What is in this article?:
Pressure is a key factor in almost every fluid-power circuit. Pressure transducers convert fluid pressure to an electrical signal for monitoring and control of pressure in electronic control systems.
Big impact from temperature
Temperature is a major consideration in the performance of pressure transducers. As temperatures change, different materials expand or contract at different rates, which creates residual stresses within the structure. These stresses can change the output of the transducer by changing its geometry, mechanical properties, and electrical characteristics. While manufacturers take great care in selecting materials and determining how they are put together, these changes are inevitable. Transducer manufacturers compensate for these changes in a variety of ways - generally, by electrically adjusting the transducer's output circuit.
Transducers are specified to have a compensated temperature range. Within this temperature range, the transducer will perform within published specifications. Transducers also have an operating temperature range. Transducers will continue to perform within this range, but an operating error likely would exceed published specifications. For example, a transducer's typical compensated range might be 30° to 130° F (1° to 54° C), while its typical operating range could as great as 60° to 200° F (51° to 93° C).
The degree of compensation is expressed by two specifications:
Thermal effect on zero gives the boundaries within which the zero value of the transducer is compensated to stay. This is usually published as ±x% of FSO within the compensated range. Some transducer manufacturers may publish the same value as ±x% of FSO/°F within the compensated range; this makes the numbers look smaller. Graphically, the thermal effect on zero can be expressed as shown in Figure 3.
Thermal effect on span gives the boundaries within which the full scale output of the transducer is compensated, to stay. This is usually expressed as ±x% of reading within the compensated range. Here, too, some manufacturers may publish the same value as ±x% of reading/°F within the compensated range. Graphically the thermal effect on span can be expressed as shown in Figure 3.
Thermal errors are separate from other errors affecting total transducer accuracy (linearity, repeatability, and hysteresis). These errors must be accounted for and specified separately. Accuracy over the compensated temperature range must include the thermal effects on zero and FSO.
It is important to realize that the performance specifications of transducer manufacturers refer to ambient temperature - that is, the temperature of the air surrounding the external case of the transducer. Users also must be aware of media (fluid) temperature, because it can have a significant effect on the actual operating temperature of the transducer and, therefore, its performance.
Materials of construction are usually selected by the transducer manufacturer, but they also are clearly important to the user. Material in contact with media (or wetted materials) includes all materials exposed to the pressurized fluid. These may include any of a variety of stainless steel, bronzes, epoxies, plastics, elastomers, glasses, and silicon. Users must satisfy themselves that the pressure medium will not adversely affect any of these materials. If this does occur, the calibration of the transducer will certainly change. Ultimately, its sealing integrity will fail as well.
Environmental specifications tend to be difficult to relate to actual working environments. OEMs often test their components through actual usage in the working environment to carefully and accurately determine operating parameters - shock and vibration levels, temperature excursions, moisture levels, etc. These approaches require time and money, but they are recommended. If lack of time or money prevents exhaustive testing, follow these steps:
- Look at manufacturers' published environmental specifications.
- Ask around. Chances are that associates may have had experience with similar applications and components.
- Talk to transducer suppliers who have extensive experience in similar fluid power applications. No test is better than actual product usage in a real-world working environment.