Signal pressure sensors

Intelligent transmitters have two major components (1) a sensor module which comprises the process connections and sensor assembly, and (2) a two-compartment electronics housing with a terminal block and an electronics module that contains signal conditioning circuits and a microprocessor. Figure 6.9 illustrates how the primary output signal is compensated for errors caused in pressure-sensor temperature. An internal sensor measures the temperature of the pressure sensor. This measurement is fed into the microprocessor where the primary measurement signal is appropriately corrected. This temperature measurement is also transmitted to receivers over the communications network. 

An additional component, fitted in the hose pipe between the pressure sensor and the suds container and fastened to the suds container, is capable of generating a pressure signal from the dynamic movement of the suds container. This is then sent via the pressure sensor to the microprocessor for evaluation (Fig. 5.57). 

Further cost reductions are possible if the pressure sensor can be fastened to the vibrating suds container instead of requiring an additional component Pressure sensors normally measure the force exerting pressure on a defined surface. Depending on the principle of the particular pressure sensor, it will require an accelerating mass which, when accelerated, produces an additional force and the resulting acceleration signal. Three possible examples are described below. 

Fig. 5.59 shows a section through a Piezo-resistive silicon pressure sensor. The ambient pressure is applied from above, while the pressure being measured is applied from below. A silicon membrane that deforms under the pressure is applied to a silicon carrier structure. Piezo-resistive structures are fitted in the membrane, which then change their resistance accordingly when the membrane deforms. A bridge circuit generates an electrical output signal which is proportional to the difference in pressure. 

Tumble dryers use pressure sensors for filter measurements by measuring the differential pressure across the lint filter signalling the user when to clean the filter. 

Thermoelectric flame failure detection Analog burner control systems Safety temperature cut-out Mechanical pressure switch Mechanic/pneumatic gas-air-ration control Thermoelectric flame supervision Thermal combustion products, discharge safety devices Electronic safety pilot Electronic burner control systems Electronic cut-out with NTC Electronic pressure sensor/transmitter Electronic gas-air-ration control with ionisation signal or 02 sensor Ionisation flame supervision Electronic combustions product discharge safety device... 

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. 

Another method to process and detect faulty responses of the redundant sensor signal is called the likelihood test. The likelihood test used is borrowed from nuclear reactor safety for pressure sensors deciding shutdown or no shutdown. Instead of shutdown or no shutdown for this case, the sensor reading would be rejected or accepted. For accuracy of the readings, the test chosen needs to reject readings that have even the smallest likelihood of being false and leading to the patient s inappropriate response.36... 

R 18] [A 1] Each module is equipped with a heater (H3-H8) and a fluidic cooling (C03-C06). Temperature sensors integrated in the modules deliver the sensor signals for the heater control. Fluidic data such as flow and pressure are measured integrally outside the micro structured devices by laboratory-made flow sensors manufactured by silicon machining. The micro structured pressure sensor can tolerate up to 10 bar at 200 °C with a small dead volume of only 0.5 pi. The micro structured mass flow sensor relies on the Coriolis principle and is positioned behind the pumps in Figure 4.59 (FIC). For more detailed information about the product quality it was recommended to use optical flow cells inline with the chemical process combined with an NIR analytic or a Raman spectrometer. 

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...

Electrochemical sensors — Electrochemical sensors belong to the family of chemical sensors. Chemical sensors are devices that convert chemical information (concentration, - activity or partial pressure of the analyte) into a measurable signal. Chemical sensors contain two basic functional units a receptor and a transducer. The receptor interacts with the analyte and transforms chemical information into a form of energy that is converted further by the transducer into a useful analytical signal. 

The data acquisition system consists of a pressure sensor attached to a nozzle on the side of the cylinder body (the sensor should not be affected by the hot gases or decomposition products and should be capable of responding to a rate of pressure increase between 690 kPa (100 psig) and 2,070 kPa (300 psig) within 5ms), an amplifier for signals from the sensor, and a digital memory scope that has a recording system the system should correspond to the input described above. 

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. 

The pressure and liquid level in the chlorine vaporizer are controlled automatically. As vapor flows out the top. the pressure in the tank begins to decrease. A pressure sensor detects the drop and sends a signal to the steam flow control valve. The valve opens and feeds more steam to the coil, causing more liquid chlorine to evaporate and raising the pressure back to the desired value. As the liquid evaporates, the liquid level in the tank begins to drop. A liquid level sensor detects the drop and sends a signal to the liquid chlorine flow control valve. The valve opens, and more liquid enters the tank to replace the liquid that evaporated, raising the liquid level back to the set-point (desired) value. 

The feed rate of chlorine to the absorber is automatically controlled to maintain the absorber pressure at a specified value. If the pressure begins to fall, a pressure sensor detects the drop and sends a signal to the expansion valve described in Step 3. The valve opens, causing the flow rate of chlorine to increase and raising the absorber pressure back to the set-point value. It the pressure rises above the set point, the pressure sensor sends a signal that adjusts the control valve to decrease the flow rate of chlorine and the pressure comes back down to the set-point value. 

A more detailed overview of signal characteristics, including the definition of fundamental terms describing sensor characteristics, is given for instance in [8] and [9], and with respect to the special behavior of pressure sensors in [10]. 

Here we show an example of applying the EFS method to a non-silicon-based pressure sensor to operate at high pressure ranges. The membrane of many high-pressure sensors (Bosch, WIKA) is manufactured of steel, with thin-film metal resistors as a measurement signal pickup (Fig. 4.1.12). 

Obviously, a sensoTs function can be tested at the end of the fabrication process after the sensor is completely assembled and fully packaged. At this time a primary input signal (pressure, acceleration, yaw rate, mass flow, etc.) can be applied directly, and a comprehensive test of the specified performance is possible. Unfortunately, by the time a defective sensor component is packaged, loss of time and capital is maximum, since a fully packaged sensor needs to be discarded. 

This can further be enhanced by the so-called lock-in technique, in which modulated signals are used, which after demodulation separate the desired signal from the noise. Both techniques are described here by examples one is a monolithic pressure sensor with moderate piezoresistive bridge signals and the other is a signal evaluation circuit for a high-pressure sensor with very small sensor signal levels. 

Fig. 6.2.14 shows the chip photo of the realized signal processing ASIC for high-pressure sensors. The signal conditioning operates in the above described manner. The parameters of the sensor are as follows ... 

Pressure sensors present a potential reliability problem when pressure signals originate from harsh environments, such as that in an engine manifold or in an automotive tire. 

High-pressure sensors differ mainly in the measuring principle used and the type of internal connection. Most sensor designs use a steel diaphragm to separate the pressure fluid from the sensor signal and environment. 

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