The measurement of temperatures in gas-cooled reac­tors requires certain specialized sensors, e. g., sensors based
on the transmission of acoustic energy through gases. For a complete discussion, see Vol. 2, Chap. 18, Sec. 18-2.2.

4- 3 PRESSURE SENSING AND TRANSMITTING

4- 3.1 Sensors

This section deals with elastic sensing elements that respond to a system pressure change and, in so doing, generate a measurable physical quantity, such as position or

•Courtesy Pall Trinity Micro Corp Values from ASMF Boiler and Pressure Vessel Code, Sec. VIII—Pressure Vessels, 1971

tASME Spec. Min Tensile = 45,000 psi $ASME Code, case 1325 (special ruling)

§ASME Code, case 132 3 (special ruling) f ASME Code, case 1321 (special ruling)

mechanical or electrical force. Each sensor is a differential element, and atmospheric pressure is constantly applied in opposition to the system pressure. To sense the absolute pressure, you must apply a second element (e. g., a calibrated spring) in opposition or place that part of the sensor that is normally at ambient (atmospheric) pressure within an evacuated containment.

(a) Materials. In out-of-core pressure sensors, materials coming in contact with the measured fluid must be noncorrosive, must not otherwise deteriorate, and must not contain elements that may become dangerously radioactive by accidental exposure to neutrons. The objective is a device capable of continuous, dependable pressure sensing over an extended period of time. Sensor materials contact­ing the measured fluid should be compatible with the fluid This is the same problem that is involved in choosing thermowell materials. Stainless steels, type 304 or better, are frequently used. Sometimes Inconel is used. Teflon materials for seals and О-rings are avoided as are any components containing cobalt. If the most highly desirable materials are not available at the sensor, diaphragm seals described in Sec. 4-3.4(b) are used.

(b) Basic Types. Elastic metal sensors, available in a variety of forms, consist of slack and rigid diaphragms, multiple or stacked diaphragms, corrugated bellows, and the Bourdon tube in a variety of forms, from single-turn and torsion-bar to helical and spiral multiple-turn designs.

Each manufacturer has his own series of ranges for the various designs based upon the sizing of components and the required performance of a complete linkage system or other device that depends on this initiating element for its successful operation. Table 4.16 gives some typical ranges, and Sec. 4-3 6 gives a sample set of performance specifica­tions.

Strain gages consist of a fine wire or an array of fine wires usually bonded into an assembly for mechanical strength. Under an applied stress the array of fine wires is stretched, this results in an increase in its electrical resistance. If this array is incorporated in a suitable arrangement, the resistance change can be made directly proportional to the imposed pressure. Close temperature control must be maintained by comparing the strain wire with unstressed wire (or compensation), electrical shielding of the sensor wire is also important. In some designs the strain wire may be mounted (bonded) on a Bourdon tube, bellows, or mechanical structure, such as a beam or ring [11] In Fig. 4.17 the strain gage is in the form of a short tube sealed by a diaphragm. Note the variable resistance is applied as a leg of a conventional Wheatstone measuring bridge Because the fractional change in strain-gage resis­tance is very small, electrical amplification and signal conditioning are usually required before use in readout and action modules.

Piezoelectric sensors are similar to strain-gage sensors with a crystal used for stress sensing instead of a wire. The crystal responds to a pressure change (usually expressed by a force in a predetermined direction with respect to the crystal axes) by generating a small electrical potential difference. The latter depends on the magnitude of the imposed stress and on the crystal properties Again, temperature control or compensation, as well as amplifica­tion and signal conditioning of the output, are essential.

Silicon wafer piezoelectric sensors now available are capable of sensing range spans from 0 to 6 psig to 0 to 1500 psig. Output is 10 to 50 mA d-c. Features include a range span adjustment of 4 1 for a given diaphragm and

the capability of elevating the range span (zero suppression) to the maximum pressure range for the unit. Thus a 0- to 1500-psi range device would be expected to calibrate for 1100-to 1500-psig range input for a 10- to 50-mA d-c output.