Tech Sheet #I 308
Selection Guide for Mechanical Thermometers
SCOPE
The purpose of this article is to outline three popular classes of Self Powered mechanical
thermometers. Having knowledge of thermometer selection criteria is important for proper use
and value to the user. Every application presents a unique set of circumstances that need to be
considered. Rangeability, the range span and the resolution in the use range, is an important
factor for choosing the instrument that will have the required accuracy. Metric (°C), English (°F),
or dual units of measure are important for the familiarity of the end users. Over range, and
storage temperature capability are of considerable concern for protection against damage and for
the long life use that these thermometers are expected to have. Measurement accuracy is most
important and can be stated in a variety of ways according to the type and class of thermometer.
Environmental conditions, such as ambient temperature and outdoor exposure, need to be taken
into account when choosing a type and style of thermometer casing. Thermowells are available
in a variety of materials and sizes and process connections; a proper fit of the thermowell to the
thermometer stem is important and is shown for each classification of thermometer. External,
internal or non-resetable adjustments may be required for field adjustment; each type of
thermometer has a field adjustment feature that is unique. Reliance on self-powered
thermometers to perform their function is enhanced with proper selection knowledge.
More detailed information on the thermometers described can be found in "Temperature
Measurements" ASME/ANSI PTC 19.3, Performance Test Codes, Part 3.
SELF POWERED THERMOMETERS
An instrument, which generates movement of a column of liquid or a pointer across a scale,
graduated to define a temperature range. The energy to move this column or pointer is derived
from thermal energy delivered via temperature differential to the thermometer bulb, without any
outside power source. There are three general types of self-powered thermometers:
1 - Liquid in-Glass
2 - Bimetallic Actuated
3 - Filled System
LIQUID-IN GLASS
A Liquid-in-glass Thermometer is one consisting of a thin-walled glass bulb attached to a glass
capillary stem closed at the opposite end, with the bulb and a portion of the stem filled with an
expansive liquid, the remaining part of the stem being filled with an inert gas. Associated with
the stem is a scale in temperature degrees so arranged that when calibrated the reading
corresponding to the end of the liquid column indicates the temperature of the bulb. The
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
operation of a liquid-in-glass thermometer depends upon the coefficient of expansion of the
liquid. As a consequence, an increase in the temperature of the bulb causes liquid to be expelled
from the bulb, resulting in a rise in position of the end of the liquid column.
ETCHED STEM LABORATORY THERMOMETER
As the name suggests, the scale is marked directly on the glass stem by etching. The etched stem
marks are made legible by filling with a pigment material. Etched stem thermometers are
manufactured for total immersion, or partial immersion. An emersion line is etched to the
thermometer tube to indicate the immersion depth on a partial immersion thermometer. Full
immersion thermometers are considered the most accurate because the stem is at the same
temperature as the bulb; it is necessary, however, to have the depth of the “bath” to include the
column of the thermometer. Etched stem thermometers are used for direct immersion into a
liquid. A series of etched stem thermometers called ASTM thermometers are made in strict
accordance to rigid specifications of American Society for Testing Materials Standard E1-03,
available in 110 versions of sizes and ranges, extreme precision with sub divisions as small as
1/20th of a degree are available. When cased in a metal “armor” with a cut out to view the scale
etched stem thermometers can be used to measure temperature of sand, powder, liquid asphalt
and similar materials. High precision thermometers of this type are made with mercury as the
expansion liquid, other liquids are now available that are non-toxic and environmentally
compatible, where this is a concern an MSDS data sheet should be requested from the
manufacturer.
INDUSTRIAL TYPE THERMOMETERS
In this type thermometer the glass bulb and a portion of the glass stem are enclosed in a metal
tube, while the glass scale section is contained in an attached metal case. The temperature scale
is printed on metal plates fastened to the inside of the case; the scale attachment allows a limited
vertical adjustment of the scale for field calibration offset. The case opening is closed by a glass
or plastic viewing window. Industrial Thermometers are available in a variety of case sizes,
stem lengths and case-stem angles. The bulb chamber, or sensitive portion may be immersed
directly in the medium whose temperature is being measured, or more common it may be
inserted in a well (separable socket), which in turn is threaded into a pipeline, tank or other
vessel. The extension of the bulb assembly incorporates a threaded swivel nut connection. It is
thru this connection, the thermometer is mounted to the well (separable socket). Metal to metal
contact between the tapered bore of the thermo-well, and the tapered sensitive portion of the bulb
chamber, provides the thermal path directly to the sensing portion and a rapid response to
temperature changes in the process being measured. Union bushings and air duct flange
connections are also available as an alternative means of mounting.
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
Industrial Thermometers are available in case stem angles of 180º or straight, the 90º back angle,
90º right and left side angles, and various oblique angles. The step angle allows the connection to
various pipe connections with the reading portion in an upright vertical position. An adjustable
angle form can be positioned to assume all of the case stem angles and has become by far the
most popular type. This field position adjustment is a valuable aid when installing the
thermometer to the equipment. The most common case sizes are 7", and 9”. A 6" case
thermometer is available for small pipelines, and other equipment where space is somewhat
limited. Stem construction, wells (separable sockets) and other fittings have their distinct thread
and sizing, different from the 7" and 9” size. 6" case thermometers have similar case angles, but
are not available in the adjustable angle configuration.
