Temperature measurements information notes

Basics

Temperature is the measurement of heat (thermal energy) associated with the movement (kinetic energy) of the molecules of a substance. Thermal energy always flows from a warmer body to a cooler body. In this case, temperature is defined as an intrinsic property of matter that quantifies the ability of one body to transfer thermal energy to another body.

Temperature Scales

Several temperature scales have been developed to provide a standard for indicating the temperatures of substances. The most commonly used scales include the Fahrenheit, Celsius, Kelvin, and Rankine temperature scales. The Fahrenheit (·F) and Celsius (·C) scales are based on the freezing point and boiling point of water. The freezing point of a substance is the temperature at which it changes its physical state from a liquid to a solid. The boiling point is the temperature at which a substance changes from a liquid state to a gaseous state. To convert a Fahrenheit reading to its equivalent Celsius reading, the following equation is used.

·C = 5/9 (·F - 32)

In order to convert from Celsius to Fahrenheit, the following equation is used.

·F = 9/5 (·C) + 32

The Kelvin (K) and Rankine (·R) scales are typically used in engineering calculations and scientific research. They are based on a temperature called absolute zero. Absolute zero is a theoretical temperature where there is no thermal energy or molecular activity. Using absolute zero as a reference point, temperature values are assigned to the points at which various physical phenomena occur, such as the freezing and boiling points of water.

International Practical Temperature Scale

For ensuring an accurate and reproducible temperature measurement standard, the International Practical Temperature Scale (IPTS) was developed and adopted by the international standards community. The IPTS assigns the temperature numbers associated with certain reproducible conditions, or fixed points, for a variety of substances. These fixed points are used for calibrating temperature measuring instruments. They include the boiling point, freezing point, and triple point.

Temperature Measuring Devices

Temperature mesuring devices are classified into two major groups, temperature sensors and absolute thermometers.

Sensors are classified according to their construction. Three of the most common types of temperature sensors are thermocouples, resistance temperature devices (RTDs), and filled systems. Typically, temperature indications are based on material properties such as the coefficient of expansion, temperature dependence of electrical resistance, thermoelectric power, and velocity of sound.

Calibrations for temperature sensors are specific to their material of construction. Temperature sensors that rely on material properties never have a linear relationship between the measurable property and temperature. The accuracy of absolute thermometers does not depend on the properties of the materials used in their construction.

The temperature of an object or substance can be calculated directly from measurements taken with an absolute thermometer. Types of absolute thermometers include the gas bulb thermometer, radiation pyrometer, noise thermometer, and acoustic interferometer. The gas bulb thermometer is the most commonly used. Temperature measuring devices can also be categorized according to the manner in which they respond to produce a temperature measurement. In general, the response will be either mechanical or electrical. Mechanical temperature devices respond to temperature by producing mechanical action or movement. Electrical temperature devices respond to temperature by producing or changing an electrical signal.

Factors Affecting Accuracy

There are several factors, of effects, that can cause steady state measurement errors. These effects include:

• Stem losses and thermal shunting
• Radiation
• Frictional heating
• Internal heating
• Heat transfer in surface mounted sensors

Thermometers

Thermometers are classified as mechanical temperature sensing devices because they produce some type of mechanical action or movement in response to temperature changes.

Liquid-in-glass Thermometers

Liquid-in-glass thermometers can be read directly and are very accurate and stable when used properly. The bulb is usually a thin-walled glass chamber that serves as a reservoir for the liquid. The stem is a glass tube that contains the capillary for the liquid. A capillary is a narrow passage within which the liquid can rise and fall. The scale is a series of markings that is used to read the temperature. There are three basic types of liquid-in-glass thermometers: partial immersion, total immersion, and complete immersion. A partial immersion thermometer is inserted to a fixed point that is indicated by the immersion ring. A total immersion thermometer is immersed to the height of the fluid column, not the entire length of the thermometer. A complete, or full, immersion thermometer is totally submerged in the fluid to be measured.

Filled Thermometers

Filled thermometers contain a gas or a volatile liquid and rely on pressure measurements to provide temperature indications. There are several types of filled thermometers, including liquid filled systems, vapor systems, and gas filled systems.

A liquid filled system is completely filled with a liquid. This type of system operates on the principle that liquid expands with an increase in temperature. When the liquid expands, it causes the pressure to increase. This causes the Bourdon tube to uncoil and move the needle on the scale.

