Page 271 - Instrumentation Reference Book 3E

Measurement techniques: direct effects 255

mal expansion bonded together to form, in the also makes it possible to keep the thermal cap-
simplest case, a cantilever. Typical metals are acity of the element and its stem at a low value, so
brass and Invar. Figure 14.14 illustrates this prin- the instrument will respond rapidly to small tem-
ciple. As The temperature rises the brass side of perature changes.
the strip expands more than the Invar side, result- The helices in the winding are so
ing in the strip curling, in this case upwards. compensated that any tendency towards lateral
In this “straight” form a bimetal strip can form displacement of the spindle in one helix is
part of a micro-switch mechanism thus forming a counteracted by an opposite tendency on the
temperature-sensitive switch or thermostat. path of one or more of the other helices.
To construct a thermometer the bimetal ele- Thus, the spindle of the instrument is fully
ment is coiled into spiral or helix. Figure 14.15 floating, retaining its position at the center of
shows a typical coiled thermometer element. the scale without the help of bearings. The
A long bimetal strip, consisting of an Invar instrument is, therefore, not injured by mechan-
strip welded to a higher expansion nickel-molyb- ical shocks which would damage jeweled
denum alloy wound around without a break into bearings.
several compensated helices, arranged coaxially This particular design also results in the angu-
one within the other, forms the temperature-sen- lar rotation of the spindle being proportional to
sitive element of an instrument which may be the change in temperature for a considerable tem-
designed to measure temperature. This method perature range. The instrument has a linear tem-
of winding the strip enables a length, sufficient perature scale, and can be made to register
to produce an appreciable movement of the free temperatures up to 300°C to within fl percent
end, to be concentrated within a small space. It of the scale range.
HEAWDIECASTBEZEL -..-

DIALS USUALLY HAVE BLACK FIGURES, EX..
ON A WHITE GROUND AND CAN BE SUPPLIED
IN ALL RANGES UPTO 560’
CELSIUS IF DESIRED

STEEL OR
SENSITIVE NICKEL STEEL MULTIPLE
DIRECTLY TO INDICATIFJG P
SYSTEM
CAP POINTOFTRANSMISSION ;
THE POINTER BEING RIGIDLY
FIXED TO SHAFT
WHEN NECESSARY CENTER
, STEADY FITTED TO KEEP HEAWPLATEGLASS ’ ,:
SHAFT CENTRAL
SHAFT RIGIDLY HEAD SEALING GASKET -‘.
FIXEDTO SYSTEM BOW CAN BE SUPPLIEPGAS, OIL, AND WATERTIGHT
Figure 14.13 Dial thermometer.
lnvar

Figure 14.14 Action of bimetal strip.