Industrial Thermometers historically used Mercury as the actuating medium; the modern
Industrial Glass thermometer uses an organic liquid as the fill media that is responsive to
temperature change.
With Mercury actuated Industrial Thermometers the limits for temperature ranges are as follows:
Adjustable Angle: -40ºF thru 750°F.
All other fixed case forms: -40ºF thru 1100ºF
With the Organic liquid ranges limits are from -300ºF thru 550ºF.
Most applications for Industrial glass thermometers are within –40°F to 550°F. Celsius and dual
scales are readily available.
See Alert Bulletin published by Fluid Controls Institute regarding mercury. A copy is available
on the Fluid Control Institute’s website: www.fluidcontrolsinstitute.org.
Please refer to ASME PTC 19.3 for additional details.
BIMETALLIC ACTUATED THERMOMETER
An analog, dial type, device utilizing a bimetal element which senses temperature and indicates it
by means of a pointer moving over a graduated scale. Fixing one end of a bimetallic element
and attaching a shaft and pointer to the other achieve a rotational motion of the pointer with
temperature. The bimetallic element is a strip of dissimilar metals bonded together, and coiled in
the form of a helix. The rotational motion of the bimetal helix depends upon the difference in
thermal expansion of the two metals. See Illustrations.
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
LABORATORY OR TEST TYPE BIMETALIC THERMOMETER
These thermometers are characterized by the absence of threaded connections, case sizes ranging
from 1" to 2-1/2", and stem diameters from 1/8" to 1/4". Standard stem lengths are 5" or 8".
Temperature ranges for the laboratory thermometer are -40°F thru 550°F.
INDUSTRIAL TYPE BIMETAL THERMOMETER
Standard case /dial sizes are 2”, 3” and 5”diameter, consider installation space limitations and
readability when choosing the size. There are three case connections adjustable angle, rear/back
and lower/bottom; all have stainless steel cases and process connections. The case style can have
an external reset screw on it so it can be recalibrated in the field or tamper proof which has no
adjustment. Threaded connections are generally ¼” NPT for 2” dials and ½” NPT 3” & 5” dials.
See Illustrations.
Such connection is welded or brazed to the case and to the top of the stem. The diameter of the
stem is 1/4". Standard stem lengths are 21/2”, 4”, 6”, 9”, 12”, 15”, 18” & 24”. Longer stem
lengths are available. This thermometer is available dry or liquid filled, for very severe
applications special “high shock” constructions are made.
Thermo-wells are the major accessories for bimetallic thermometers. Where pressure, velocity,
corrosion, or erosion indicates the need for greater protection of the thermometer stem, wells
should be used, where process condition warrants it. Temperature ranges for the bimetallic
Industrial thermometer are -80ºF thru 1000ºF. (Continuous service up to 800ºF with intermittent
service from 800ºF to 1000ºF). Liquid filled bimetallic industrial thermometers are available in
ranges from -20 to 550ºF.
Refer to ASME B40.3 and ASME PTC 19.3 for further details.
FILLED SYSTEM THERMOMETERS
Filled system thermometers are suitable for a wide variety of applications and are therefore
offered in various categories called classes. Filled system thermometers are the only selfpowered temperature instruments that can be produced for direct mounting to equipment or as a
remote mounting, with the sensing element (bulb) mounted up to 100 feet away from the case.
The common elements are they all use a bulb which is completely or partially filled; a
connecting tubing or capillary; an elastic element which may be in the shape of a "C", helical,
spiral, or other form; and a movement that converts elastic element motion to a rotary pointer. If
the elastic element is a direct drive spiral, or helix, a movement is not used. Case sizes for filled
system thermometers are offered from 2" Diameter thru 81/2" Diameter in a variety of materials
and mounting configurations. Sensing bulbs are available in different styles and sizes, bulbs with
a union connection mount directly into a process or thermowell, plain bulbs for use in open tanks
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
or air and special air bulbs that have a quick temperature response and can measure the average
temperature across an air duct. See Illustration.
CLASSIFICATION SUMMARY
CLASS
Filling
1
Organic Liquid
Range Limits
-100/750ºF
-
70/400ºC
2
Vapor
-40/450ºF
-
40/230ºC
3
Gas
-320/1200ºF
-
200/650ºC
4
Gas w/Adsorbent
-320/1200ºF
-
200/650ºC
5
Mercury
-40/1000ºF
-
40/540ºC
6
Non-organic liquid
-40/300ºF
-
40/150ºC
In addition to the basic class designations, there are sub classifications that consider
compensation to correct the effects of ambient temperature changes on the elastic element (case
compensation) or the effect on the entire length of the capillary.
Consideration should be given to the following:
*Accuracy required
*Case size and means of mounting
*Process compatibility
*Installation into process
*Operating Range
Please refer to ASME PTC 19.3 for addition details
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
REFERENCED SPECIFICATIONS AND PERFORMANCE TEST CODES
All ASME (American Society of Mechanical Engineers) standards noted refer to temperature
measuring instruments described in this article.
