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 FIGURE 6-1.
Internal Mechanism of a Bourdon Type Pressure Gauge
e n d of the tube is closed and the other end is expanded into a bulb filled
w i t h a suitable liquid, such as mercury or alcohol. When the bulb is heated,
t h e liquid expands and rises in the tube. The reverse is true when the bulb
i s cooled. I n each case, the displacement of the liquid through the tube is
p r o p o r t i o n a l to the temperature. Divisions marked in the tube, or on an
a d j a c e n t surface, i n d i c a t e the degrees of temperature. I n d u s t r i a l
thermometers are made in armored and unarmored styles, and in straight, angle,
a n d oblique stem styles. F i g u r e 6 - 2 i l l u s t r a t e s a s t r a i g h t s t e m i n d u s t r i a l
thermometer.
2.3.2 Analog Thermometers. Analog thermometers use a thermocouple or
platinum resistance temperature detector (RTD) with a cable type probe and a
m e t e r which has a pointer and a graduated dial for reading the temperature in
e i t h e r Fahrenheit or Celsius. F i e l d analog thermometers have self-contained
power sources and are hand-carried. F i g u r e 6-3 illustrates an analog
thermometer.
2 . 3 . 3 Digital Thermometers. Digital thermometers use a thermocouple or
platinum RTD with a cable type probe and a digital LED or LCD readout in
e i t h e r Fahrenheit or Celsius. D i g i t a l thermometers have self-contained power
s o u r c e s and have the advantages of holding the last reading for viewing later
and changing the precision desired between whole degree and one-tenth of a
degree readings. S o m e digital pyrometers utilize infrared sensing for
temperature measurement. F i g u r e 6 - 4 i l l u s t r a t e s a t y p i c a l d i g i t a l t h e r m o m e t e r .
2 . 4 Flowmeters. F l o w m e t e r s are instruments that measure the rate of flow of
a f l u i d . F l o w m e t e r s are categorized into the following general
c l a s s i f i c a t i o n s : d i f f e r e n t i a l p r e s s u r e , positive displacement, velocity, and
6-2
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