What it means: Alicat mass flow control response specifications
Watch this one-minute video to see Alicat’s control response between setpoint extremes:
Physical time constant (τ)
The physical time constant (τ) is one of the few standards that exist when it comes to reporting response times. This constant is equal to the amount of time it takes the controller to reach 63.2% of the commanded setpoint. While this number can be useful, it reveals no information about the time required to achieve the remaining 36.8% of the setpoint change.
The oscilloscope image below shows that the Alicat reached 63.2% of the commanded setpoint in just 7.4 ms.
Arbitrary setpoint percentage change
Many mass flow controller specifications designate an arbitrary percentage change to define their speed of response. A few of these have become common across manufacturers, such as the time required to move from 10% to 90% of the setpoint. Since the bottom and top 10% of a control response curve tend to be the most variable, this specification misses critical information about the actual performance of the control system.
For example, the image below shows that the Alicat achieved 100% of the setpoint in 11.4 ms. You can see that the oscilloscope steps are the longest at 50% of the setpoint change and are much smaller below 10% and above 90%. In this image, a 10-90% specification would have resulted in a response time of 6-7 ms.
Because getting to the setpoint does not guarantee that the controller will not also overshoot the intended setpoint, many manufacturers specify control response times based on settling time. This is the amount of time the controller takes to reach the setpoint and then reduce further oscillations to a given error band. For example, an error band of 2% means that settling is not achieved until the flow rate no longer oscillates outside of 2% of the setpoint.
Accuracy specifications for mass flow controllers commonly fall within the 0.5% to 2% range, but some specifications define error bands for settling as large as 10%. In the oscilloscope image below, a common-sense settling error band equivalent to the Alicat controller’s accuracy specification (1% at full scale) is achieved after just 27.6 ms.