Page 144 - Instrumentation Reference Book 3E

128 Measurement of pressure

i- -Po'n1er

Figure 9.9 Precision absolute pressure gauge. Courtesy Figure 9.10 Helical Bourdon tube. Courtesy lnvensys
US. Filter Corp. Inc.

basic design and materials of construction, but
also because of the conditions under which they
are used. The principal sources of error are hys-
teresis in the Bourdon tube, changes in its sensi-
tivity due to changes of temperature, frictional
effects, and backlash in the pointer mechanism.
A typical accuracy is f2 percent of span.
Much higher precision can be achieved by atten-
tion to detail, and one example is illustrated in
Figure 9.9, which shows a gauge for measuring
absolute pressure. It includes two Bourdon tubes,
one being completely evacuated and sealed to pro-
vide the reference while the unknown pressure is
applied to the other Bourdon tube. The free ends
of the Bourdon tubes are connected by a ratio
linkage which through a push rod transmits the
difference in the movement of the free ends to a
rack assembly which in turn rotates the pinion and
pointer. Jewel bearings are used to minimize fric- Figure 9.11 Spiral Bourdon tube. Courtesy, lnvensys Inc.
tion and backlash is eliminated by maintaining a
uniform tension for a11 positions of the rack and
pinion through the use of a nylon thread to con- ness and is dependent on the cross-sectional
nect a spring on the rack with a grooved pulley on shape. It also varies directly with the angle sub-
the pinion shaft. tended by the arc through which the tube is
The Bourdon tubes are made of Ni-Span C, formed. By using a helix or spiral to increase the
which has a very low thermoelastic coefficient effective angular length of the tube, the move-
(change in modulus of elasticity with tempera- ment of the free end is similarly increased and
ture) and good resistance to corrosion. As both the need for further magnification is reduced.
Bourdon tubes are subjected to the same atmos- Examples of these constructions arc shown in
pheric pressure, the instrument maintains its Figures 9.10 and 9.11. They avoid the necessity
accuracy for baromctric pressure changes of for the toothed quadrant with the consequent
+130 mmHg. The dial diameter is 216 mm and reduction of backlash and frictional errors. In
the full range of the instrument is covered by general, the spiral configuration is used for low
two revolutions of the pointer, giving an effective pressures and the helical form for high pressures.
scale length of 1.36m. The sensitivity is 0.0125
percent and the accuracy 0.1 percent of full scale.
The ambient temperature effect is less than 0.01 9.2.3.3 Diuplzrugni prcssure elements
percent of full scale per Kelvin. There are two basic categories of diaphragm
elements, namely stiff metallic diaphragms and
9.2.3.2 Spirul und helirul Bourdon tubas slack diaphragms associated with drive plates.
The simplest form of diaphragm gauge is the
The amount of the movement of the free end of a Schaffer gauge shown in Figure 9.12. It consists of
Bourdon tube varies inversely as the wall thick- a heat-treated stainless-steel corrugated diaphragm