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Pneumatic Flow Measuring

Pneumatic Flow MeasuringDownload a printable version of this page.

Lake’s rugged, high pressure pneumatic flow meters are designed for permanent installation in compressed gas systems. These products provide a low-cost means to measure compressor volumetric outputs, pneumatic tool consumptions and other industrial gas flow rates.

Lake's flow meters operate using the variable annular orifice method with compression spring return – the identical method used in our field proven liquid flow meters. The product’s follower, where the measurement is indicated, is magnetically coupled through a high pressure casing to the flow meter’s internal orifice assembly.

Benefits of these design features are:

  • high operating pressure
  • linear displacement of the follower with respect to flow rate
  • high turn-down ratios
  • have a measuring accuracy of ± 2.5% of full scale in the center third of measuring range, and ± 4% in upper and lower thirds
  • operation in any mounting orientation

Lake Monitors are available in three standard materials of construction:

  • aluminum for standard monitoring applications to 600 PSIG
  • brass for media/material compatibility
  • stainless steel for compatibility and operation to 1000 PSIG

Measuring ranges cover 1.5-12 SCFM through 150-1300 SCFM. Twenty-four port sizes from 1/8" through 2" in NPT, SAE and BSP can be ordered to meet specific plumbing requirements. Lake’s pneumatic monitors are also available in alarm and transmitter configurations for electronic monitoring applications.

Standard Cubic Feet

Lake’s flow meters are calibrated to measure the flow of compressible media (gases) in SCFM – standard cubic feet per minute. A “standard” cubic foot is defined as a cubic foot of dry air at standard atmospheric conditions: 70°F and 14.7 PSIA (0 PSIG) measured at sea level.

Density Correction Factors

When a standard cubic foot of air is compressed, its actual volume will decrease proportionally as absolute pressure increases. For example, a standard cubic foot of air’s actual volume will decrease by 50% and density will increase by 100% as the air is compressed from atmospheric pressure 14.7 PSIA (0 PSIG) to 29.4 PSIA (14.7 PSIG). See FIGURE 1.

PSIAThere are three factors that affect the Flow Meter Calibration: specific gravity, pressure and temperature. Lake flow meters are calibrated for air (specific gravity of 1.0) at 70°F and 100 PSIG. Most low pressure rotameters are calibrated at 0 PSIG and require corrections for use at any other pressure.

Lake products are designed for pneumatic systems where pressures between 90-110 PSIG are used. In these common applications, a Lake monitor with a standard calibration can be read directly without applying corrections.

Correction Factors

If a Lake flow meter is installed in a system where conditions differ from the standard listed above, correction factors will need to be applied to retain the design accuracy of the monitor. The appropriate correction factor equations are detailed in Chart 1 on the back of this page. To assure the best monitoring accuracy, pressure and temperature measurements should be taken directly at the monitor’s inlet port.

Special Scales

Special calibrations can be performed by Lake Monitors to correct for the following system characteristics:

  • system pressure
  • system temperature
  • media specific gravity
  • various measuring units (i.e. LPM, LPS, m3/hr, etc.)
  • any combination of the above

Consult Lake’s factory or your distributor for details and prices.

Selecting the Proper Monitor

To order a pneumatic flow rate meter the following information is required:

  • pipe size and port style
  • media (air, nitrogen, argon, etc.) – for material compatibility and specific gravity considerations
  • approximate flow range required1
  • system pressure: nominal, maximum, minimum
  • system temperature

Flow Range

Estimating the flow rate in a compressed gas system may seem complicated, but with some research and a few simple equations an educated guess can be made. Two suggested methods are:

Method 1

A compressor is typically rated in SCFM output at a certain pressure and efficiency. If the rating cannot be located or is unknown, an estimate of compressor output can be obtained by the following formulas:

  • 1-stage compressors: motor HP/0.179 = SCFM @ 100 PSIG
  • 2-stage compressors: motor HP/0.164 = SCFM @ 100 PSIG
  • 3-stage compressors: motor HP/0.159 = SCFM @ 100 PSIG

Method 2

If all of the potential of a compressor is not being used (the unit cycles on and off) or if flow rate in excess of compressor capacity is being consumed (the compressor cannot meet the demand), a summation of machine usages can be totaled to determine the maximum flow rate. Most machine tools that use compressed air specify the maximum consumption of the tool.

INSTALLATION DO'S AND DON’TS

To obtain satisfactory operation from a Lake pneumatic flow rate meter, the following points should be considered:

DO…

  • install a pressure gauge near the inlet of the meter
  • place throttling valves at the outlet of the meter
  • use pipe sealer on the connections
  • install a union on one side of the meter for easy removal for maintenance and calibration
  • install solenoid valves at the meter outlet (as far downstream as possible)
  • mount in any orientation: vertical, horizontal or upside down

DO NOT…

  • install restrictions between pressure gauges and the meter inlet
  • install solenoid valves at the meter inlet
  • place restrictions between the meter’s pressure gauge and the meter inlet
  • use in systems where reverse flow is possible
  • place meter in non-aligned piping
  • over-flow the meter by more than 150% of maximum reading
  • operate at pressures and temperatures greater than specified

©2011 Lake Monitors

8809 Industrial Drive, Franksville, WI  53126
Phone: 262-884-9800 | Toll Free: 800-850-6110
Fax: 262-884-9810

info@lakemonitors.com