Offset pressure sensors

Pressure sensors that give temperature-corrected, linear, analog voltage output are available from Motorola and other manufacturers. In such sensors, the on-chip electronics correct any temperature effects and nonlinearities in the output of the piezoresistors. The on-chip electronics replace a shoebox-size collection of printed circuit boards. The price of this kind of smart sensor is considerably less than 100. The integration of a large amount of circuitry on the chip allows functions like amplification, offset correction, self-testing, autocalibration, interference reduction, and compensation of cross-sensitivities (6). 

Figure 4.1.16 shows the same comparison for the offset of Bosch s high-pressure sensor. 

Fig. 4.1.16 Comparison of measured distribution (left) and results of Monte Carlo simulation (right) for the offset of Bosch s high-pressure sensor (adopted from [27])...

With the pressure sensor, linear errors (i.e., offset voltage, offset voltage temperature characteristics, sensitivity, and sensitivity temperature characteristics) can be corrected and reduced with a simple circuit. 

The pressure sensor may have changed its offset correction value compared with the laboratory calibration. The negative of the pressure sensor s pre-cast deck value replaces the laboratory offset correction. Calibration may be performed for all sensors at this stage (see Section 3.6.3). However, to save computing time it may be delayed until after the data have been reduced, but before smoothing and interpolation starts. 

Differential pressure sensors are special pressure sensors with two chambers separated by a membrane. The pressure difference between those two chambers is detected. This setup allows determination of pressure differences significantly smaller than either one of the applied pressures with high accuracy. Fields of application are the pressure losses over certain unit operations such as separation columns or the use of differential pressure to monitor two other process parameters flow and level. The latter applications are discussed in the subsequent chapter. In high-pressure technology, two aspects have to be observed closely. First, the maximum allowable differential pressure of the transducer can easily be exceeded due to high system pressures, thus destroying or offsetting the sensor. Second, fluid... 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

The pressnre sensor is qnite fast, whereas the process (change in pressure for change in vent valve stem position) and the actnator are generally the slowest elements in the feedback system therefore, this is also a relatively fast-responding process. The P-only controller can be used if offset elimination is not important, and a PI controller can be used when offset elimination is important. 

Functional parameters FP of a sensor include the sensitivity S, cross sensitivities, temperature coefficient TC, temperature coefficient of sensitivity TCS, offset O, and corresponding TCO. Nonhnearities of TCS and TCO are NLTCS and NLTCO. And we also have hysteresis, burst pressure, hermeticity, and other parameters. These functional parameters need to be described by model parameters MPj, j= l...n, which are appropriate for the processes used to fabricate the device (layer thickness, etching profiles, residual layer stress, etc.). 

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. 

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