A vapor system contains a volatile liquid and vapor. This type of system operates on the principle that pressure in a vessel containing only a liquid and its vapor increases with temperature and is independent of volume. In a vapor system, temperature is measured at the interface between the liquid and the vapor.

Gas filled systems are commonly used for industrial applications. The ideal gas law is an approximation at normally encountered temperatures and pressure. According to this law, the pressure of an ideal gas confined to a constant volume is proportional to absolute temperature. In a typical gas filled system, the gas (usually nitrogen) is not perfect, so there may be a slight change in volume. However, these differences are minor and do not prevent the use of pressure measurement to indicate temperature.

Bimetallic Thermometers

Bimetallic thermometers use the differences in thermal expansion properties of metals to provide temperature measurement capability. Stripes of metals with different thermal expansion coefficients are bonded together. When temperature increases, it causes the assembly to bend. When this happens, the metal strip with the large temperature coefficient of expansion expands more than the other strip. The angular position versus temperature relation is established by calibration so that the device can be used as a thermometer.

Selection

Selection of a particular type of thermometer depends on the specific application and the degree of accuracy required.

Thermocouple basics

Most thermocouples are composed of metal wires. Thermocouples are constructed in a variety of styles to provide direct or differential temperature measurements, ruggedness, durability, or circuit isolation. Thermocouples have a rapid dynamic response to temperature change. However, for protection and ease of calibration and removal, most thermocouples are installed in insulated thermowells which delays dynamic response considerably.

The Seebeck Effect

The principle of the thermocouple was first described by Seebeck in 1821. Seebeck discovered that when wires of two dissimilar metals were jointed together to form a circuit of at least two junctions, a current would flow when the junctions were at different temperatures. This phenomenon, called the Seebeck Effect, is the basis upon which thermocouples are designed.

A thermocouple consists of two wires, each made of a different homogeneous metal or alloy. The wires are joined at one end to form a measuring junction. This measuring junction is exposed to the fluid or medium being measured. The other end of the wires are usually terminated at a measuring instrument where they form a reference junction. When the two junctions are at different temperatures, current will flow through the circuit. The millivoltage resulting from the current flow is measured to determine the temperature of the measuring junction. The reference junction is held at a constant, or reference, temperature. In many cases, the junction is kept at the temperature of melting ice, which allows temperature to be read directly from an indicator without the need for calculating a correction. The 0·C reference temperature can be maintained using an ice bath. In many thermocouple installations, the measuring junction is several hundred feet from the voltage measuring instrument where the reference junction is located. Extension, or lead, wires that have the same thermoelectric characteristics as the thermocouple wires are typically connected to the thermocouple and run to terminals at the measuring instrument.

Typically, reference junction compensation is an integral feature of the instruments that measure the millivolt signals from thermocouples. In these instruments, a temperature sensitive component is thermally bonded to the reference junction connections. The resistance-temperature curve of the bonded component matches the millivoltage-temperature curve of the thermocouple wires.

Types of Thermocouples

About a dozen types of thermocouples are commonly used in industrial applications. Seven of these have been assigned letter designations by the Instrument Society of America (ISA). By convention, a slash mark is used to separate the materials of each thermocouple wire. For example, copper/constantan identifies a thermocouple with one copper wire and one constantan wire. The order in which the wire materials are listed identifies the polarity of the wires. The first wire, on the left of the slash, has a positive polarity when the measuring junction is at a higher temperature than the reference junction.

Thermocouples can be divided into three functional classes: base metal thermocouples, noble metal thermocouples, and refractory metal thermocouples. Base metal thermocouples are useful for measuring temperatures under 1000·C. This class includes thermocouples made of iron/constantan (Type J), copper/constantan (Type T), Chromel/Alumel (Type K), Chromel/constantan (Type E), and alloys of copper, nickel, iron, chromium, manganese, aluminum, and other elements. Noble metal thermocouples are useful to about 2000·C. Refractory metal thermocouples are useful to about 2600·C. This class includes tungsten-rhenium alloy thermocouples as well as thermocouples made of tantalum, molybdenum, and their alloys.

