20.08.2015   Handbook Nuclear Terms

Range Selection

A common practice is to specify the pressure interval in which the sensor is to be used so that the sensor is normally operating at a pressure between % and of the range span. Data for the elastic-metal sensors are given in Table 4.16. The basic ranges shown in the table may be modified.

Table 4.16—Typical Ranges of Elastic-Metal Pressure Sensors

Range span

Maximum over-range pressure, psig

Design

available

Min.

Max.

Thin diaphragm

316 s. s

0 5 in HjO

10 in. H20

50

Thick diaphragm

316 s. s.

25 psig

2000 psig

500 to 4000*

Bellows

Inconel or AM350 steelt

10 in H20

1000 in h2o

6000$

Bourdon tube

Ni-Span C,§ 316 s. s., or beryllium — copper

30 psig

8000 psig

Max. range x 1.25

‘See discussion of force balance in Sec. 4-3 2

tModified type-304 stainless steel adapted for welding and stress relieving. $And 100 psig on gases § Constant-modulus alloy.

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Fig. 4.17—Catenary-diaphragm sensing element and typical bridge circuit. (From D. M. Considine, Process Instruments and Controls Handbook, p. 3-42, McGraw-Hill Book Company, Inc., New York, 1957.)

 

Modifications include ranges having an elevated zero (com­pound range) and suppressed ranges where the minimum pressure is greater than atmospheric zero.

Motion balance refers to the mechanical system where­by the sensor motion is transferred by linkage or other means to a pointer, recording pen, or transducer mecha­nism, such as an armature or core (see Fig. 4.24). Typical motion-balance mechanisms are shown in Figs. 4.18, 4.19, and 4.20. Where restraining members are used, they are limited to providing means for adjusting /его or permitting calibration within the initial range capability of the sensing
element, in no way should the restraining members restrict the sensing action within the intended range span of the device.

Forte balance refers to the system whereby the free motion of the sensor is limited and actively opposed by some mechanical or electrical means. In effect this reduces the actual mechanical motion of the sensor to a very few thousandths of an inch throughout its range. An example of such a device is shown in Fig. 4.21. The mechanism is capable of highly elevated ranges. One design features four interchangeable diaphragms or capsules (Table 4.17).

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BELLOWS ABSOLUTE-PRESSURE GAGE

SLACK-DIAPHRAGM DIFFERENTIAL UNIT

Fig. 4.18—Typical bellows and diaphragm pressure gages. Sensor motion is transmitted to indicator by mechanical linkage (motion-balance mechanisms).

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LINK TO POINTER OR LVDT MECHANISM

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Fig 4.19—Sectional view of a motion-balance pressure gage. (I mm D. M. Considine, Process Instruments and Controls Handbook, p. 3 15, McGraw-Hill Book Company, New York, 1957.)

 

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Fig. 4.20—Types of Bourdon gages, (a) C type, (b) spiral, (c) helical. (From D M. Considine, Process Instruments and Controls Handbook, p 3-16, McGraw-Hill Book Company, New York, 1957.)