Correcting for temperature and pressure effects on sensors

 by Alicat Scientific

Alicat recently rolled out a major update to the main circuit board and sensors on our mass flow devices. Improvements in our sensors and advanced our understanding of flow physics have brought about changes in our circuitry to ensure you’re getting the maximum performance out of every part of your Alicat mass flow device.

The high resolution information we get from our new circuit board gives us a better picture of what’s happening in the flow path of our devices than ever before. One thing this lets us see very clearly is how tiny changes in operating conditions alter the behavior of our sensors. Compensating for these changes lets us build devices which give more repeatable measurements and are less prone to drift.

Separating the signal from the sensor

A perfect sensor would map to external physical properties 1:1 with its output signal under any conditions.  But sensors and circuits are made of matter and their behavior changes as their physical state changes. Alicat sensors are no exception, as our electronics warm up and our sensors are compressed by process gas the signal we are interested in changes.

Alicat compensates for this variation in sensor behavior using the one of the strengths of our measurement technique: multiple independent sensors. In each of our flow instruments a differential pressure sensor provides our main tool to measure flow while absolute pressure and temperature sensors allow us to correct this volumetric flow rate to a mass flow rate. Read more on our theory of operation of laminar differential pressure flow measurement. As long we characterize the response of each sensor to pressure and temperature shifts beforehand we can use software to correct sensor output based on operating conditions.

From straight lines to curves

The previous generation of Alicat instruments used a linear compensation technique for temperature and pressure. Each device had its sensor offset measured at ambient and high temperature/pressure. We interpolated these two points to make a straight calibration line. In our new release, we’ve improved our modeling to more effectively model the physical systems we’re dealing with to create a non-linear interpolation of sensor state changes.

On a new Alicat device each sensor and circuit board combination is characterized in an environmental chamber at the full range of the device’s expected operating temperatures and pressures. This produces smooth compensation curves which better account for real sensor behavior.  The improvement is profound and especially noticeable at low temperature:

Shift in absolute pressure reading with varying temperature. Current generation in blue and previous generation in red.

By pairing better hardware with this software compensation the newest generation of Alicat devices is more capable than ever before. We’re now running into the limits of flow physics instead of sensor noise. Ongoing research into gas behavior and improvements to our flow channel will help us unlock the full potential of our devices moving forward.

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