Absolute pressure sensors

The sensors are manufactured as differential or absolute pressure sensors. The differential pressure sensors are connected on both sides to absolute pressure sensors, of which the difference between the two pressures is to be measured. They are then evacuated down to a pressure lower than 1 x 10 7 mbar on one side (a chemical getter protects the vacuum of 10 7 mbar from residual degassing). 

Most pressure sensors detect the difference between the measured value and a reference. In the case of absolute pressure sensors, the reference chamber cannot be evacuated to absolute zero, because it can only be approached within a few thousands of a millimeter of mercury (torr). In the case of positive pressure detectors, when the barometric pressure is the reference, atmospheric pressure variations can cause errors up to 25 mmHg. Intelligent transmitters can also operate with multiple references and switch them as required. 

We have built a prototype of an interferometric pressure transducer that was designed to be attached to a non-commercial room temperature sorption instrument [3] equipped with a differential membrane sensor (MKS, Baratron 220CD, full range 10 mbar, pressure resolution 0,15 % of full range value). An additional absolute pressure sensor (MKS, Baratron 220 CA) integrated into the set-up provides the average pressure in the manifold and... 

Figure 3 shows the test section and instrumentation. Ten wall temperatures on the tube external surface were measured with 0.5 mm diameter calibrated type E thermocouples electrically insulated from the aluminium. Fluid inlet and outlet temperatures were measured with 1 mm diameter calibrated type K thermocouples. Cah-bration was carried out with a Rosemount 162-CE platinum thermometer. Due to the high thermal conduchvity of the aluminium and the low thickness of the tube walls the measured temperature is very close to the wall temperature in contact with the fluid (the difference less than 0.01 K). The inlet fluid pressure was measured with a calibrated Rosemount type 11 absolute pressure sensor. Two calibrated differential pressure sensors measured the pressure loss through the test section. A Rosemount Micro-motion coriolis flowmeter was used to... 

M. Esashi, Y. Matsumoto, S. Shoji Absolute pressure sensors by air-tight electrical feedthrough structure. Sensors and Actuators A21—A23 (1990), pp. 1048-1052. 

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

Fig. 5.1.17 Absolute pressure sensor for good media compatibility with pressure inlet port on the bottom of the silicon diaphragm and an on-chip vacuum reference volume, made possible by triplestack anodic bonding...

SensoNor asa, Data Sheet SP13 Absolute Pressure Sensor, SensoNor, Horten, Norway, 1997. 

Gauge pressure sensor Absolute pressure sensor Differential pressure sensor... 

Pressmes can be measured in environments from the near vacuum of space to more than 1,400 megapascals (MPa) and from steady state to frequencies greater than 100,000 cycles per second. Sensors that measure with respect to zero pressure or absolute vacuum are called absolute pressure sensors, whereas those that measure with respect to some other reference pressure are called gauge pressure sensors. Vented gauge sensors have the reference side open to the atmosphere so that the pressure reading is with respect to atmospheric pressure. Sealed gauges report pressure with respect to a constant reference pressure. 

Currently plastics represent around 13% of the total weight of a medium-sized car and industry experts believe that 90% of future car design improvements and innovations will have an electronic content such as electronic tyre pressure monitors installed to improved road safety. The ultimate tyre pressure monitor has been fitted to the world s fastest road car, the Bugatti Veyron 16.4. Its wheel electronics comprise a military grade battery, absolute pressure sensor, temperature sensor, accelerometer micro-controller and radio transmitter/receiver which are housed in a compact wheel housing made from PEEK. 

Absolute pressure is pressure measured relative to a perfect vacuum, an absolute 2ero of pressure (2). Like the absolute 2ero of temperature, perfect vacuum is never reali2ed in a real world system but provides a convenient reference for pressure measurement. The acceptance of strain gauge technology in the fabrication of pressure sensors is resulting in the increased use of absolute pressure measurement in the CPI (see Sensors). The pressure reference... 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

In addition to absolute pressure measurements, pressure sensors can be used to determine flow rates when combined with a well-defined pressure drop over a microfluidic channel. Integration of optical waveguide structures provides opportunities for monitoring of segmented gas-liquid or liquid-liquid flows in multichannel microreactors for multiphase reactions, including channels inside the device not accessible by conventional microscopy imaging (Fig. 2c) (de Mas et al. 2005). Temperature sensors are readily incorporated in the form of thin film resistors or simply by attaching thin thermocouples (Losey et al. 2001). 

