TIPS ON SPECIFYING PRESSURE TRANSDUCERS •
Pick a robust pressure transducer. Hydraulic applications can be remarkably difficult for pressure sensors. A proper transducer design must survive mechanical stress at the installation point and hydraulic pressure spikes. Think total cost. Troubleshooting, downtime and replacements can dwarf the original purchase cost. If the data sheet states nothing about mechanical shock and vibration testing, ask for details or keep looking. Consider potential pressure spikes. A rapid pressure increase that greatly exceeds normal working pressure is a pressure spike. Severe pressure spikes can exceed working pressure, offset calibration and damage electronics inside the device. Think of a pendulum swinging, where the raised ball represents this pressure peak. It is about to strike your thin metal disc, which has a circuit device glued to the opposite side. Some of the worst pressure spikes occur in manual operation during a system startup.
the distance the signal must travel and possible interference that can be found in the environment around the system. Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally. The maximum rated pressure is the highest allowable pressure that the pressure transducer is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value. A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; operating outside of this temperature may significantly affect the accuracy of pressure sensing.
6 • 2016
FLUID POWER WORLD
Remember location. Mount the pressure sensor close to the main line, actuator or volume of interest, but keep location serviceable. If you think the sensor is exposed to severe mechanical shock, high vibration or physical damage, move it. Add a short length of hose and find a more stable location. Minimize the risk of early transducer failure by selecting the right device and putting it in the correct location. Keep it short. Avoid using test or measurement hose longer than 1 or 2 ft, unless you only plan to measure static pressures. Never use long measurement hose if the transducer provides feedback to an active closed-loop system, as it slows pressure feedback, which can make your PID system unstable. If possible, install the hydraulic port facing up or sideways. This prevents trapped air inside the transducer, which causes a delay in reading the actual hydraulic pressure. If you commission a system with maximum pressure of 1,400 psi or lower pressure, you must manually bleed air from all high points. Above this pressure, the remaining air becomes entrained in the hydraulic fluid, where it eventually escapes in the tank. (Entrained air … it sounds rather pleasant, except for the violent implosion that occurs as these air pockets collapse at high pressures. This is another kind of pressure spike that you might wish to avoid inside your pressure sensor.) Avoid using transducers with built-in orifices. Or add a gauge snubber. These are used to improve transducer life or to hide poor pressure regulation. However, adding a hydraulic delay leads to pressure instability in a closed-loop control system.
6/17/16 9:32 AM
Fluid Power World Handbook 2016