SELECTION GUIDE: ETCHED STEM THERMOMETERS
(Options)
Length: pocket 6”, 12”,16”,18”,and 24”
Immersion: Partial or full
Range: select between –150F and 750F or specify C
Accuracy: sub divisions of 5°, 2°, 1°,1/2°,1/5°, 1/10°, or 1/20°
Fill Media: Mercury, Spirit, or Environmental safe liquid
Accessories: Open Face Armor, Solid Armor, Certificate of accuracy
(Example): 12”, 3” immersion, –30/0/120F, 1°sub divisions, mercury, with armor
SELECTION GUIDE: INDUSTRIAL TYPE THERMOMETERS
(Options)
Scale/ Case Size: 7”, 9”
Stem Connection: Back, Bottom, or Adjustable
Temperature Range: -40F to 750F
Stem Length: 3.5, 6, 9, 12
Thread Size: 1 ¼ - 18 swivel nut (industry standard) or air duct flange
Fill Media: Mercury, Spirit, Environmental safe liquid
Thermowell (if required): to fit stem length with NPT at hex or on thermowell neck supplied as
standard in Brass, optional Stainless Steel or Steel
(Example): 9”, Adjustable form, 30/240F, standard connection, Environmental safe liquid,
3.5”well ¾”NPT Brass
SELECTION GUIDE BIMETALLIC THERMOMETERS
(Options)
Type: Test or Industrial
Dial/Case Size: Test type 1” or 1 ¾” Industrial Type 2”, 3” or 5” diameter
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
Connection Location: Test type Back connected Industrial Back, Bottom or Adjustable
Connection Size: Industrial Type ½”NPT standard
External Reset Adjustment: With reset or without reset
Stem Length: Test Type 5” or 8” Industrial Type 2.5, 4, 6, 9, 12”
Stem Diameter: Test Type .150” Industrial Type ¼”
Range: Specify from standard ranges between –80 to 100F or equivalent C or dual scale
Accuracy: 1% of scale range
Thermowell (if required) To fit stem length Brass or Stainless steel with ½”NPT, ¾”NPT,
1”NPT at the hex or with lagging extension neck
(Example): 5” Industrial Type, adjustable form, ½”NPT, with reset screw, 4”stem X ¼”
diameter, range 0/250°F, nominal 4” SS well ¾”NPT
SELECTION GUIDE FILLED SYSTEM THERMOMETERS
(Options)
Dial/Case Size: 2”, 2 ½”, 3 ½”, 4 ½”, 6” and 8 ½”
Case Material: Stainless Steel (all sizes), Die cast Aluminum (4 ½ & 6”), Phenolic (4 ½, 6”)
Mounting Style: Remote Mount (Front Flange, U-clamp, or Back Flange), Direct Mount
(Adjustable with thermowell or Union connection)
Range: Depends upon classification available in °F, °C, or dual scale
Accuracy: 1% or 1 scale division
Capillary: Remote Type-1 to 100 ft copper, stainless steel, braded copper, armored stainless steel
Connection: Includes fitting for thermowell or Union, plain no fitting
Sensing Bulb: Depends upon class ¼”, 3/8”, 7/16”, ½”, 9/16” and ¾”
Fill Media: Depends upon classification
Thermowell (if required): Size/style to fit sensing bulb, Brass and stainless steel available
(Example): 4 ½” case, Stainless Steel case, Remote with Back Flange, Range 50/750°F &
0/400ºC dual scale, 1% full scale accuracy, 20ft capillary, union connection for, 6” SS
thermowell with ½”NPT, 3/8”dia X 3” sensing bulb, Enhanced Gas filled (class 4)
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
3/3/06 Page 7 of 7
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Tech Sheet #I 308
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
3/3/06 Page 8 of 8
This sheet is reviewed periodically and may be updated. Visit www.fluidcontrolsinstitute.org for the latest version.
Tech Sheet #I 308
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
3/3/06 Page 9 of 9
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Tech Sheet #I 308
Industrial Type Glass Thermometer
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
3/3/06 Page 10 of 10
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Tech Sheet #I 308
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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Tech Sheet #I 308
INSTRUMENT SECTION MEMBERS and WEBSITES
AMETEK, U. S. GAUGE DIVISION
BURKERT FLUID CONTROL SYSTEMS
MID-WEST INSTRUMENT
MOELLER INSTRUMENT CO., INC.
NOSHOK, INC.
ORANGE RESEARCH
THUEMLING INSTRUMENT GROUP, INC.
TREND INSTRUMENTS INC.
WEISS INSTRUMENTS, INC.
This Tech Sheet was developed by the members of the Fluid Controls Institute (FCI) Instrument Section. FCI is a trade association comprising the
leading manufacturers of fluid control and conditioning equipment. FCI Tech Sheets are information tools and should not be used as substitutes for
instructions from individual manufacturers. Always consult with individual manufacturers for specific instructions regarding their equipment.
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