Thermocouple Reference Tables

Many thermocouple circuits contain devices that amplify the millivolt signals from the thermocouple and convert the voltage signals into direct temperature indications. However, there are some thermocouple circuits, that do not have converting circuits for direct temperature readout. In these cases, temperature-EMF reference tables are available for use in converting voltmeter readings to the equivalent temperature values. Since deferent types of thermocouples have different thermoelectric relationships, there is a reference table for each type of thermocouple. The values in these tables are based on the International Practical Temperature Scale and U.S. legal electrical units. Tables are available from a variety of sources, including NBS Monograph 125, ANSI Standard MC 96.1, the SATM Annual Book of Standards, and ISA standards.

Thermocouple details

Thermocouple Design

The smaller the diameter of the wire, the quicker the response time of the thermocouple. Bare wire thermocouples are typically insulated with hard fired ceramic insulators. Sheathed thermocouple wires are insulated with a crushed mineral oxide that is compacted within a protecting sheath. To ensure accuracy when extension wire is used with a thermocouple, the extension wire must have the same temperature-EMF characteristics as the thermocouple. The insulation used on extension wires may be divided into four general classifications: waterproof, moisture resistant, heat resistant, and radiation resistant. To assist in visual identification of wires and to avoid inadvertent cross wiring, many thermocouple wires and extension wires are color coded. Note that the color red is always used to identify the negative lead.

Thermocouple Assembly Components

A thermocouple assembly consists of a thermocouple and one or more associated parts, such as a terminal block, connection head, and protecting tube. Thermocouples are typically installed in a protecting tube or a thermowell.

Protecting tubes and thermowells prevent contamination of the thermocouple by protecting it from the process. They also provide mechanical protection and support. Protecting tubes are thin walled metal or ceramic tubes and are used in low pressure applications. A protecting tube may have external threads designed for direct attachment to a connection head, or a bushing or flange may be provided for attachment to a vessel.

A thermowell or protecting tube will increase the radial heat transfer resistance and increase stem loss. The heat transfer in the gap between the thermocouple and the wall of the well or tube is usually a major factor in increasing radial heat transfer resistance. For this reason, a filler material is often used to improve heat transfer. However, it should be noted a filler is subject to aging and/or redistribution. Therefore, its effect on response time of the thermocouple is uncertain. Filler materials used to improve heat transfer should have certain characteristics, such as:

• High thermal conductivity.
• Chemical compatibility with the thermocouple, thermowell, and process medium in case of a leak.
• Long term stability of chemical and physical properties at operating temperatures ¾ Some fillers can improve heat transfer when new, but reduce heat transfer after aging.
• Adequate fluidity or plasticity ¾ A filler must not compact at the tip of the well because it will prevent complete insertion of the thermocouple. Plasticity is required to prevent the filler from running out if the thermowell is installed with its tip higher than the head.

To improve the response time, a thermocouple is usually bottomed in the protecting tube or thermowell. Bottoming ensures that the measuring junction is pressed tightly against the end of the tube or well. This may ground the thermocouple, causing difficulties with some types of installations.

Installation Considerations

Depth of immersion is critical to the accuracy of the measurement. Immersion length should not be confused with insertion length, which is the length from the free end of the well or tube to, but not including, the external threads or other means of attachment to a vessel or connecting head. A minimum immersion length of 8 to 10 times the tube or well diameter is recommended in order to minimize conduction errors. However, when a thermocouple is used in a high velocity liquid, it may not need to be immersed as deeply.

The position of the thermocouple is also an important installation consideration. Thermowells are usually installed perpendicular to an angle, or in an elbow. In gas applications, the thermocouple should be located where the mass velocity is as high as possible. In high temperature applications, such as ovens or furnaces, vertical installation of the thermocouple through the top of the vessel will prevent the thermocouple from bending or sagging. It is recommended that the protecting tube or thermowell extend beyond the outer surface of the vessel furnace or processing equipment so that the temperature of the connection head is closed to the ambient atmospheric temperature.

Thermocouple extension wire should be installed in such a way that it is protected from excessive heat, moisture, and mechanical damage. Extension wire should be run from the connection head to the measuring instrument terminal in one continuous length. Thermocouples may be connected in parallel to provide an average temperature measurement. Thermocouples can also be installed in series to form a thermopile. In this case, the thermocouples are connected so that alternate junctions are at a known temperature. In a thermopile, the voltages of all the thermocouples are added so the output is n times the number of measuring junctions, providing a way to detect small changes in temperature.

