Ceramic sensors perform well in tough applications

Precision and reliability are key concerns when selecting pressure transmitters. Using pressure measuring devices that utilise a ceramic measuring cell rather than an oil-filled pressure membrane is one way instrument engineers can increase accuracy, reliability and long-term stability.

In addition to improved performance, the ceramic cell is more robust than traditional oil-filled pressure cells. In many applications the ceramic cell is a better choice for measuring pressure because it has no oil-fill fluid and can operate much longer under process conditions where traditional oil-filled pressure cells require maintenance on a routine basis. Oil-filled pressure transmitters are very delicate instruments and careful consideration must be given to which oil fill is most suitable for the application. In many cases, multiple spares are needed to accommodate the various types of oil fills that are needed in a typical plant.

The problems of oil-filled pressure sensors Traditional pressure measuring cells that utilise a metal isolation diaphragm require an oil fill that surrounds a chip, strain gauge or other signal generator which responds to force and transfers the pressure from the metal isolation diaphragm that is in contact with the process medium. This technique is the most common way to measure pressure and is used by virtually every device manufacturer in the industry. The oil fill in a traditional pressure transmitter is critical to ensure a uniform exchange of force from the process medium to the pressure measuring cell, thus it is critical to the accuracy of the transmitter in the application for which it is applied. When utilising oil-filled pressure cells, there are many process conditions and changes that can interfere with pressure measurements providing false results that can lead to inadequate process control, such as air bubbles in the measuring cell.

Figure 1: Traditional metal isolation diaphragm.

Most plant personnel do not realise that a metal diaphragm cell may be filled and calibrated perfectly at the factory but once a vacuum beyond about 40 mbar is pulled on the diaphragm, there may be air trapped inside the diaphragm. This phenomenon is called outgassing, and is the release of absorbed or occluded gases or water vapour contamination under elevated temperatures or vacuum.

It is a natural condition of stabilisation where materials try to reach equilibrium inside the measuring cell. These liberated gases are then present in a pressure cell and under normal conditions the operator may never realise unless the measurement cell is being turned down to the low end of the range. What this means is that under high pressure conditions, the gas will diffuse into the oil and will likely go unnoticed; conversely, when a vacuum occurs on the face of the diaphragm, the gas expands inside the measuring cell creating a positive pressure. Two things happen when the gas expands: firstly, the transmitter does not measure accurately in this condition because the gas is compressible and proper force is not exerted on the electronic strain gauge; and secondly, the oil is outgassing causing a bubbling effect and exerting pressure inside the measuring cell that is being exerted on the back side of the diaphragm. This negative pressure can cause the diaphragm to take the shape of a dome as shown in Figure 2. Once this happens, the measuring cell must be replaced, leading to costly repairs and downtime.

Figure 2: Metal isolation diaphragm under a vacuum condition.

Temperature can also greatly affect the measurement of a pressure cell that utilises an oil-filled metal diaphragm because as the process temperature increases or decreases, the oil inside may expand or contract creating pressure inside the measuring cell. This is usually identified by the technician as zero drift in the pressure transmitter. Some manufacturers of oil-filled pressure cells have addressed this problem by convoluting the isolation diaphragm, thus allowing the diaphragm to expand when the process temperature increases and decrease when the temperature decreases, having a minimal effect on the sensing element, the full deflection of the metal diaphragm being only about 0.05 mm.

The selection of the oil which fills the diaphragm is tremendously important when it comes to measuring pressure. The higher the process temperature, the thicker the oil required, while the lower temperature applications require lighter oil fill. This is why it is extremely important that the oil fill is matched to the process conditions. There may also be process compatibility issues; for example, the isolation diaphragm may rupture due to a pressure spike, fatigue, abrasion, chemical erosion or some other process condition. It is therefore imperative the oil fill be compatible with the process media.

Overpressurisation and vacuum service are also serious considerations when it comes to the selection of a pressure measuring cell that uses a metal isolation diaphragm. If the process cycles from pressure to vacuum, the metal isolation diaphragm may fatigue, which can lead to premature failure. Overpressurisation is commonly combated by forming metal stops directly behind the diaphragm which act as a backstop should the pressure become too great. Although the backstop approach is not a guarantee the diaphragm will not be damaged due to overpressurisation, it greatly reduces the chances of causing significant damage to the diaphragm provided the overpressurisation is not excessive. The manufacturer’s specification will usually rate the cell for a maximum allowable pressure.

Chemical compatibility of the metal diaphragm with the process media is another great concern when it comes to the selection of a traditional pressure transmitter. If the proper material is not selected, the process media may damage or destroy the metal diaphragm, rendering the instrument useless. Additionally, metal sensors are delicate and must be handled very carefully, ensuring the measuring membrane is never touched.

The benefits of ceramic sensors

A capacitive ceramic gauge and absolute sensor is a dry cell requiring no oil fill and provides a high degree of accuracy and reliability. The Endress+Hauser Ceraphire sensor, for example, consists of a ceramic body, a ceramic process membrane, a measuring electrode, a reference electrode and a spacer ring. The distance between the membrane and the sensor body is 40 μm. Signal processing, temperature compensation and conditioning is accomplished by an onboard application specific integrated circuit (ASIC) shown in Figure 3. This technique can be used for gauge and absolute pressure.

Figure 3: Ceraphire ceramic sensor.

A differential pressure cell is also possible in a ceramic design. The differential ceramic cell has an added benefit, which is a self-monitoring circuit that will alarm should the ceramic sensor fail.

