Pneumatic pressure transducers

Another type of transducer is the pneumatic pressure transducer. It has good robustness, but poor temperature sensitivity, poor dynamic response, and average measurement error. The capillary transducer has fair robustness, fair temperature sensitivity, and fair dynamic response. The total measurement error varies from 0.5 to 3% depending on the quality of the transducer. The pushrod is similar to the capillary transducer, except that it tends to have poor temperature sensitivity and poor total error. The piezo-resistive transducer has good robustness because of its relatively thick diaphragm, good temperature sensitivity, good dynamic response, and low measurement error. A comparison of different pressure transducers is shown in Table 4.1. 

Transducers The ciirrent-to-pressiire transducer (I/P transducer) is a conversion interface that accepts a standard 4-20 rnA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range (normally (),2-L0 bar [3-15 psig] or, less frequently, 0,4-2,0 bar [6-30 p.sig]). The output pressure generated by the transducer is connected directly to the pressure connection on a spring-opposed diaphragm actuator or to the input of a pneumatic valve positioner. 

Systems with electronic pneumatic control use pressure transducers at the inlet and outlet, the column dimensions and physical properties of the carrier gas to determine the gas hold-up time and the flow rate. 

Transducer housing and conb ols for amplitude and duration of ultrasound. Also provides pneumatic pressure... 

FIG. 8-87 Current-to-pressure transducer component parts. (a) Direct-cur-rent-pressure conversion. (b) Pneumatic booster amplifier (relay), (c) Block diagram of a modern I/P transducer. 

Dynamics of Positioner-Based Control Valve Assemblies Control valve assemblies are complete, functional units that include the valve body, actuator, positioner, if so equipped, associated linkages, and any auxiliary equipment such as current to pneumatic signal transducers and air supply pressure regulators. Although performance information such as frequency response, sensitivity, and repeatability data may be available for a number of these components individually, it is the performance of the entire assembly that will ultimately determine how well the demand signal from the controller output is transferred through the control valve to the process. The valve body, actuator, and positioner combination is typically responsible for the majority of the control valve assembly s dynamic behavior. On larger actuators, the air supply pressure re lator capacity or other airflow restrictions may hmit the control valve assembly s speed of response. 

FIG. 8-73 Current to pressure transducer components parts (a) direct current to pressure conversion (b) pneumatic booster amplifier (relay) (c) block diagram of a modem I/P transducer. 

Hornbcek Pneumatic gauge from 1890. Float in stilling well from app. 1959. Differential pressure transducer — CTD (Conductivity, Temperature, Depth) — from June 2000. 

The following two references can be consulted for further details on the variable capacitance differential pressure transducer and the pneumatic control valve ... 

Fig. 12.4 Electronic flow controller (EFC) in a gas chromatograph, (a) Capacity pressure transducer, (b) Pneumatics of the EFC. (c) Control loop regulation by means of two microprocessors. (Courtesy of Hewlett-Packard).

Transducers The current-to-pressure transducer (I/P transducer) is a conversion interface that accepts a standard 4- to 20-mA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range [normally 0.2 to... 

The Varian Model 8500 pump (Fig. 5) is the most sophisticated syringe-type pump. There are two gas solenoid valves, the first of which pressurizes the solvent container to re-fill the chamber and the second actuates a pneumatic valve downstream of the pressure transducer on the column tubing. The operating controls are selected automatically by depressing push-button switches on the front panel of the pump controller. These switches control the opening of the... 

Fig. 5. Diagram of Varian Model 8500 pumping system, a = valve to pressurize the reservoir, c = solvent reservoir, -flow valve, f = pressure transducer, g = pneumatic valve...

Each position command issued to the motor controller caused the pneumatic pressure sensor to expand and contract to simulate respiratory movement. Note that while the motor position was controlled in a close-loop manner, the loop was not closed on the linear displacement. In a reasonable assumption, all mechanical dynamics from the motor shaft to the pressure transducer were neglected. 

Fig. 35. Diagram of the system used to pressure-eject peptides from multibarrelled pipettes. A pressure of (50 Ib/in ) was supplied to the system from a nitrogen cylinder through a standard gas regulator in series with A, an electronic/pneumatic valve, and an adjustable needle valve control B. A toggle switch two-way valve C was used either to connect the remainder of the system to the pressure cylinder, or to open it to the atmosphere through an exhaust. A pressure transducer (Statham PM6) was connected at D, to each of the individual pipette lines E. Pressure tubing was secured to the side barrels of the multibarrelled electrode by O ring connectors. Z and Y represent a second means of control in which the pressure application was controlled by the output of the conventional electrical stimulator, using a circuit identical to that developed by McCaman et al. (1977). (From Dingledine et ah, 1980.)...

The reproducibility for electric systems is about 0.1 to 0.2%, for pneumatic systems about 0.5%, and about 1 to 2% for mechanical pressure transducers. The hysteresis with electric pneumatic transducers is about 0.1 to 0.2%, while it is as high as 4 to 5% with mechanical systems. 

Measurement, comparison, and adjustment constitute an entity called a control loop. Very schematically one may describe the control loop in terms of sensors, controllers, actuators, and the process to be controlled. Shown in Figure 12.18 is a level control system for a tank. Broken lines denote control signals. The controller (denoted by LC) receives the level signal from the level transducer (LT) that sends a signal to the current-to-pressure transducer (I/P) that then applies a pneumatic pressure to the valve diaphragm. 

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. 

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