Selecting The Right Flowmeter Size

Selecting The Right Flowmeter Size - There are certain factors which affect the measurement of a fluid’s
flow rate with a rotameter. The fluid’s temperature, pressure and specific gravity all impact gas flow measurements.


Flow capacities (ranges) for the flowmeters described in this catalog are given for air at standard conditions —- 14.7 psia (101.3 KPa Abs) and 70°F (21°C). Sizing a meter for a gas other than air, or for your specific application pressure and/or temperature, requires that you first determine the equivalent flow capacity in air at standard conditions. Once determined, the flow capacity tables in this catalog can be applied directly. Reference Scale tables can be requested for each flowmeter ordered which will provide conversion to your desired fluid or conditions.

Note that for flowmeters calibrated at standard conditions with a valve on the inlet, readings on the tube are correct provided that the outlet pressure is close to atmospheric. When the valve is on the outlet, readings are correct if the inlet gas pressure is equal to the pressure for which the tube was calibrated.


To estimate which flow tube should be purchased when measuring the flow of a gas other than air, multiply the flow rate desired by its factor above to find the air flow equivalent. The flow tube whose range (capacity) covers this flow rate should be the one purchased. Be sure to keep units consistent.

Air Equivalent= Gas Flow Rate Desired x Factor.

These factors assume standard operating conditions. Temperature 70°F/21°C; pressure 14.7 psia (101.3 K Pa Abs).