Pressure drop is the amount of line pressure permanently lost as gas passes through an instrument. This loss is due to the frictional resistance of the components the gas touches. Every instrument and fitting in a line induces some pressure drop.

## Calculating pressure drop

Pressure drop is determined by calculating the difference between the pressure of the gas when it enters the instrument and when it leaves the instrument. The easiest way to make this measurement is to plumb the inlet and outlet of the device to a differential pressure transducer.

When making these measurements, it is important to consider the following:

• Under laminar flow conditions, pressure drop is proportional to volumetric flow rate (doubling the flow rate doubles the pressure drop).
• When flow is turbulent, pressure drop increases as the square of the volumetric flow rate (doubling the flow rate quadruples the pressure drop).
• Increasing common mode pressure increases pressure drop.
• Pressure drop increases as gas viscosity increases. Since increasing the temperature of the gas increases its viscosity, pressure drop also increases as gas temperature increases.

## Maximum pressure drop – flowing at full-scale

Each Alicat device specification sheet lists the maximum pressure drop at full-scale flow when venting to atmosphere under standard conditions. This pressure drop indicates the minimum amount of inlet pressure needed to run an instrument at full-scale flow while venting to atmosphere.

Different mass flow technologies cause different levels of pressure drop, as each has significant differences in frictional resistance. For example, a laminar DP mass flow meter will use a relatively wider flow channel than a low-flow Coriolis mass flow meter, in which the gas passes through a very small tube. The diagram below provides an overview of the relative levels of each technology.

Maximum static pressure vs. differential pressure for various mass flow technologies.

For any gas process to work, the available system pressure must be greater than the total pressure drop of the components in the system at the expected operating flow rates and temperatures. If too little pressure is provided at the system inlet, there will not be enough gas pressure to pass through all the components of the process at full-scale flows.

As an example, a 20 SLPM MC-Series mass flow controller has a full scale pressure drop of 20 PSID. At pressures < 20 PSIA, the device will be unable to reach full-scale flow.

## Low pressure drop applications

In processes with little available inlet pressure or a large amount of back pressure, it is particularly important to use a device with a low pressure drop. Alicat’s Whisper low pressure drop mass flow meters and controllers have maximum pressure drops as low as 0.07 PSID.

These devices have a larger flow body and uniquely arranged laminar flow elements, resulting in pressure drops up to 10x lower than standard devices. Whisper instruments also utilize a much more sensitive pressure sensor package to take full advantage of the lower pressure readings.

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