In this simple design, the pressure measurement is expressed as:

P = (C_p – C_r) / C_p

where:

C_p = Process capacitance
C_r = Reference capacitance

Ceraphire ceramic sensors are formed of 99.9% aluminium oxide (Al2O3) and begin as crystalline particles which are smaller than 400 nm. This type of ceramic membrane has a microscopic finish that, with a diffusion coefficient seven to ten times smaller than metal diaphragms, assures that no permeation of small molecules from the process can diffuse into the sensor. Furthermore, the absence of metal, and nickel in particular, ensures that ceramic sensors are suitable for use in processes where hydrogen diffusion can be a problem for metal diaphragm sensors. Traditional metal diaphragm sensors must be conditioned for hydrogen diffusion by plating them with noble metals such as gold, but gold-plated metal isolation diaphragms are very expensive and generally take a long time to procure because they are made to order and the materials are not generally kept on hand by the manufacturer.

Figure 4: Construction of the ceramic differential pressure sensor.

The gauge pressure sensor shown in Figure 4 is a vented cell which measures pressure relative to atmospheric conditions by way of a protected vent. The vent is filter protected within the transmitter and allows the sensor to be compensated for atmospheric conditions. Moisture is kept out of the measuring cell with the use of a Goretex membrane.

In the absolute sensor there is no vent. Absolute pressure within the sensor is achieved by evacuation of the measuring cell when the sensor is produced.

Ceramic pressure measuring cells are insensitive to temperature effects under extreme process conditions. According to the McGraw-Hill Sensors Handbook, the typical temperature effects on sensitivity on the ceramic cell are <1% per 100 °F and as much as 12% per 100 °F for silicon metal diaphragm sensors (Soloman, 1998). Although ceramic material can withstand temperatures up to 2000 °C, there are limitations to the electronic components and the gasket material located between ceramic sensor and process connection. Ceramic sensors are typically fully resistant to temperature shock and will operate up to 150 °C without affecting the performance or long-term stability of the cell.

Vacuum applications are no problem for ceramic sensors due to the fact the sensor has perfect memory. When the cell is exposed to pressure or vacuum, the sensor deflection responds proportionally and returns to the rest position when the pressure or vacuum is released or the cell returns to atmospheric pressure. Ceramic sensors do not fatigue or bend out of shape as metal will under vacuum conditions, and because the ceramic gauge and absolute sensor have no oil fill, no bubble effect can occur in the cell which would create a positive pressure in a metal diaphragm pressure cell.

Overpressure is of key concern when selecting a pressure measuring device for any process. Constant overpressure on the measuring cell or pressure shocks such as water hammer caused by pump starts and valve actuation under normal process conditions can cause fatigue in a metal isolation diaphragm.

In addition to fatigue that may occur from constant movement of a metal isolation diaphragm, when maximum operating pressure is reached, a metal sensor will begin to bend out of shape. This bending of the diaphragm is permanent and will cause errors in the pressure reading, as well as zero offset. As shown in Figure 5, the ceramic sensor will operate consistently and accurately up to as much as 40 times the stated range of the cell without affecting the zero point or the accuracy of the measuring cell.

Figure 5: Graph of burst pressure for metal and ceramic diaphragms.

Ceramic sensors are also well suited to abrasive process conditions because of the durable material of construction. In addition, these sensors are an excellent choice for processes where coating occurs because they can be cleaned with a wire brush or even scraped with a sharp tool with little concern for damaging the ceramic surface. Chemical compatibility is of minor concern when selecting ceramic sensors due to the resistive nature of the ceramic material.

Vibration and pulsating pressure on the gauge and absolute sensors is of minimum concern because the sensor has no fill to transmit the vibration from the outside in - as a result, the deflection on the sensor only occurs when force is exerted on the outside of the measuring surface. The sensor is also immune to EMI and RFI.

Beyond the consideration of the process temperature, diaphragm material and what type of oil the transmitter must be filled with, when it comes time to consider replacement of a pressure transmitter one must consider total performance as well as cost of ownership. Recommended spare parts, calibration interval, chemical compatibility of materials, set-up time, reliability, ease of operation as well as local service are all major factors in the selection of a new transmitter.

Conclusion

What does this mean in dollars and cents? The traditional metal diaphragm sensor is delicate and has limitations in vacuum service, overpressurisation and vibration. Chemical compatibility and hydrogen diffusion applications using metal diaphragm pressure cells can lead to using extremely expensive noble metals such as Monel and gold.

Additionally, cleaning metal diaphragms in dirty or coating applications can be time consuming and risky due to the delicate nature of the metal diaphragm. Because ceramic sensor technology is rugged and built for industrial applications, combined with the ease of use, long-term stability, chemical compatibility and lower purchase price, it is easy to understand why ceramic sensors are a good alternative to the traditional metal isolation diaphragm sensor.

References:

• McMillan GK, Considine DM 1999, Process/Industrial Instruments and Controls Handbook, New York, McGraw-Hill.
• Soloman S 1998, Sensors Handbook, New York, McGraw-Hill.

Author John Van Nostrand is Pressure Product Manager for gauge, absolute, differential and DP flow products at Endress+Hauser. Formally trained in instrument theory and application, John has worked in an NIST Type III calibration lab. He has more than 20 years’ experience working in pressure measurement.

Endress+Hauser Australia Pty Ltd
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