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PI Lecture 15

Industrial mercury thermometers have an accuracy of about ±1% of their span. They work best when the bulb is immersed to minimize heat loss and the surrounding temperature is near room temperature. Response time ranges from 0.01 to several minutes depending on factors like size and heat transfer medium. They are used to measure temperatures of liquids, steam, and gases. Bimetallic thermometers use two metals with different expansion coefficients welded together. When heated, the strip bends in proportion to temperature change. Invar alloy is commonly used as the low expansion metal paired with brass or nickel alloys. Bending direction depends on whether the temperature is above or below the bonding temperature.

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21 views6 pages

PI Lecture 15

Industrial mercury thermometers have an accuracy of about ±1% of their span. They work best when the bulb is immersed to minimize heat loss and the surrounding temperature is near room temperature. Response time ranges from 0.01 to several minutes depending on factors like size and heat transfer medium. They are used to measure temperatures of liquids, steam, and gases. Bimetallic thermometers use two metals with different expansion coefficients welded together. When heated, the strip bends in proportion to temperature change. Invar alloy is commonly used as the low expansion metal paired with brass or nickel alloys. Bending direction depends on whether the temperature is above or below the bonding temperature.

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pk2003158
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CH2007D

PROCESS INSTRUMENTATION
Lecture – 15
Temperature Measurement – 2
Characteristics of Industrial Mercury
Thermometers
• Accuracy
– The accuracy of the industrial thermometer, when properly installed
and used, is about ±1 percent of span.
– The thermometer bulb should be installed in such a way that the
speed of flow of the surrounding medium past the bulb is sufficient to
provide rapid heat transfer.
– It should be immersed to a sufficient length that the heat loss along
the thermal well to the surroundings is a minimum; otherwise, what is
known as "immersion error" will result.
– The temperature of the scale and surroundings should be relatively
near ordinary room temperature, so that errors due to expansion or
contraction of thermometer bore and calibrated scale are small.
• Range
– –38°F to 950°F
Characteristics of Industrial Mercury
Thermometers
• Response
– The speed of response of the industrial mercury thermometer depends on:
• the characteristics of the fluid medium in which it is inserted
• the size of the thermometer
• the size of the thermal well.
– The space between the bulb and well may be filled with a conducting liquid
such as mercury or oil, in order to increase the rate of heat transfer.
– The time constant will range from 0.01 min to several minutes.
• Applications
– open tanks containing liquids
– cooking kettles
– certain molten-metal baths
– steam lines
– pipe lines for fluid flow
– air ducts
Bimetallic Thermometer
• Commonly used wherever the industrial mercury thermometer is employed.
• A bimetal is composed of two strips of metal welded together, each strip made
from a metal having a different coefficient of thermal expansion.
• For a bimetal in the form of a straight cantilever beam, temperature changes
cause the free end to deflect, and this deflection can be related quantitatively to
the temperature change.
• The deflection of the free tip is directly proportional to temperature.
• The deflection with temperature is nearly linear, depending mainly on the
coefficients of linear thermal expansion.
• Invar, an iron-nickel alloy containing about 36 percent nickel, is universally
employed as the low-expansion metal.
• The temperature coefficient of expansion is about
1
that of ordinary metals.
20
• As the high-expansion metal, brass is used at lower
temperatures and nickel alloys at higher temperat-
ures.
Temperature Measurement Using Bimetallic
Thermometer
• When the strip is subjected to a temperature higher than the
bonding temperature, it will bend in one direction; when it is
subjected to a temperature lower than the bonding temperature, it
will bend in the other direction.
Temperature Measurement Using Bimetallic
Thermometer
• When the thicknesses of low-expansion material and high-expansion
material are same in the strip, 𝑚 = 1.
• When Young’s moduli of the materials are approximately equal, 𝑛 =
1.

2𝑡 1
𝑇 = 𝑇0 +
3(𝛼2 − 𝛼1 ) 𝑟

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