Volume Flow Rate vs Standardised (Mass) Flow Rate in

Gas Flow Measurements

By: M Dlamini

D. Jonker

M. Molefe

Background

Volume Flow

• It is the measurement of a volume of a gas flowing through a conveyance per quantity of time.

• SI units are cubic metres per second (m3/s).

• Common measurement units are lpm and ccm.

• Its advantage is that it is independent from gas composition.

• Disadvantage when a volume flow instrument is used at conditions or gas different from calibration conditions or gas.

• It does not take into account change in gas density.

• Gas volume changes with temperature and pressure.

Background

Standardised Volume Flow (Volumetric Mass Flow)

• This deals with quantifying the movement of a gas relative to its mass over time.

• A fixed amount of gas has a mass that is constant regardless of changing temperature and pressure.

• Mass flow is independent of temperature and pressure unlike volume flow.

• Mass flow measuring instruments are configured for certain gas composition and it is very difficult to measure gas mixtures and gases with different phases.

• SI units are kilograms per second (kg/sec).

Background

• However, common units for mass flow measurements are volumetrically type such as standard cubic centimetre per minute (sccm) or standard litres per minute (slpm).

• Volumetrically based mass flow defines mass flow in terms of the quantity of a gas that occupies a volume under standard conditions of temperature and pressure.

• These units can be more appropriately referred to as “reference conditions”.

• Preferably, the standard temperature and pressure (STP) during calibration must be chosen to match those at which the instrument will most like be used.

Converting Volume Flow to Standardised Flow

• In a gas system under steady state, volume flow is likely to be different throughout the system.

• It is highly dependent upon the gas pressure and temperature at the point where it is measured.

• However, mass flow would still be the same throughout the system.

• Therefore, when calibrating a volume flow device, the gas temperature and pressure at the unit under test (UUT) location must be measured for easy conversion to standardised flow.

• The conversion is performed using the combined gas law –Boyle’s law, Charles’ law and Amonton’s law.

Converting Volume Flow to Standardised Flow

• The combined gas law can be written as:

𝑃𝑉 = 𝑘𝑇 where k is a constant.

• This can also be written as𝑃1𝑉1

𝑇1=

𝑃2𝑉2

𝑇2

𝑄2 = 𝑄1𝑃1𝑇2

𝑇1𝑃2

• Where 𝑄1 is in volume flow units (ccm, lpm, etc.) and 𝑄2is mass flow in volume units or standardised volume flow (sccm, slpm).

• Therefore, volume flow conditions will now be denoted by subscript v and standardised volume conditions flow by subscript s as follows:

𝑄𝑠 = 𝑄𝑣𝑇𝑠𝑃𝑣

𝑇𝑣𝑃𝑠

Calibration of Volume Flow Meters

• The NMISA flow laboratory measures the gas temperature and pressure at the UUT location when calibrating volume flow meters.

Figure 1: Shows calibration of a volume meter setup

Calibration of Volume Flow Meters

• The volume flow is converted to mass flow (standardised volume flow) in volume units using client given “reference conditions” or standardised conditions.

• For example, a bubble flow meter was calibrated at 25 oCand 101,325 kPa standard conditions as per client’s request.

• The UUT read Qv=2 998 ccm at 20,1 oC and 86,2 kPa.

• Therefore, the mass flow in volume units is calculated as ff.:

𝑄𝑠 = 𝑄𝑣𝑇𝑠𝑃𝑣

𝑇𝑣𝑃𝑠= 2 998 ×

273,15+25 ×86,19

(273,15+20,1)×101,325

𝑄𝑠 = 2 593 sccm

Calibration of Volume Flow Meters

Unit Under Test

ReferenceFlow Rate at

760 mmHg and 25 °C

(mL/min)

Correction (Standardised

Flow Rate)

(mL/min)

% difference between

volumetric and standardised

volume flow rate

Volumetric FlowRate

(mL/min)

Calculated Standardised Flow

Rate for 760 mmHg and 25 °C

(mL/min)

603,2 522,0 512,7 -9,3 15,6

1 797 1 554 1 542 -12 15,6

2 998 2 592 2 581 -11 15,6

4 499 3 890 3 881 -9 15,6

5 707 4 932 4 934 2 15,6

Table 1. Shows bubble flow meter calibration results

Calibration of Volume Flow Meters

• Volume flow measuring devices, such as bubble flow meters, cannot distinguish temperature and pressure changes.

• Therefore, measuring mass flow using these devices require additional temperature and pressure sensors, and some computation to compensate for the variations in these process conditions.

• Thermal mass flow meters are virtually insensitive to variations in temperature and pressure.

• NMISA calibration certificates report both volume flow and mass flow in volume units at the client’s specified temperature.

Conclusion

• For flow as low as 6 L/min, the percentage difference between volume flow and standardised flow is already 15,6 %.

• This difference depends on the actual and reference conditions.

• Therefore, it is beneficial to measure the absolute temperature and pressure at the UUT location in the gas flow line when the gas flow instrument is to be used at conditions different from the calibration conditions.

• This helps when flow correction is to be applied.