Transducers pressure measurement

Fig. 7. Instmment components of a control loop, where A = process measurement devices, in this case, pressure measurement B = transducer ...

Strain-gauge pressure transducers are manufactured in many forms for measuring gauge, absolute, and differential pressures and vacuum. Full-scale ranges from 25.4 mm of water to 10,134 MPa are available. Strain gauges bonded direc tly to a diaphragm pressure-sensitive element usually have an extremely fast response time and are suitable for high-frequency dynamic-pressure measurements. 

At each of the measurement locations, pressure probes may be attached to a harness, and these probes will direct the air flow to external pressure transducers for measurement while serving as a sheath for the appropriate thermocouple at that location (each thermocouple will be seated inside a pressure probe). 

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

Pressure measurements are made with four differential pressure transducers which produce a 0- to 5-VDC output signal from an unregulated 24-VDC Input. The range of three of the transducers 1s 0 to 500 psi and the other 1s 0 to 320 psl. The signal from these transducers 1s connected to an analog Input card 1n the HP 3497. 

The pitot tube is a device for measuring v(r), the local velocity at a given position in the conduit, as illustrated in Fig. 10-1. The measured velocity is then used in Eq. (10-2) to determine the flow rate. It consists of a differential pressure measuring device (e.g., a manometer, transducer, or DP cell) that measures the pressure difference between two tubes. One tube is attached to a hollow probe that can be positioned at any radial location in the conduit, and the other is attached to the wall of the conduit in the same axial plane as the end of the probe. The local velocity of the streamline that impinges on the end of the probe is v(r). The fluid element that impacts the open end of the probe must come to rest at that point, because there is no flow through the probe or the DP cell this is known as the stagnation point. The Bernoulli equation can be applied to the fluid streamline that impacts the probe tip ... 

Hz, providing 20 pressure measurements per rotation. Typicai pressure measurements for transducers positioned in meiting sections and metering sections that are fiiied with moiten resin are shown in Fig. 12.2. 

Figure 12.2 Typical pressure measurements for transducers positioned in the barrel for a screw speed of 60 rpm and a positive downstream pressure gradient (c>P / dz > 0) ...

SORPTION EXPERIMENTS. Sorption experiments were carried out using a Cahn 1000 recording balance, with an accuracy of 0.03 mg. The balance and its accessories are shown in Figure 1. The instrument is equipped with an MKS Baratron pressure transducer (0-100 torr, 0.15% accuracy) with a digital readout for pressure measurements. 

Apparatus. A conventional static vacuum apparatus was used. Reactants were introduced into cylindrical borosilicate glass reaction vessels (250-ml. capacity) suspended in a furnace maintained at a temperature within 0.1°C. Pressure measurements were made by a transducer (Consolidated Electrodynamics) linked to a recorder (RE 511 Servo-scribe). 

Detonation Velocity by Metallic Transition of Sulfur. Joigneau Thouvenin (Ref 1) reported a large increase in the elec conductivity of cryst sulfur when it was subjected to high transient pressure, but detected no sudden or discontinous transition to metallic condition. It was inferred from their results that a modified system should permit the use of sulfur as an active element of a pressure transducer for measurements in the kilobar range Hauver (Ref 2) prepd a modified system using a thin disc of sulfur, 0.00 50 inch thick and 9/32 inch in diam, insulated in Teflon. A number of tests were performed... 

Figure 25 Integrated thermocouple and pressure transducer to measure temperature as well as pressure during extrusion. Source Photo courtesy of Heaster Industries.

If measurements of the shock velocity in water are available at various distances from the charge, one can compute shock pressures at these distances. This approach can be used close to the charge where direct transducer measurements present formidable problems. An exptl arrangement for measuring shock velocity is shown in Fig 16 (Ref 1). Measurements thus obtained are compared with theoretical predictions in Fig 17 (closed circles). Also shown are direct pressure measurements (open circles). 

The apparatus and procedure described require great care in setting up and in operation. The effort is eased considerably if an automatic pressure measuring device operating at effectively constant volume is used instead of the manometer9. Probably, all apparatus now has some form of pressure transducer. Improvements as regards accuracy and sensitivity can also be obtained by, for example, having a vacuum instead of atmospheric pressure on the low pressure side. 

Cavity Type Transducer suitable for making dynamic pressure measurements of ordnance items was discussed by C.L.Pataky, PATR 232"1(1956)(U)... 

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

C. Flow Measurement by Pressure Drop across an Orifice. Another common scheme for the measurement of flow is based on the determination of the pressure drop on either side of a constriction, such as an orifice or venturi. Either a liquid-filled differential manometer or a pressure transducer with associated digital readout may be used for this pressure measurement. The flow rates determined by these meters are in units such as cm3/s, and it is necessary to make a correction for total pressure to convert these to standard cm3/s or mol/s. 

Electronic manometers provide a convenient method of pressure measurement in a tensimeter, and the general arrangement may be very simple (Fig. 9.3). The one problem which must be anticipated is long-term zero pressure drift, which can be encountered with an electronic pressure gauge. Drift is minimized by maintaining a constant temperature on the pressure transducer and by avoiding mechanical vibration at the transducer. 

Electrical Methods Electrical methods for pressure measurement include strain gauges, piezoresistive transducers, and piezoelectric transducers. 

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