Pressure And Temperature Compensation To Optimize Flow Control

Various external factors can have influence on the measurement accuracy and control stability of mass flow controllers. But what are the most common challenges in pressure and temperature control? And how can you fix these?
Real-time pressure- and temperature compensation can be the answer. As a Gas Flow Technology Manager at Bronkhorst, I know that efficiency and yield in a process require a stable gas flow. This gas flow can be measured and controlled by a thermal mass flow controller.

In this blog I will explain the challenges you can encounter in pressure and temperature control and share some tips on how to optimize your flow control by using real-time pressure and temperature compensation.

Challenges in temperature and pressure compensation

Some examples of external factors are:

• Temperature fluctuations
• Line-pressure fluctuations

These fluctuations can occur due to reduced pressure in a gas cylinder or due to cross talk between multiple flow controllers. How does Bronkhorst solve these problems, and what solutions do we offer?

Cross talk with mass flow controllers

What is cross talk? Cross talk typically arises when multiple mass flow controllers are positioned in close proximity in the same pipe, or installed upon the same rail or frame. The line-pressure from a gas regulator is affected by the flow demand of the flow controllers. When the flow instrument is changing its setpoint, it will affect the line-pressure. Due to this pressure change, the flow measuring section in a conventional flow controller is affected, indicating an incorrect flow measurement that does not represent the actual flow through the MFC.

The smaller the nominal flow of the flow controller, the bigger the effect will be to a setpoint change of a larger, parallel installed MFC.

What is the difference between static and dynamic pressure compensation?

Static pressure compensation is the compensation for slow pressure changes, for example the slowly reduced pressure from a gas cylinder. By integrating a pressure transmitter to the mass flow controller, together with an on-board conversion algorithm, real-time calculation of the actual fluid properties can be performed. For semi-caloric measurement the density, viscosity, thermal conductivity and heat capacity are used in the calculation. Under influence of pressure and temperature, these properties change. Thus, actual temperature and pressure are measured and processed, resulting in accurate flow measurement and control stability.

Dynamic pressure compensation is the compensation for rapid pressure changes. This can occur when a higher-flow mass flow controller on the same supply line changes setpoint, an undesired effect which is also known as ‘cross talk’. The moment that these rapid pressure changes are identified by the pressure sensor, the valve control will be adjusted accordingly so that the flow remains stable.

Stable flow control with on-board conversion

The on-board conversion algorithm makes it possible to convert the stored calibration fluid into one of the 100 on-board gases (multi-fluid multi-range functionality).

The actual measured temperature and pressure is used in the on-board conversion model to compensate for changes in the process conditions. This leads to a more reliable and accurate conversion and control stability.

Benefits of using real-time pressure and temperature compensation

1. Firstly, due to the improved and accurate flow measurement and control, optimized and more constant process conditions are possible, resulting in an improvement of your process yield.
2. Secondly, ease of installation since there is no need for exactly providing/meeting the process conditions the instrument was ordered for.
3. Thirdly, because the supplied line pressure becomes less important for the accuracy and control stability of the instrument, less accurate components or even reduction of components in the supply line are needed. This allows saving costs on, for example, a pressure regulator.