Sensitivity pressure sensors

With these or similar sensitive pressure sensors possible excess foaming can also be registered and an appropriate program modification activated, as already mentioned. 

Cheap, very sensitive pressure sensors will be used to monitor the behavior of ventilation systems. These will be based on silicon sensors with built-in signal conditioning. 

Miniaturized and integrated sensor systems were developed early for pressure and accelerometer sensors. The technology of silicon micromachining leads to sensitive pressure sensors which were marketed early [4]. Also accelerometers were developed mainly driven by the huge market of air bag application and crash sensors [5]. 

A dedicated very high pressure vessel (max 350 bar) is connected to a high sensitivity pressure sensor through a special capillary tube. This vessel allows to record simultaneously the heat and pressure released by the sample during the reaction as a function of time or temperature [32],... 

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). 

An electronic torque balance transmitter incorporating an LVTD (Fig. 6.13) has a sensitivity of 1 mA output current per 1 kN/m2 change in measured differential pressure, where the current is measured across an output impedance of 100 kO. This transmitter is connected to a recorder which has an input impedance of 10 kf) and a sensitivity of 1 kN/m2 per mA change in current from the transmitter (Fig. 6.73). If the pressure sensor is measuring a true pressure differential of 5 kN/m2, what will be the corresponding reading on the recorder ... 

Some characteristic values of the pressure sensors mentioned here have been summarized in table 10. As mentioned before, the value r ldX/dp, denoting the sensitivity of the sensor with respect to a pressure change, is the highest for SrFCl Sm2+. However, if an experiment requires high pressures and high temperatures, a closer look at the value of r 1dX/dp/(dX/dT) is necessary. This value can be regarded as a measure for the overall performance of the sensor with respect to pressure and temperature. From this point of view YAIO3 Nd3+ would be the best sensor due to its extremely low temperature shifts. 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

The bulk modulus cell consists of a hollow, cylindrical steel probe with a projecting stem (Figure 3.142). When exposed to a process pressure, the probe is compressed, and the probe tip is moved to the right. This stem motion is then converted into a pressure reading. The hysteresis and temperature sensitivity of this unit are similar to that of other elastic element pressure sensors. The main advantages of this sensor are its fast response and safety in effect, the unit is not subject to failure. The bulk modulus cell can detect pressures up to 15,000 bar (200,000 psig) with 1-2% full-span error. 

Fig. 16.2. Integration of organic transistors with pressure-sensitive rubber sheets, (a) A cross-sectional view of the device structure and (b) a circuit diagram of one sensor cell. The device is formed by laminating four different functional films. The pressure sensors are...

As shown in Fig. 16.2, the pressure sensors are formed as pressure-sensitive rubber (3) sandwiched between two electrodes and connected to the transistors though via holes (2). Voltage bias Vdd is connected to the transistor when pressure is applied to the sensors, enabling detection of the distribution of pressure. 

There are several applications of ZnO that are due to its excellent piezoelectric properties [28,164]. Examples are surface-acoustic wave (SAW) devices and piezoelectric sensors [28,165-167]. Typically, SAW devices are used as band pass filters in the tele-communications industry, primarily in mobile phones and base stations. Emerging field for SAW devices are sensors in automotive applications (torque and pressure sensors), medical applications (chemical sensors), and other industrial applications (vapor, humidity, temperature, and mass sensors). Advantages of acoustic wave sensors are low costs, ruggedness, and a high sensitivity. Some sensors can even be interrogated wirelessly, i.e., such sensors do not require a power source. 

Fig. 6. Schematic design of a pressure sensor. A flexible stainless steel membrane interfaces the pressure-sensitive elements (bridged piezo-resistors) from the measuring liquid. Some products contain the amplifier electronics in the housing and are (somehow) temperature compensated. The shown 2-strand cabling mode resulting in a current signal is very convenient...

Due to the limited response time of suitable sensors fast sorption or gas transport processes on a time scale below a second are hard to monitor. To significantly improve the resolution in time an interferometric pressure sensor can be applied. The central part of the interferometric pressure sensor presented is a Michelson-interferometer this set-up is sensitive to changes in gas pressure as the index of refraction, and thus the optical path length for a laser beam within the interferometer, is a function of the gas density. 

Gas pressures in vacuum applications are usually either recorded via membrane transducers, systems that monitor the gas density via partial ionisation of the gas or sensors that make use of the fact that the thermal conductivity or diffusivity of a gas is pressure dependent. The first type of transducer is sensitive to the total gas pressure while the other methods yield gas dependent signals. In terms of application properties such as the response time of the sensor, the sensitivity and the pressure range that the sensor covers are important technical specifications. The response of a membrane pressure sensor to a step-like pressure change is essentially an exponential function characterized by a relaxation time r for a MKS transducer, type Baratron 220 [1], r was determined to be 0.227 s (see Fig. 1), the actual pressure and the value as recorded by the transducer therefore do not match within the error bars given for the sensor until more than a second passed. 

Figure 1 Recorded gas consumption during methane hydrate formation at 3°C in various porous media (see also Table 1). Small pressure fluctuations are caused by the sensitivity of the pressure sensor to fluctuations of the room temperature.

The analysis of thin supported films by N2 adsorption is often difficult due to the very small percentage of pore volume contributed by the thin layer relative to that of the support. Usually it is necessary to scrape off most of the bulk support layer to increase the pore volume percentage of the thin film. Recent technical improvements in pressure sensors on commercial apparatus (reaching now a sensitivity of 5.10 mmHg) or new sophisticated detection techniques using surface acoustic waves may in some cases solve this problem. 

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