This document provides an introduction to process instrumentation and discusses different types of filled system thermometers. It describes the construction, operating principle, and classification of filled system thermometers. The four main classes described are liquid filled, vapour filled, gas filled, and mercury filled thermometers. The document also discusses sources of error in filled system thermometers and compares their advantages and disadvantages to other temperature measurement instruments.

1. PROCESS INSTRUMENTATION (CH255)
INTRODUCTION
Dr Sourav Poddar
Department of Chemical Engineering
National Institute of Technology
Warangal, TS

33. GF = gauage factor = (∆R/R)/ɛ
ɛ = strain = ∆l/l
Eb =battery voltage

34. Hint:

73. 𝑆𝑡𝑎𝑡𝑖𝑐 𝑆𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 =
𝐶ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝑂𝑢𝑡𝑝𝑢𝑡 𝑆𝑖𝑔𝑛𝑎𝑙
𝐶ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝐼𝑛𝑝𝑢𝑡 𝑆𝑖𝑔𝑛𝑎𝑙
=
∆𝑞𝑜
∆𝑞𝑖

99. Suppose a reading of 27.5 mV results from the sensor used in the previous problem. Find the temperature that
could provide this reading.

112. 1.
2.

114. 1.
2.

116. Basic concepts of response of order instruments

174. Bio metallic strip

180. FILLED
SYSTEM
THERMOMETER

181. INTRODUCTION
Filled system thermometer are temperature measuring device
which works on pressure or volume change of a gas or changes in
vapour pressure of a liquid.

183. CONSTRUCTION
• The bulb contains a fluid that is sensitive to temperature
changes.
• The capillary tube connects the bulb to the element that is
sensitive to pressure or volume changes.
• The pressure-sensitive or volume-sensitive element may be
Bourdon tube, a helix, a diaphragm or bellows.
• The motion of the temperature or volume-sensitive element
couples mechanically to the indicating, recording, or controlling
device.

184. OPERATING PRINCIPLE
• Almost all liquids, gases or vapors expands when heated &
contracts when cooled. This phenomenon is utilized to measure
temperature by filled system thermometer.
• Bulb contains liquid, gases or vapour whose pressure or volume
changes in response to a change in temperature.
• This variation in pressure is transmitted through capillary tube to
the bourdon tube.
• The movement of bourdon tube is used to drive a pointer by
linkages for indicating temperature.

185. CLASSIFICATION
• As per the classification made by Scientific Apparatus Makers’ Association (SAMA) of
the USA, filled system thermometers are divided into four classes as shown below

186. Class - I - LIQUID FILLED
• These types of thermometers are completely filled with a liquid other than Mercury &
operates on the principle of volumetric expansion of liquid with an increase in
temperature.
• The filling fluid are usually inert hydrocarbon liquids such as Xylene, Toluene, Alcohol
etc. which has coefficient of expansion six times of Mercury so that system has high
sensitivity.
• Range: -87 to 371oC
• Accuracy : + 0.5 of full range
• Advantages: (i) Wide temperature span, (ii) Smaller bulb size and (iii) Lower
cost
• Disadvantages: (i) Compensation of errors is difficult and (ii) It has a shortest
capillary.

187. Class - II - VAPOUR FILLED
• Range: -184 oC below ambient temperature & Above ambient temperature to 343 oC
• Accuracy: 1% of differential range
• Speed of response : 4 - 5 seconds except when passing through ambient
• Advantages: (i) Ambient temperature effect is negligible.
(ii) Good response against temperature.
(iii) Long capillary length available.
• Disadvantages:
(i) Nonlinear scale
(ii) Difficult to measure ambient temperature.

188. Class - III - GAS FILLED
• These thermometers operates on the principle of ideal gas law which states that volume of a gas
increases with increase in temperature if pressure is maintained constant or vice versa i.e. pv = RT
• These thermometers are filled with gases such as N2, H2, He which makes system more sensitive to
temperature changes.
• As N2 is chemically inert & inexpensive and also has good coefficient of thermal expansion, it is used
mostly.
• Range: - 268 oC to 760 oC
• Accuracy: ±0.5% upto 320oC & ±1% above this range
• Speed of response : 4 - 7 seconds
• Advantages: (i) Greatest working range of temperature., (ii) No head or elevation effect.
• Disadvantages: (i) Larger bulb size, (ii) Generates less deflection force for controlling device.

189. Class - IV - MERCURY FILLED
• Mercury is a liquid & in this respect a mercury filled thermometer is similar to liquid filled
thermometer.
• These two are separated due to the unique characteristic of Mercury & its importance as a
temperature measuring medium.
• Range: -40 to 649oC
• Advantages over liquid filled thermometer:
(i) Rapid response
(ii) High accuracy
(iii) Large power for operating elements.
(iv) At high temperature, pressure is also high which reduces head or elevation effect error.
(v) As mercury is incompressible, ambient temperature effect error is also very less.
(vi) Least difficult to compensate.

190. SOURCES OF ERRORS
• The common sources of errors in the filled system thermometers are :
1) Ambient temperature effect : The capillary tube & bourdon tube are
exposed to surroundings & subjected to a variable temperature
widely different from that of the bulb, which results as an error in
temperature measurement.
2) Head or elevation effect : If the long capillary tube is used &
thermometer bulb is placed at a different height with respect to the
bourdon tube, then due to elevation difference between bulb &
bourdon tube deflecting end, pressure head is created which results as
an error in temperature measurement.

191. 3) Barometric effect : The tip travel of a bourdon tube is due to
pressure difference between atmosphere & filled system pressure. So
if any variation in atmospheric pressure from designed pressure, the
bourdon tube tip travels & causes an error in temperature
measurement.
4) Immersion effect : If the bulb is not properly immersed or head of
bulb is not properly insulated, then heat from the bulb may be lost
due to conduction from bulb & hence temperature at the bulb may be
reduced, so that, it indicates lower temperature than the actual.
5) Radiation effect : This error occurs due to temperature difference
between the bulb & other solid bodies around it.

192. ADVANTAGES
Advantages of filled system thermometer :
1. Simple & low cost.
2. Quite rugged construction & less chances of damage in handling.
3. Self operated, no power requires & generate sufficient power for controller
mechanism.
4. Good response, accuracy & sensitivity
5. Vapor actuated thermometer are most widely used as they are less costly &
simple. It has good speed of response.
6. It can be used for remote indication.

193. DISADVANTAGES
Disadvantages of filled system thermometer :
1. Lower accuracy, sensitivity & temperature span compared to electrical temperature
instruments.
2. Compensation is required in case of variation in ambient temperature.
3. The transient response is primary dependent on the bulb size & thermal properties of the
filled fluid.
4. To get good accuracy, requires large size of bulb results in poor response.
5. In case of damage, entire system has to be replaced.
6. As filled system thermometers don’t produce electrical signals, it is difficult to implement
them in process controls that rely on electrical and computerized control.

201. Comparison of RTD and Thermistors
Characteristic RTD Thermistors
Change in resistance with
temperature
Positive, Resistance increase
with increase in temperature
Negative, Resistance
decreases with increase in
temperature
Temperature resistance
relationship
Approximately linear Non-linear
Operating range -160°C to 600°C -100°C to 300°C
Composition Platinum, Copper, Nickel Metallic oxides like
Manganese,
Nickel, Copper, Iron,
Titanium,
Uranium
Sensitivity Lower Higher
Cost High Low

229. THANK YOU