Checking Thermocouple Accuracy

The output of a new, unused thermocouple will be determined solely by the temperature of its measuring junction. However, after the thermocouple has been used, the thermocouple material will no longer be homogeneous. In this situation, the output of the used thermocouple will not be determined solely by the temperature of its measuring junction. Thus, there are different procedures for checking the accuracy of unused and used thermocouples.

Various methods for testing new thermocouple materials are described in Publication 300, Volume II, "Precision Measurement and Calibration ¾ Temperature", published by the National Bureau of Standards, and in E2220-72, "Calibration of Thermocouples by Comparison Techniques," published by the American Society for Testing and Materials. One common method of testing the accuracy of a new thermocouple involves placing the reference junction in an ice bath to establish the reference temperature and placing the measuring junction in a variable calibrating temperature bath.

Used, or installed, thermocouples which are exposed to high temperatures in various atmospheres may change characteristics. This thermoelectric nonumiformity results from contamination or deterioration of the thermocouple wires and/or junction. The measuring junction will deteriorate or become contaminated. It is good practice to check thermocouples at regular intervals.

The purpose of checking an installed thermocouple is to determine the temperature error in actual service, not the temperature-EMF characteristics of the thermocouple. Therefore, a used thermocouple should always be checked in its normal installed location. If a thermocouple were removed from its installed location and placed in a calibrating furnace for checking, it is highly improbable the temperate gradiant in the furnace would match the temperature gradiant of the normal installation.

The accuracy of an installed thermocouple is checked by comparing its readings with the readings of a new, or checking, thermocouple of the same type. When the diameter of the protecting tube is large enough, the checking thermocouple may be inserted beside the service thermocouple. When the protecting tube is not large enough to accommodate another thermocouple, the service thermocouple can be removed and the checking thermocouple temporarily inserted in its place.

If the installed thermocouple is used to measure a wide range of temperatures, it should be checked at more than one temperature within the range of its use. while testing a thermocouple of one temperature provides some information, it is not safe to assume that the changes in the EMF of a thermocouple are proportional to the temperature or to the EMF. Noble metal thermocouples used as checking thermocouples may normally be relied upon for a considerable period of use.

Resistance Temperature Detectors

Resistance thermometers, respond to temperature by changing their electrical resistance. Two common types of resistance thermometers are resistance temperature detectors (RTDs), which have metallic sensing elements, and Thermistors which have semiconductor elements.

RTD Principle of Operation

An RTD consists of a sensing element fabricated of metal wire or metal fiber which responds to temperature change by changing its resistance. The sensor is connected to a readout instrumentation that monitors the resistance, typically through the use of a bridge circuit, and then converts resistance to a temperature value.

Sensor Designs

The sensing element of an RTD usually consists of a wire cut to a length that provides a predetermined resistance at 0·C. The wire may be coiled within or wound around an insulating material.

RTD Readout Instrumentation

Temperature measurement with an RTD is actually a measurement of the sensor's resistance, using the sensor calibration to convert the measurement into temperature. This is achieved by connecting the sensor to a transducer that has a bridge circuit, typically a wheatstone bridge or Mueller bridge.

RTD Accuracy

Accuracy problems can occur when RTDs from different manufactures are used in the same system, or when an RTD from one manufacturer is replaced with an RTD from another manufacturer. Self heating can also affect accuracy.

RTDs for Specialized Applications

Designs include averaging RTDs, annular element RTDs, and combination RTD-thermocouples. An averaging RTD has long resistance elements. In annular element RTDs, the sensors are made with annular elements that provide a tight fit against the inner wall of a thermowell. Combination RTD-thermocouple designs are available with both an RTD and a thermocouple enclosed in the same sheath.

Thermistors

Themistors are made of solid semiconductor materials having a high coefficient of resistivity. The relationship between resistance and temperature, and linear current-voltage characteristics are of primary importance. Typical thermistors are suitable for temperature measurements in the range of -100·C to 300·C. However, some thermistors can be as high as 600°C.

Characteristics of Thermistors

Thermistors are semiconductors formed from complex metal oxides, such as oxides of cobalt, magnesium, manganese, or nickel. They are available with positive temperature coefficients of resistance (PTC thermistors) and with negative temperature coefficients of resistance (NTC) thermistors). NTC thermistors are used almost exclusively for temperature measurement. Despite the nonlinear nature of thermistors, readout instrument circuits have also been developed to provide a nearly linear output voltage versus temperature or resistance versus temperature.

