need a lot ofroom.

Contact manometerUp until the 1990s, it was considered state-of-the-artto use mechanical contact manometers for differen-tial-pressure measurements, e. g. to monitor filters orto control compressor systems.

Please note:

• in order to achieve sufficient resolution, the optimalmeasuring range should be close to the operatingrange.

• Electrical connections result in mediocre repeat-ability and complicated adjustments of max. fourusable contacts.

Facts Measurement TechnologyPage 2 of 5

Electronic pressure sensorThe compressors of modern compressor systemsshould be controlled based on the pressure meas-urements of electronic pressure sensors which con-vert the pressure values into analogue signals.

Please note:

• pressure sensors with an output signal of 4 to20 mA help to avoid cable breaks.

• If the maximum of the measuring range is close tothe range of parameters to be controlled, a higherresolution should be aimed at.

• These very robust and reliable systems are char-acterised by their high repeatability as much astheir compact construction.

Measuring the volume flow rateMeasuring the volume flow rate is used to detect thecapacity of compressors both with regard to the totalair consumption of a plant as well as with a view toindividual air consumption of local production facili-ties.

It must be taken into account that the volume flowdata of compressors and air consumers refer to theoperational environment, but that the measurementis made in the system conducting the pressure. It istherefore necessary to convert the measured data tothe operational environment.

In order to obtain an absolutely exact result, not onlythe volume flow rate, the temperature and the pres-sure of the compressed air, but also the atmosphericpressure, the atmospheric temperature and the hu-midity of the inlet air have to be determined (see Fig.1). This is indispensable for performance testingcompressors.

Inlet temperature T1

Inlet pressure p1

Inlet humidity Frel1

Outlettemperature T2

Outletpressure p2

Outletvolume V2

Power input in kW

12

1221 pT

TpVV×

××=

Fig. 1: Measuring the inlet volume flow rate

Volume flow rate measurements for in-plant ac-counting or planning a compressor system do notjustify the cost of parallel measurements of the am-bient temperature, humidity and atmospheric pres-sure. However, a backward projection should be

made based on average pressure and temperatureconditions at the installation site.

Temperature and pressure compensationPressure and temperature are rarely constant in acompressed air system. Therefore when measuringair consumption, the pressure and temperatureshould be determined as well as the volume flowrate, so that a correct backward projection of themeasured operating state can be made based onthe ambient conditions (see gas equation, Fig. 1).This is essential for an accurate measurement.

Without temperature and pressure compensationIf a volume flow rate measurement is made withoutparallel pressure and temperature measurement andwithout backward projection of these factors to therelaxed state, it is only possible to determine thevolume flow at actual temperature and pressureconditions. Otherwise pressure and temperaturefluctuations occurring during the measurement resultin errors when projecting backwards to the ambientstate.

Direct measurement of the volume ormass flow rateMeasuring the dynamic pressure makes it possibleto determine the volume flow rate with a high degreeof accuracy. A venturi nozzle can be used or alter-natively a dynamic pressure sensor (see Fig. 2).

Fig. 2: Measuring dynamic pressure

dP

v1 v2 v11

2

pstatpstatPdyn +

Measurement withpressure sensor

Venturi nozzle

Measurementwith orifice

Facts Measurement TechnologyPage 3 of 5

Please note:• In order to reach sufficient resolution, the optimum

measurement range should be close to the oper-ating range.

• Electrical connections result in mediocre repetitiveaccuracy and complicated adjustments of the max.four usable contacts.

• The correct lengths of the inlet and outflow zonesare important as is the insertion of the measuringdevice into the distribution system andthe exact geometrical data of the pipe.

• Attention!: contamination hazard.• If the flow rate falls below 10 per cent of the maxi-

mum measured value, this results in lower meas-urement accuracy.

Volumetric measurementVolumetric measurements are highly accuratemeasurements which are used, e. g. to determinethe capacity of compressors. The most importantmeasuring devices are rotary displacement gasmeters and turbine gas meters. Whereas the rotarydisplacement gas meter should be applied in ameasurement range from 10 to 90 % of its max.volumetric flow rate, the turbine gas meter is veryaccurate in the lower measurement range as well.

Please note:

• these measurement devices are complex me-chanical components requiring intensive monitor-ing.

• Not overload-proof (danger in depressurised com-pressed air system).

Calorimetric measurementSo-called hot-wire anemometers can measure thevolume flow rate as a function of the mass flow ratein a compressed air pipe by relating the heat re-moved to the volumetric flow rate (see Fig. 3).

Fig. 3: Calorimetric volume flow rate measurement

Please note:

• Without temperature and pressure compensation,deviations from the design point in temperature,humidity and pressure fluctuations have a stronginfluence on the result.

Coriolis mass flow rate measurementIs based on exploiting the controlled generation ofthe coriolis force. These forces occur where transla-tory (straight) and rotary (rotating) movements over-lap. The magnitude of the forces depends on themasses moved and their velocity and thus on themass flow rate (see Fig. 4).

ω = Angular velocityt A, B = SensorsF = Coriolis force y = Amplitude∆ϕ = Phase shift t = Time

Fig. 4: Coriolis mass flow measurement

OthersIn addition to the classical methods of measuringvolume flow rate, there are several new measure-ment systems available today.

Karman Vortex StreetThe volume flow rate measurement is based on theKarman vortex street (see Fig. 5).

Fig. 5: Karman vortex street

An exactly defined body fixed in a compressed airsystem generates a vortex and thus vibrations whichcan be recorded. They vary analogue to the changesin the volume stream shed from the diverting body.

