When controlling pressure using a precision pressure controller in a calibration laboratory or in a production environment, there are inherent trade-offs between the speed at which a pressure setpoint is reached, the stability of that setpoint once reached, and the volume of the system where the pressure is being introduced. For a production line where throughput is important this is critical to understand. In calibration labs, which typically allow for slower speeds and longer dwell periods, the relationships between these three variables still exist but have little effect on operations.
For this discussion, we will focus on a system containing a precision pressure calibrating controller like the CPC4000 and a device, or multiple devices, being tested or calibrated. Precision pressure controllers regulate pressure into a volume consisting of tubing, a manifold, and the transducer(s) being tested. The stability of the pressure output is controlled by an algorithm that reads the controller's internal reference sensor and adjusts the regulator to add or exhaust gas to and from the system to achieve a stable pressure.
In this example, the trade-offs between speed, stability, and volume can be significant and affect the efficiency of the whole system.
Speed Vs. Stability
First, let’s examine the speed and stability of a system with a constant volume. The control stability specification of a precision pressure controller is defined as the expected maximum pressure fluctuation around a set point after a period of time. For example, the CPC6050 stability window is 0.003% of the active transducer range and will indicate a stable condition if the pressure remains in this window for a user-defined stable dwell time, usually 3 or 4 seconds.
The speed is defined as the time it takes to reach a setpoint while satisfying the stability window specification and dwell time.
In a high-speed production testing environment, pressure calibration instruments must reach a setpoint quickly to increase the throughput of the component being tested. The quicker this is done, the more product can be produced. An example of this is testing tire pressure sensors that are standard equipment in all new vehicles.
Many times sensors are tested in groups and may be connected via a manifold or chamber with a constant volume. In general, the faster that a volume is filled the harder it is to achieve a stable pressure within a short dwell time. It is a matter of momentum and, thermodynamics. A mass of gas flowing into a system at a high velocity will act like a spring when shut off abruptly causing the pressure to oscillate around a point until the oscillation is dampened over time. In addition, gas going from low to high pressure will heat up, then cool off over time, causing the pressure to decrease from the initial value.
The oscillation described above can be controlled by throttling down the flow of gas as the setpoint is approached and then controlling the inlet and exhaust of gas to maintain stability at the prescribed setpoint. The controller’s algorithm must be tuned to know when to start this throttle-down process to avoid overshoot and how to react to small fluctuations in pressure around the setpoint. Every system will have an optimal set of parameters that control the inflow and outflow of gas into the system to achieve the highest possible speed and stability.
However, the faster the setpoint is reached the more difficult it will be to control the fluctuation around the setpoint. The system variables include the volume and geometry of the piping and valve system, the change in pressure, the temperature of the gas, and the temperature of the surrounding environment. The operator must weigh the speed against the stability requirements. Stability is important because the reading of the pressure standard and the reading of the device being tested may be slightly different in some cases when the dwell time in minimal. The operator will decide the acceptability of the readings given the speed, stability, and accuracy of the device being tested.
Speed Vs. Volume
A system where sensors are being tested contains a set volume into which pressure must be controlled. The greater the volume the more gas is needed to reach any given setpoint. The implication of this is that bigger volumes are slower to control than smaller volumes, which is pretty intuitive. However, larger volumes can accommodate a higher number of sensors, so the slower speed might be mitigated by the number of sensors that can be tested at once.
Volume Vs. Stability
In a small volume, small changes in the amount of gas entering or exiting the system will cause a relatively large change in the pressure. On the other hand, in a large volume small changes in the amount of gas entering or exiting the system will produce little change in the system pressure. With this in mind, many applications prefer a larger volume to establish better system stability.
The importance of understanding how speed, stability, and volume affect overall system performance is critical. Once the goal is established relating to these variables, the next step is finding the most suitable pressure controller for the desired outcome. The Mensor sales team can assist in choosing the right controller for your application. Control to a set point quickly with the desired stability to increase the efficiency and speed of your system.