Thermistor Designs

The bead thermistor is made of a small bead of thermistor material to which a pair of leads is attached. The bead is usually enclosed in glass. A disc thermistor consists of a disc of thermistor material and a pair of leads. The leads may be attached radially or axially to the top and or bottom of the disc. Some disc thermistors have no leads, and are fabricated with metal plated faces that can be clipped or soldered in the circuit. A washer thermistor resembles a disc thermistor but has a center hole and metal plated faces for contact. The center hole enables the thermistor to be held by a mounting bolt or stacked with other washer thermistors and electrical components. A rod thermistor is basically a stick of thermistor material to which a pair of leads are attached. The leads may be attached axially or radially to each end of the rod.

Thermistor Accuracy

The most common problem related to thermistor accuracy is interchangeability. Thermistor accuracy can also be affected by several mechanical or chemical actions that change its electrical resistance.

Radiation Pyrometers

Noncontacting temperature measurement can be achieved through the use of radiation, or optical, pyrometers. The high temperature limits of radiation pyrometers exceed the limits of most other temperature sensors. Radiation pyrometers are capable of measuring temperatures to approximately 4000·C without touching the object being measured.

Principles of Radiation Pyrometry

Temperature measurement with radiation pyrometers is based on the factthat all objects emit radiant energy. Radiant energy is emitted in the form of electromagnetic waves, considered to be a stream of photons traveling at the speed of light. The wavelengths of radiant energy emitted by a hot object range from the visible light portion (0.35 to 0.75 microns) to the infrared portion (0.75 to 20 microns) of the electromagnetic spectrum.

In the visible light portion of the spectrum, radiant energy appears as colors. The expression "red hot" is derived from the fact that a sufficiently hot object will emit visible radiation. Common examples include a piece of red hot steel and a tungsten filament lamp. Radiation pyrometers measure the temperature of an object by measuring the intensity of the radiation it emits. The intensity and wavelength of the radiation emitted by an object depends on the emittance and the temperature of the object. Emittance is a measure of an object's ability to send out radiant energy. It is inversely related to reflection of the object's surface. Since emittance will differ from one object to another, a standard, called a blackbody, is used as a reference for calibrating radiation pyrometers and serves as the basis for the laws that define the relationship of the intensity of radiation and wavelength with temperature. A blackbody is an object having a surface that does not reflect or pass radiation. It is considered a perfect emitter because it absorbs all heat to which it is exposed and emits that heat as radiant energy.

• The intensity of radiant energy increase as temperature increases.
• The peak of radiation moves to lower wavelengths as temperature increases. In the visible light portion of the spectrum, this effect can be seen by the change in color of heated metals. They change from red to yellow to white to blue-white as temperature increases.

Types of Pyrometers

A radiation pyrometer consists of optical components that collect the radiant energy emitted by the target object, a radiation detector that converts the radiant energy into an electrical signal, and an indicator that provides a readout of the measurement. The optical pyrometer, also known as the brightness pyrometer, requires manual adjustment based on what is viewed through a sighting window. Because it relies on what can be seen by the human eye, an optical pyrometer is designed to respond to very narrow band of wavelengths that fall within the visible light portion of the electromagnetic spectrum.

Another type of pyrometer that is commonly used for industrial temperature measurement is the total radiation pyrometer. A total radiation pyrometer responds to wavelengths in both the visible and infrared portions of the spectrum. Ideally, it would measure all wavelengths within this range. However, the glass window filters out some wavelengths. Any gases or vapors between the target and pyrometer will also attenuate certain wavelengths. Total radiation pyrometers are based on the Stefan-Boltzmann law which states that total radiation is proportional to the fourth power of temperature. These pyrometers are calibrated using a blackbody and therefore measure the temperature based on the total radiation a blackbody would emit.

Pyrometer Accuracy

One technique for ensuring that emitted radiation rather than reflected radiation is being observed is to drill a hole in the target object and aim the pyrometer into the hole. It is recommended that the depth of the hole be about five times its diameter. Measurement accuracy can also be affected by the presence of gases or vapors between the target and pyrometer. Gases and vapors can filter out some radiation wavelengths. One technique for resolving this problem is to use fans to disperse any gases or fumes. A film of dirt on the viewing window or lens will also affect measurement accuracy. In some applications, it may be necessary to use a purge to prevent soot or other particles form being deposited on the viewing window or lens.