Selection Guide for Mechanical Thermometers

Transcription

1 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: LIQUID-IN GLASS 1 - Liquid in-glass 2 - Bimetallic Actuated 3 - Filled System 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 3/3/06 Page 1 of 1 This sheet is reviewed periodically and may be updated. Visit for the latest version.

2 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 1/20 th 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. 3/3/06 Page 2 of 2 This sheet is reviewed periodically and may be updated. Visit for the latest version.

3 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: 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. 3/3/06 Page 3 of 3 This sheet is reviewed periodically and may be updated. Visit for the latest version.

4 LABORATORY OR TEST TYPE BIMETALIC THERMOMETER Tech Sheet #I 308 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 3/3/06 Page 4 of 4 This sheet is reviewed periodically and may be updated. Visit for the latest version.

5 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 Range Limits 1 Organic Liquid -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 3/3/06 Page 5 of 5 This sheet is reviewed periodically and may be updated. Visit for the latest version.

6 REFERENCED SPECIFICATIONS AND PERFORMANCE TEST CODES Tech Sheet #I 308 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 3/3/06 Page 6 of 6 This sheet is reviewed periodically and may be updated. Visit for the latest version.

7 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) 3/3/06 Page 7 of 7 This sheet is reviewed periodically and may be updated. Visit for the latest version.

8 3/3/06 Page 8 of 8 This sheet is reviewed periodically and may be updated. Visit for the latest version.

10 Industrial Type Glass Thermometer 3/3/06 Page 10 of 10 This sheet is reviewed periodically and may be updated. Visit for the latest version.

12 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. 3/3/06 Page 12 of 12 This sheet is reviewed periodically and may be updated. Visit for the latest version.