Understanding Mass Flow Meters and Mass Flow Controllers Specifications

The amount of information provided in the a spec sheet for mass flow meters and mass flow controllers can be a little overwhelming for users, especially if they are not familiar with the terminology. Some of the key specifications of mass flow meters and mass flow controllers are covered in this article.

Accuracy

Accuracy is a measure of how precisely a flow device performs at various flow ranges. It is typically expressed as percentage of reading or percentage of full scale flow. Error as a percentage of full scale is obtained by the multiplication of the error percentage by the full scale flow. Devices with error described as a percentage of full scale provide most accurate measurements when flowing at full scale.

Error described as a percentage of reading states error as a percentage of what the flow meter is actually flowing. If the accuracy of a device is rated to +/-1% of the reading, then the device will be accurate to +/-1% of whatever it is flowing. At 100SLPM, the device will be precise to within +/-1SLPM, and within +/-0.1SLPM at 10SLPM of flow the unit.

Accuracy, irrespective of the measurement technique, typically relies on operating conditions, such as the temperature and pressure of the gas flowing through the device.

Manufacturers will define the error rate of their devices based on certain predefined set of operating conditions, generally standard temperature and pressure. Hence, the accuracy of the unit can vary drastically when the gas pressure and/or temperature fail to satisfy those parameters specified by the manufacturer.

Some devices, such as those from Alicat, are internally compensated, meaning that sensors inside the instrument measure pressure and temperature conditions and perform real time corrections for differences in gas conditions. Hence, users need not worry much about maintaining constant process conditions.

Turndown Ratio

Turndown ratio describes the useable range of a flow device, defined as the ratio between maximum flow and minimum flow.

In essence, it is the least amount of fluid that is measurable by the device, indicating how much of the device range is capable of producing accurate readings. This is crucial in the measurement and control of a broader flow without the requirement for changing devices.

Repeatability

Repeatability is a measure of the ability of a flow device to accurately repeat flow functions. The repeatability of a device is usually measured by monitoring its readings at a specific flow rate, stopping the flow to allow the device to return to zero within a specific time, and then resuming the same flow.

The device’s repeatability is established by monitoring the variations between the original flow reading and the flow reading subsequent to stopping and resuming the flow. Simply, repeatability is defined as the ability of the device to produce the same reading at the same flow rate.

Pressure Drop

Pressure drop is defined as the loss in pressure due to the friction between the fluid particles and with the pipe walls when a fluid passes through a channel or pipe. Pressure drop is roughly proportional to the distance the fluid travels and is influenced by every component that the fluid comes into contact with, such as every bend, every pipe wall, and every fitting.

Since the pressure drop affects the fluid flow, it is essential to ensure that the components produce the least pressure drop possible.

Warm up Time

Warm up time is defined as the amount of time required for a device to become stable for use. Thermal units likely to have prolonged warm up times.

Some instruments may require up to 30min to become stable to within 2%FS. Warm up time is a key specification if the device is turned off at the end of the day.

Zero Shift or Offset error

Zero shift or offset shift (Figure 1) describes how far from zero a flow device will move when there is a change in temperature and/or pressure. The slope of the calibration curve will not be affected by offset error as it will be the same across the flow range. Offset error is measured in %FS (or %reading)/degree change in temp (or psi change in pressure). Generally, the calibration is offset by the percentage of error for every change in psi or change in degree temperature.

Figure 1. Zero Shift

Span Shift or Span error

Span shift or span error (Figure 2) describes the shift in the slope of the calibration curve with no change in zero. At different flow ranges, a flow device’s calibration curve will be affected differently. Span error is measured in %FS (or %reading)/degree change in temp (or psi change in pressure).

Actually, the calibration is offset by the percentage of error for every change in psi or change in degree temperature. Zero shift or span shift can also be described as ‘Temperature coefficients’ or ‘Pressure Coefficients’ and can be measured in the same way.

Figure 2. Span Shift

Dead Band

Dead band (Figure 3) describes the area of a signal range or band wherein no action takes place, i.e., the band where the system is dead. Relating to a pressure switch, dead band is the band where the switch resets and in between which the switch trips.

Figure 3. Dead band

This information has been sourced, reviewed and adapted from materials provided by Alicat Scientific.

For more information on this source, please visit Alicat Scientific.