This test set-up has similar features to dynamicpressure measurement systems

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Please note:

• Vibrations caused by the pipeline construction caninfluence the measurement result.

Ultrasonic measurementUltrasonic measurement devices such as thosecommon in gas and water engineering, have not yetbeen widely used in compressed air systems (seeFig. 6).

Ultrasonic transducerSonic beams

Focusing surface

Fig. 6: Ultrasonic flow measurement

Indirect measurementsWhereas the direct measurements already de-scribed can be applied centrally and locally to meas-ure the air consumption in companies and to deter-mine the performance data of compressors, indirectmeasurements with the aid of the compressors serveto determine air consumption values and character-istics of complete compressed air systems.

Digitally recording the load time of compressorsDiscontinuously regulated compressors are con-nected to a data logger which records the full load,no-load and downtime of the compressors (seeFig. 7).

Optocoupler

digital Data logger

analogP-Transmitter

40-20 mA

Fig. 7: Digital load time recording

After these data have been entered into a computer,the capacities of the individual compressors and thetotal air consumption value of the plant can besimulated.

Please note:

• One advantage of this indirect measurementmethod compared to direct measurements is that itnot only provides information on air consumptionvalues but also data on the efficiency and runningperformance of the compressors.

• Easy to set up.• Min. measurement phase 1 sec to include con-

sumption peaks.

Other methodsSimple air consumption measurement or load meas-urements of compressors can also be determined byreading the counter for load operating hours andmeasuring the time needed to drain the boiler.

Please note:

• Very personnel-intensive and rather vague.

Measuring leaks using pressure measurements

Using a pressure sensor which can be easily inte-grated into the compressed air system the pressurecan be measured and recorded over a longer periodof time at short intervals. In order to do so, the sys-tem does not have to be separated, a coupler or aone inch connection are sufficient.

The pressure curves are subsequently processedusing a mathematical method so that the contractorknows exactly how high the share of leaks and howlarge the load capacity share (in per cent) is for eachindividual measurement point. This is done by cal-culating the pressure losses and their gradients,which result in a perfect curve using a mathematicalalgorithm. The perfect curve is then compared withthe actually measured curves (see Fig. 8).

7.0

7.2

7.4

7.6

7.8

8.0

1:40:00 1:40:28 1:40:57 1:41:32 1:42:01 1:42:30

Time

Pres

sure

0%

20%

40%

60%

80%

100%

Pressure Share of useful air use

Fig. 8: Leckage measuring during plant operation

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The results are the relative shares of the load ca-pacity or the leaks at each point in time. If the flowrates or compressor. operating times are recorded atthe same time, the relative values can be convertedinto absolute losses.

Please note:

• The advantage of the method is that it is possibleto calculate the leaks during operation. It is there-fore particularly suitable for plants operating incontinuous production.

Measuring leaks by drainingcompressed air receiver

A simplified leak measurement is also possible viathe air receiver. The pressure here is increased tothe maximum required in the system and the time ittakes for a pressure drop of 1 to 2 bar due to leaks ismeasured (see Fig. 9).

7 9

Sealedinlet

Amount of leaks

Tools not in use

tppV EAB )( −×

= VL�

time Measuringtpressure final Receiverppressure initial Receiverp

volume Receiver Vleaks of AmountV

E

A

B

L

=====�

Fig. 9: Measuring leaks by draining compressed air receiver

Air quality measurements under ISO 8573

The manner of taking samples is particularly impor-tant for exact air quality measurements.

If there is a turbulent stream in a compressed airpipe and especially if boundary flows are present,the sample must be taken at a point at which it canbe ensured that it contains a representative andutilisable mix of all the components of the com-pressed air. This can only be guaranteed with a so-called isokinetic sample (see Fig. 10).

1. Sampling probe in main pipe2. Adjustable socket to fix probe3. Compressed air pipe cross section “D”4. Screw-in depth min. „3 x D“5 Direction of flow6. Min. length of inlet zone = 10 x D

Fig. 10: Isokinetic sampling

For the individual groups of contaminants, e. g.

• ISO 8573-2: Oil aerosol content• ISO 8573-3: Water content• ISO 8573-4: Particle content• ISO 8575-5: Oil vapour and hydrocarbon content• ISO 8573-6: Gaseous contaminants• ISO 8573-7: Microbiological contaminantsthe measuring systems described in each of thenorms should be introduced downstream of thesampling point.

The air qualities are classified under ISO 8573-1.

The campaign “Druckluft effizient“ aims to motivate the operators of compressed-air systems to optimise their systems and save substantialcosts. It is conducted by the German Energy Agency (dena), the Fraunhofer Institute Systems and Innovation Research (FraunhoferISI; project management), and the Federation of the Engineering Industries (VDMA) with support of the Federal Ministry for Economicsand Labour (BMWA) and the following industrial enterprises:

Atlas-Copco BEKO Technologies BOGE Kompressorendomnick-hunter Energieagentur NRW Gardner Denver WittigGASEX Gebr. Becker Ingersoll-RandKaeser Kompressoren Legris – TRANSAIR METAPIPESchneider Druckluft systemplan, Karlsruhe Thyssen Schulte – MULTIPLASTultra air ultrafilter International ZANDER Aufbereitungstechnik

Further information can be found at www.druckluft-effizient.de Druckluft effizient, Fraunhofer ISI, Karlsruhe/Germany, October 2003