Knowledge of the sample pressure is essential in all high-pressure experiments. It is vital for determinations of equations of state, for comparisons with other experimental studies and for comparisons with theoretical calculations. Unfortunately, one cannot determine the sample pressure directly from the applied force on the anvils and their cross-sectional area, as losses due to friction and elastic deformation cannot be accurately accounted for. While an absolute pressure scale can be obtained from the volume and compressibility, by integration of the bulk modulus [109], the most commonly-employed methods to determine pressures in crystallographic experiments are to use a luminescent pressure sensor, or the known equation of state of a calibrant placed into the sample chamber with the sample. W.B. Holzapfel has recently reviewed both fluorescence and calibrant data with the aim of realising a practical pressure scale to 300 GPa [138]. 

The vibrating cylinder version of this type of transducer employs a sensing element of the straight tube type and is similar to the vibrating cylinder density meter (Section 6.6.1). The cylinder is maintained in oscillation by a feedback amplifier/limiter combination and its frequency of oscillation varies with the pressure of the fluid within it, which alters the hoop stress on the cylinder. As with the vibrating wire, the frequency of oscillation increases with the fluid pressure. In order to be able to use the sensor for the measurement of absolute pressures, it is surrounded with a cylindrical housing and the annular space between the two is evacuated. Such sensors give excellent repeatability and have a relatively low temperature sensitivity. 

In the last 20 years, several different types of oxygen sensors based on 02 pumping have been developed(lO). All these sensors have the common characteristic that their signal output is linearly proportional to the ambient oxygen partial pressure. Their other characteristics, however, such as response time, temperature and absolute pressure dependence, and effect of gas flow depend on the specific design and mode of operation of the sensor. 

Under this condition, there is complete depletion of O2 at the electrode next to the porous layer of Fig.6a or inside the cavity of Fig.6b. The constant depends on the diffusion constant of O2 (in its particular carrier gas), Dq, and the geometrical characteristics of the diffusion barrier. In the device of Fig.6b (sensor with integral cavity), the diameter of the aperture C (usually greater than 50 microns) is much larger than the mean free path of the gas molecules at 1 atm (about 1 micron) and bulk diffusion dominates. In this case(ll-12). D0 - K Ta/P and cr - (DgA)/(kTd), where K] is a constant, P is the absolute pressure, a is a constant having a value between 1.5 and 2 and A and d are the cross-sectional area and length of the aperture C. Representative values for D0 are about 1.5 cm2/s at 700 °C and 0.15 cm2/s at 20 °C. Since Pg — cP with c the percentage of O2 molecules in the gas, we have... 

Of the various types of oxygen pumping sensors, the double-cell devices appear to have the best overall performance characteristics. These devices can be constructed to achieve the best combination of response time, independence of temperature and absolute pressure, insensitivity to electrode properties and operation not limited by the resistance of the Zr02 material. 

The first range can be adequately handled with a simple mercury manometer. ( 1 ) However, absolute pressure transducers, such as the Baratron 200 Series manufactured by MKS Instruments, offer much greater accuracy over a wider pressure range.(8) If accurate and precise pressure measurements are commonly conducted on the vacuum line, the transducers are the sensors of choice. If not, mercury manometers are the economical alternative. 

Fig. 5.1.15 Anodic-bonded silicon sensor with sensor for absolute pressure (left) and relative pressure (right) on the same chip...

Wafer bonding or cavity sealing is present on many MEMS products. Wafer-level packaging was first used in pressure sensors (Fig. 7.1.13a) to create an absolute... 

Fig. 7.1.13 a) Wafer bonding to create an ab- automotive accelerometers, including the Mo-solute vacuum cavity for an absolute pressure torola version (b or Fig. 7.1.1 b) sensor device [21] was adapted to several... 

Level. Level sensors that depend on a pressure difference between two points, one submerged and one in the vapor, are subject to the same installation considerations described for pressure measurement. The inferred level measurement is more accurate when determined by a differential pressure transducer using remote diaphragms than when calculated from the difference between two absolute pressure transducers due to errors from calibration and transducer drift. In either case, the measurement is affected by changes in the weight percent solids in the solution an increase in the liquid density would be interpreted as an increase in the level. Ultrasonic level sensors are not affected by the slurry density but are sensitive to fouling. 

Some BPBs will be fabricated with a pressure transducer mounted at one end. The initial version of this sensor is about 3 mm in diameter and is sensitive to pressures along the axial dimension of the BPB. Future versions will be sensitive to lateral pressure, and may be mounted remotely from the BPB. The present full-scale absolute pressure range is 400 to 900 mmHg. This signal can be read out either AC or DC coupled. When DC-coupled, it reads the absolute pressure. Since ambient pressure varies with altitude changes, this offset can be accounted for by placing a reference sensor of the same type in the MCU, then subtracting off this baseline. 

---