Pressure transducers characteristics

The important characteristics of a transducer used in conjunction with an electronic measurement system are accuracy, susceptibility, frequency, impedance and, if appropriate, the method of excitation. The transducer is likely to be the least accurate component in the system, and it should be calibrated (and recalibrated) at frequent intervals. It is likely to be subject to a range of different physical conditions, some of which it is there to detect and others by which it should remain unaffected (for example, a pressure transducer should be unaffected by any changes in temperature which it might be called upon to experience). Some types of transducer are not suitable for use under D.C. conditions and all will have an upper limit of frequency at which accuracy is acceptable. Many types of transducer are also affected by stray electromagnetic fields. 

The calibration characteristics of the pressure transducer are described by the following relationship ... 

Pressure drop across the tar filter trap is measured by means of Kulite HEM-375 Piezoresistive Pressure Transducers. The sensing element is dielectrically isolated "silicon-on-stlicon" and exhibits excellent stability and thermal characteristics. They can be operated between -50 C up to 260 C. Pressure readouts at the inlet and the outlet of the filter trap are displayed in two Druck DPI 280 Series Digital Process... 

As a first step, the characteristics of the spinning valve have been investigated. The output of a pressure transducer and hot wire located downstream of the modulated jet was recorded for different forcing frequencies, and frequencies were identified where the amplitude of forcing would be nearly the same. Detailed temperature maps were then recorded at these selected frequencies. 

IV.l Consider the flow control loop shown in Figure 13.2a. The following information is also available (1) An orifice plate is used to measure the flow (2) a variable capacitance differential pressure transducer is employed (see Appendix 11 A) to sense and transmit the pressure difference developed around the orifice plate (3) the controller is PI and (4) the control valve is of equal percentage, with the valve flow characteristic curve given by... 

IV.2 Consider the liquid level control loop of Figure 13.2d. The differential pressure transducer cell exhibits second-order dynamics, the controller is proportional, and the control valve is linear with flow characteristic curve given by... 

Response times of 15-40 ms seem to be currently in use. Fry et al. (1957) analyzed the dynamic characteristics of three types of commercially available, differential-pressure pneumotachographs which employed concentric cylinders, screen mesh, and parallel plates for the air resistors. Using a high-quality, differential-pressure transducer with each, they measured total flow resistance ranging from 5 to 15 cm HjO. Frequency response curves taken on one model showed fairly uniform response to 40 Hz the second model showed a slight increase in response at 50 Hz, and the third exhibited a slight drop in response at this frequency. 

All of these pressure transducers offer frequency response characteristics acceptable for most or all respiratory applications. In situations requiring the measurement of a single pressure (for example, the pressure at the mouth during breathing), one of the two input ports to the transducer is left open to atmospheric pressure. In other situations requiring the measurement of a pressure differential (for example, measuring the output of a pressure-drop flow sensor), the two ports of the transducer are attached to the two taps of the sensor. 

Characteristic oscillation frequencies produced in the pressure by transverse wave interactions can be estimated as fo = Ucj/L, where L is the cell length, approximately 1.65. In the present experiments, fo = 4.5 MHz, corresponding to a period of 0.22 /is. Both the time and length scales associated with the transverse waves are much smaller than could be resolved by the instrumentation in the present experiments. The pressure transducers were 3 mm in diameter and the digitizing sample intervals were all larger than 1 fj.s. 

The transient pressure field in the water just above the tin layer is recorded by four pressure transducers flush mounted in the channel wall. The pressure record yields information regarding the hydrodynamic flow generated during the explosion. Figure 4 shows a typical pressure record for a trial with a water height of 12.7 cm that illustrates several common characteristics of most trials. The rise time of the pressure pulses is typi-... 

A typical functional block diagram (refer Fig. VIII/1.4 2 for explanation of functional block) for a simple pressure transmitter (courtesy ABB) is presented in Fig. VIII/2.0 1. Here, it is shown how even in a simple pressure transducer the characteristics are attached to each block and these are connected to the output via the software system. This simple example has heen chosen to show the role of hardware and software and their interrelationship in PE. 

High Speed Pressure Transducer. The characteristics of the Datametrics Type 531 Barocel pressure transducer are shown in Table Jj[. It can be operated as a differential or absolute type up to 450 C without cooling. 

Table II. Characteristics of Pressure Transducer Sensing Element capacitive potentiometer Rise Time 2-3 milliseconds Hysteresis Less than 0.2%...

A large-scale pneumatic conveying test rig was employed to transport the above four test materials. Two Afferent pipelines were connected to the test rig. The characteristics of the pipelines are listed in Table 2. The product mass flow rate, mg was determined from load cell readings and the air mass flow rate, mf was determined by an air flow sensor, Annubar. The static air pressmes were measured by the pressure transducers. 

Figure 8.4 illustrates a few of the combinations of force-summing device-link and electrical conversion devices that are embodied in various commercial pressure transducers. Table 8.2 lists the general characteristics of some common pressure transducers as compiled by Arvidson and Brennan. The latter compilation is recommended as a comprehensive treatment of pressure measurement. 

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