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SD compressed air flow meters

Technology overview

Thermal flow sensing technology relies on the cooling effect of flowing fluids, including gases. For the SD design, one ceramic probe is heated (the measuring resistor) and the other is not (reference resistor). Flow velocity is determined by the voltage difference between the two resistors when the flow dissipates the heat.


Compressed air systems

How much is your utilities bill? Compressed air is expensive to generate and leaks can be difficult to detect.

The SD compressed air meters can detect small leaks and monitor changes in the compressed air consumption of machines over time.

Welding gas systems

What if you could control weld quality by maintaining proper flow of argon?

The SD flow meter can accurately measure the flow rate and total flow consumption of welding gases like argon, carbon dioxide and nitrogen.


Q. The SD measures mass flow rate in SCFM (standard cubic feet per minute).  What is the difference between SCFM and actual cubic feet per minute (ACFM)?

A. Compressed air is commonly rated in SCFM, the flow rate of a gas corrected to some set of standard conditions of pressure, temperature and humidity. This provides a measurement of mass flow, which does not change as pressure, temperature and humidity changes. For the SD family, standard conditions are 1013 hPa, 15 °C and 0% relative humidity. ACFM is the volume of gas that is flowing at the actual conditions of the application. ACFM can be converted to SCFM (and vice versa) assuming the ideal gas law:

Q. Is accuracy of the meter affected by particles, oil, etc.?

A. Yes. Because the meter relies on thermal technology, varying amounts of contaminants will affect it. The chart below reflects air quality per the ISO 8573-1 spec.

Class Particles Humidity Oil
µm mg/m3 °C g/m3 mg/m3
1 0.1 0.1 -70 0.003 0.01
2 1 1 -40 0.12 0.1
3 5 5 -20 0.88 1
4 15 8 3 6 5
5 40 10 7 7.8 25
6 - - 10 9.4 -

We specify accuracy based on air quality according to this standard and our datasheets provide accuracy for Class 141 and Class 344 air quality. Class 141, for example, means Particle Class 1, Humidity Class 4 and Oil Class 1. Accuracy for other air quality classes can be estimated from these two values.

Q. Can the SD be used for any gas?

A. The SDx000 and SDx001 are calibrated for compressed air. Because air is ~78% nitrogen, these meters can also be used with nitrogen, but accuracy will be about ± 5%. SD610x is calibrated for nitrogen, carbon dioxide and argon specialty welding gases. If used with other gases, the display will not be accurate. The parameter “CGA” can be adjusted somewhat if the flow rate is known. Note: helium gas molecules are very small. The SD meters are not helium tight.

Q. What is a pulse output and why is it used?

A. The SD flowmeters have a pulse output that is used for totalizing applications. A 24 VDC signal is sent every time a specified volume (e.g., 1 scf entered as ImPS in the menu) flows through the meter regardless of the flow rate. This pulse can be further evaluated with a counter, PLC, etc.