What is a flowmeter ? Simply a device that can monitor and measure the actual amount of ‘product’ being sent through it. Typically flowmeters in industry are used for monitoring use of water, chemicals, ingredients, etc.

1. An Introduction to Testo Compressed Air Flowmeters -
6440 and 6446/6447 series

2. What / why / how ?
What is a flowmeter ? :
Simply a device that can monitor and measure the actual amount of ‘product’ being sent
through it. Typically flowmeters in industry are used for monitoring use of water, chemicals,
ingredients, etc. Some flowmeters have a basic ‘indicator’ built in to show value, more
sophisticated will have ‘outputs’ that can be fed into measurement & control systems.
Why use a flowmeter ? :
To accurately determine how much of ‘something’ is being used for cost/performance
reasons. Often they are used as part of a overall process plant control system.
How does a flowmeter work ? :
There are many technologies used depending upon the product to be monitored and its
physical variables, common types for airflow would be insertion pitot, turbine/vane, variable
area, venturi, etc.
All technologies have good/bad points – it is usually a case of matching the best technology
to the required application.
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3. Testo compressed air flowmeters – basic info
We have a range of flowmeters marketed for use within the compressed air industry – these
have been designed around specific requirements found within compressed air systems,
however with a ‘correction factor’ applied they can be used for nitrogen (N2), carbon dioxide
(CO2), and argon (Ar).
Why use a flowmeter to monitor compressed air ? :
Unlike all other ‘utilities’ used in a factory or production line, compressed air is usually NOT
metered, so usage is unknown. The compressed air comes from having a compressor
somewhere in the facility, and this compressor is using electricity. Also you may have leakage
within the air system. Compressor systems typically represent between 5%-40% of total
facility energy consumption.
Surveys have found that on average 20% of the compressed air produced on a system is
wasted through leakage.
At 7 bar (100psi) :
3mm hole = 12 l/s = £3,260 per annum.
6mm hole = 47 l/s = £12,770 per annum.
A 2 l/s leak is to small to hear but would still cost £590 annually.
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4. Typical compressed air system
Diagram below shows a basic schematic for a typical industrial compressed air system.
The flowmeter must be placed after the dryer so that the air flowing through is as dry and
as clean as possible.
Note the amount of condensate drains in a typical system – this is due to the fact that
when ambient air is compressed there is a huge amount of moisture generated. This
needs to be removed by a dryer system otherwise this ‘wet’ air will destroy
downstream equipment. – Testo also make specialised sensors for measuring trace
humidity in compressed air systems.
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5. Testo compressed air flowmeters – basic info
What can we offer :
2 main versions of product.
6441 – 6444 suitable for most
small bore pipework.
6446/6447 for larger plant
pipework and also special feature
of replacement ‘sensor’ whilst
system is under pressure.
Common process conditions for
both series are :
0 to 60°C
16 bar max (PN16)
<90% RH
Air quality ISO 8573
Rec. classes 1-4-1
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6. Testo flowmeters – principle of operation
The optimal method for measuring airflow within gases is ‘thermal mass flow’ measurement
Only this technique is independent of temperature and pressure changes within the process,
and also has the added benefit of minimal back pressure within the system.
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7. Testo flowmeters – principle of operation
Conversion of ‘mass’ flow to volumetric flow :
For consumers of the air, normalised volume flow is the most important function.
The Testo 6440 divides the ‘mass’ flow value by the normalised density value (according to
ISO 2533) to give a value of normalised volumetric flow which is also independent of
temperature and pressure.
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8. 6441 / 6444 - overview
 Supplied as pre-formed flow tube, with precise internal dimensions, and threaded process
connections.
 Uses ‘calorimetric’ thermal mass flow measurement principle for high accuracy,
independent of temperature and pressure fluctuations, and almost zero backpressure
(approx 1mbar).
 Compact electronics module has built-in digital display to allow for programming and
operation of unit. (Display can be turned through 180° to suit flow direction).
 24VDC supply configured in 4-wire format.
 Output options include 4..20mA analogue output proportional to flowrate, PNP alarm
outputs for Hi/low flow warning, pulse output for use with flow totalisers. Max of 2 outputs
possible.
 Unit can be configured to display units of Nm³/h, Nl/min, m³, °C.
 High accuracy : for class 1-4-1 air, ±3% of reading & ±0.3% of final value.
 Can measure bi-directional flow within compressed air ring main.
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9. 6446 / 6447 - overview
 Supplied as pre-formed flow tube, with precise internal dimensions, and DN flanged
connection (DN65…DN250)
 Uses ‘calorimetric’ thermal mass flow measurement principle for high accuracy,
independent of temperature and pressure fluctuations, and almost zero backpressure.
6446 has fixed sensor and electronics module, 6447 version allows removal of sensor unit
whilst system is under pressure (also called ‘hot-swap’)
 Compact electronics module has built-in digital display to allow for programming and
operation of unit.
 24VDC supply configured in 4-wire format.
 Output options include 4..20mA analogue output proportional to flowrate, PNP alarm
outputs for Hi/low flow warning, pulse output for use with flow totalizers. Max of 2 outputs
possible.
 Unit can be configured to display units of Nm³/h, Nl/min, m³, °C
 High accuracy : for class 1-4-1 air, ±3% of reading & ±0.3% of final value.
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10. Technical issues
Technology used is tried and tested, so no issues there.
Robust construction with precision dimensions of flowtube, give good accuracy compared to
other technologies (clamp-on meters, etc)
Watch out for mounting position guidelines, and also the required inflow / outflow conditions
recommended within our installation manual.
Also if oil is used, then the unit must be placed in front of the lubricator to remove possibility
of contamination of sensors.
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11. Applications
Why use a compressed air flowmeter ?
Cost transparency – clear information on usage overall and to individual machines/areas.
Leakage detection – installed on supply to individual machines the 6440 will detect the
smallest amount of compressed air volume flow. If the machine is switched off but flow is
continuing then a leak or leaks are occurring (costs money).
Machine usage – also the flowmeter can be used as part of maintenance regime by
monitoring flow when system is new, against gradual increase in flow rate as leakage starts
to occur over time.
Peak load management – allows production systems to be balanced to make sure maximum
air usage is within compressors range.
Protection – Hi/Low switch outputs can protect expensive plant against under/over supply of
air.
Precise dosing of air quantity – filling of container, etc.
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12. Applications
Typical applications / customers include :
 Any processing or manufacturing site using compressed air.
Packaging lines, beverage, plastics moulding, production lines, etc
 Manufacturers of machinery which uses compressed air.
 Any company involved in compressed air systems installation/maintenance/upgrades.
 Compressor manufacturers/suppliers.
 Companies involved in energy audits within compressed air systems.
 Suppliers of components/parts for compressed air/pneumatic systems.
 Potential for use on special welding systems with Argon purge.
 Manufacturers of nitrogen gas generators.
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