A user reported to us that the flow rate measured by their mass flowmeter at half atmospheric pressure was inaccurate. What's going on here? Due to the variation of volume flow rate, measuring mass flow rate in environments below atmospheric pressure is indeed challenging. Let's explore what happens under negative pressure and how to choose a suitable range of mass or volume flow meters.
The Influence of Pressure on Static Gas Mass and Volume Flow Rate
Pressure refers to the force acting on the surface of an object. According to the ideal gas law PV=nRT, the volume (V) of a gas decreases with increasing pressure (P). Because gases are compressible, as the pressure increases, the spacing between gas molecules decreases; On the contrary, the molecular spacing increases, resulting in an increase in gas volume. However, in any case, the number of air molecules (i.e. molar mass) contained within it remains constant.
Imagine filling a resilient and inelastic container (such as a polyester film balloon) with 500 cm3 of air at standard atmospheric pressure (1 atm ≈ 14.696 psia) of 25 ° C. If the pressure is increased to 2 atm, the spacing between air molecules will decrease and the volume will be compressed to 250 cm3; If the pressure is reduced to 0.5 atm, the air volume will expand to 1000 cm3; If the pressure continues to decrease to 0.25 atm, the volume will increase to 2000 cm3; Throughout the entire process, the amount of air itself did not increase or decrease, and the molar mass of air inside the container remained constant at 500 scm3.
The influence of pipeline pressure on dynamic gases
When air enters a state of motion, its volume constantly changes with pressure, as mentioned earlier. When the pressure doubles, the volume flow rate decreases by half, and when the pressure decreases, the volume flow rate increases, but the number of flowing air molecules (i.e. mass flow rate) remains unchanged.
Selecting a suitable range of mass flow meters for negative pressure applications
The pressure differential mass flow meter has a predetermined path for the maximum volume flow that may flow through it. When the flow meter is used for negative pressure applications, this path must be increased to ensure the smooth passage of the expanded volume flow.
Taking an ALICAT mass flow meter with a full scale of 500 sccm (scm3) as an example, if a gas with a mass flow rate of 500 sccm is passed through at twice the atmospheric pressure, its volumetric flow rate is only 250 ccm. The reduced volumetric flow rate does not have any impact on the measurement of flow rate, as the flow meter can actually handle a flow rate twice that value.
If the mass flow rate of the gas at half atmospheric pressure is 500 sccm, the volumetric flow rate will double to 1000 ccm. At this point, using a mass flow meter with a full scale of 500 sccm is too small and 1000 sccm must be selected. To ensure that the mass flow rate still has the best measurement resolution, we will calibrate the device's volumetric flow rate to 1000 ccm and the mass flow rate to 500 sccm. By analogy, a mass flow meter four times the target range must be used at a quarter atmospheric pressure, and the equipment's mass flow rate should be calibrated to 500 sccm and volume flow rate to 2000 ccm.
Generally speaking, ALICAT mass flow meters do not require range expansion when used under operating conditions of no less than 0.8 atmospheres (i.e. 11.5 psia); The maximum capacity can be expanded to 5 times the range for operating conditions of 0.2 atmospheres (i.e. 2.9 psia).
Selecting a suitable range of mass flow controllers for vacuum and negative pressure applications
Unlike mass flow meters, differential pressure mass flow controllers do not require range expansion when used in negative pressure applications. Instead, the proportional control valve needs to be placed behind to form a sonic barrier to avoid the impact of gas volume expansion on the flow sensor. From the following graphic example, it can be seen that the measurement terminal inside the mass flow controller is connected to the atmosphere (1 atmospheric pressure), and the gas enters the negative pressure (1/4 atmospheric pressure) working condition and undergoes volume expansion only after passing through the valve. As mentioned earlier, when the working condition changes from 1 atmospheric pressure to 1/4 atmospheric pressure, the mass flow rate of the gas remains constant.