How can the stability of a pressure transducer affect measurements over time?

Long-term stability in a sensor is normally associated with the change in zero offset due to aging of the component and relaxation of the metallic diaphragm over a period of time. It normally causes the zero reading to go higher or lower over time. Independent testing have shown this number to be <.25% under the influence of temperature and pressure cycling for 1500 hours.

What is the difference between gauge, absolute, and differential pressure?

• Gauge pressure is referenced to the barometric pressure conditions. Changes in barometric pressure do not change the output signal of the sensor.
• Absolute pressure sensors and transducers are referenced to a full vacuum; the output signal of the sensor will change with changes in elevation and with changes in barometric pressure.
• Differential pressure is the difference in pressure between two points; this is commonly used in filtration applications.

What is a compound pressure transducer?

A compound pressure transducer is a gauge or sealed gauge sensor that is calibrated to emulate an absolute range. A pressure transducer with a pressure range of -14.7 to 30 PSIG can be referred to as a compound pressure transducer, 30V30 (referring to -30" of mercury) transducer, or a vacuum to 30 PSIG sensor. For more information on gauge pressure measurement, please read the Compound Pressure Transducer Calculation example.

What is the difference between analog and digital transducers?
In the analog version of the transducer, the digital signal from the internal A/D is adjusted in several ways. Calibration factors are applied to confirm the sensor meets accuracy specifications. Then temperature correction factors are used to adjust the signal and compensate for ambient temperature. Finally, zero and span calibrations are added that set the output signal in the desired range as dictated by the transducer part number. The result is then converted back to an analog signal by an internal D/A converter, run through a unity gain buffer, and sent to the output pin. The output signal is continuously variable, just like the pressure applied to the sensor.

In the digital version of the transducer, a digital signal-processing core manipulates the data where various compensations and corrections are applied. This processed digital data is then stored in registers to be transmitted to the system later. The most common digital communication protocol used by sensors and transducers is InterIntegrated Circuit (IIC or I2C). This communication technique is designed so a transducer doesn’t take a pressure reading or report it until the system master controller sends a request for the data. Because the need for pressure data is intermittent, the sensor can go to “sleep” (a very low power mode) between requests for data. This helps conserve system energy, an important capability in both battery powered and wireless applications.

Read this whitepaper "Analog and Digital Transducers — the Advantages of Both" to learn more about the similarities, differences, and how to choose the right transducer for your application.

What is the most common material for pressure transducers?

Pressure transducers are most commonly manufactured in either 316L stainless steel or 17-4 PH.  Both materials are relatively inexpensive, easy to machine in comparison to high strength nickel alloys, offer good material strength, and good elasticity which allows for movement of a diaphragm. There are differences in their material properties that relate to both industrial and hazardous location applications that drive a preference for each material. While variations of these materials exist, TE has chosen the most basic and common forms used in the sensor marketplace today for comparison. The difference in iron content contributes to the magnetism and corrosion resistance. 17-4 PH stainless steel is magnetic and less corrosion resistant than 316L stainless steel. Standard 316L is slightly magnetic, but non-magnetic versions are available.

In comparison of 17-4 PH and 316L stainless steel, which has the higher material strength?

17-4 PH has a higher material strength than 316L stainless steel. For many hydraulic pressure systems, where pressure surges and high cycles are common, 17-4 is more frequently used since it is a good spring material. Pressure sensors and transducers can be specified with proof (typically 2 times the rated pressure) and burst (typically 5 times the rated pressure) pressure ratings that are equal between the materials; yet there is a greater chance that 17-4 PH will measure accurately for a longer period of time when higher pressures and pressure transients above the rated pressure are present.

What is the chemical compatibility of 17-4 PH and 316L stainless steel?

17-4 PH is used in various non-corrosive or mildly corrosive liquids and gases. Hydraulic fluid, brake fluid, fuels, and other standard industrial liquids will work well with 17-4 PH stainless steel. With higher nickel content, 316L stainless steel will work for these fluids in addition to many liquids and gases with more corrosive properties. For example, natural gas with low H₂S content will need 316L to survive corrosion.

Water (excluding salt water) is often considered a non-corrosive liquid, although 316L is preferred for pressure measurement. Various pH levels can cause 17-4 PH material to generate mineral deposits and clog process connections.

Gases, such as hydrogen, require 316L material. Hydrogen ions are small enough to penetrate the grain structure of 17-4 PH stainless steel, thus breaking down the diaphragm over time as a result of embrittlement.

For ultra-high purity applications such as semiconductor process equipment, 316L VAR (Vacuum Arc Melting) material is introduced to reduce non-metallic impurities. Further, material surface finish is put through a process called electro-polishing. This further reduces impurities from coming in contact with the liquid or gas by removing non-metallic imperfections as well as a small amount of the metallic surface. 

What are the differences in pressure sensor technologies?

Technology selection is an important factor. Certain technologies are limited in the material to which they can be applied.

• The strain gauges applied to the diaphragm should match the thermal properties of the material to which it is applied.
• 17-4 PH stainless steel has a mean coefficient of thermal expansion of 6.0 x 10-6 in/in/°F with H900 heat treatment, whereas 316L ranges from approximately 9 to 11 x 10-6 in/in/°F. 
• Thin film is limited to 17-4 PH because the temperature of the sputtering process is too high at the diaphragm. 

The difference in sensor material and technology can play a critical role in pressure transducer selection. Be prepared with information regarding the liquid or gas being measured and its application to help find the better option between materials. If neither 17-4 or 316L suffice, special alloys can be offered.