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Guide to Measurement and Instrumentation -- Temperature Measurement



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1. Introduction

2. Thermoelectric Effect Sensors (Thermocouples)

3. Varying Resistance Devices

4. Semiconductor Devices

5. Radiation Thermometers

  • Broad-band (unchopped) radiation pyrometers
  • Chopped broad-band radiation pyrometers
  • Narrow-band radiation pyrometers
  • Two-color pyrometer (ratio pyrometer)
  • Selected waveband pyrometer

6. Thermography (Thermal Imaging)

7. Thermal Expansion Methods

8. Quartz Thermometers

9. Fiber-Optic Temperature Sensors

10. Color Indicators

11. Change of State of Materials

12. Intelligent Temperature-Measuring Instruments

13. Choice Between Temperature Transducers

14. Calibration of Temperature Transducers

15. Summary

16. Quiz


1. Introduction

We’re probably well aware that temperature measurement is very important in all spheres of life. In engineering applications, it’s particularly important in process industries, where it’s the most commonly measured process variable. It’s therefore appropriate for us to devote this first section on measurement of individual physical variables to the subject of temperature measurement.

Unfortunately, temperature measurement poses some interesting theoretical difficulties because of its rather abstract nature. These difficulties become especially apparent when we come to consider the calibration of temperature-measuring instruments, particularly when we look for a primary reference standard at the top of the calibration chain. Foremost among these difficulties is the fact that any given temperature cannot be related to a fundamental standard of temperature in the same way that the measurement of other quantities can be related to the primary standards of mass, length, and time. If two bodies of lengths, l1 and l2, are connected together end to end, the result is a body of length l1 + l2. A similar relationship exists between separate masses and separate times. However, if two bodies at the same temperature are connected together, the joined body has the same temperature as each of the original bodies.

This is a root cause of the fundamental difficulties that exist in establishing an absolute standard for temperature in the form of a relationship between it and other measurable quantities for which a primary standard unit exists. In the absence of such a relationship, it’s necessary to establish fixed, reproducible reference points for temperature in the form of freezing and triple points of substances where the transition among solid, liquid, and gaseous states is sharply defined. The International Practical Temperature Scale (IPTS)* uses this philosophy and defines a number of fixed points for reference temperatures. Three examples are:

• The triple point of hydrogen: _259.35_ C

• The freezing point of zinc: 419.53_ C

• The freezing point of gold: 1064.18_ C

A full list of fixed points defined in the IPTS can be found in Section -14.

If we start writing down the physical principles affected by temperature, we will get a relatively long list. Many of these physical principles form the basis for temperature-measuring instruments. It’s therefore reasonable for us to study temperature measurement by dividing the instruments used to measure temperature into separate classes according to the physical principle on which they operate. This gives us 10 classes of instrument based on the following principles:

• Thermoelectric effect

• Resistance change

• Sensitivity of semiconductor device

• Radiative heat emission

• Thermography

• Thermal expansion

• Resonant frequency change

• Sensitivity of fiber-optic devices

• Color change

• Change of state of material

We consider each of these in the following sections.

2. Thermoelectric Effect Sensors (Thermocouples)

Thermoelectric effect sensors rely on the physical principle that, when any two different metals are connected together, an EMF, which is a function of the temperature, is generated at the junction between the metals. The general form of this relationship is

e = a1T + a2T2 + a3T3 +___ + anTn

…where e is the EMF generated and T is the absolute temperature.

This is clearly nonlinear, which is inconvenient for measurement applications. Fortunately, for certain pairs of materials, terms involving squared and higher powers of T (a2T2 , a3T3 , etc.) are approximately zero, and the EMF-temperature relationship is approximately linear according to e _ a1T

Wires of such pairs of materials are connected together at one end, and in this formare known as thermocouples. Thermocouples are a very important class of device as they provide the most commonly used method of measuring temperatures in industry.

Thermocouples are manufactured from various combinations of the base metals copper and iron; the base metal alloys of alumel (Ni/Mn/Al/Si), chromel (Ni/Cr), constantan (Cu/Ni), nicrosil (Ni/Cr/Si), nisil (Ni/Si/Mn), nickel-molybdenum, and nickel-cobalt; the noble metals platinum and tungsten; and the noble metal alloys of platinum-rhodium, tungsten-rhenium, and gold-iron. Only certain combinations of these are used as thermocouples, and most standard combinations are known by internationally recognized type letters, for example, type K is chromel-alumel. The EMF-temperature characteristics for some of these standard thermocouples are shown in Fgr. 1: these show reasonable linearity over at least part of their temperature-measuring ranges.

A typical thermocouple, made from one chromel wire and one constantan wire, is shown in Fgr. 2a. For analysis purposes, it’s useful to represent the thermocouple by its equivalent electrical circuit, shown in Fgr. 2. The EMF generated at the point where the different wires are connected together is represented by a voltage source, E1, and the point is known as the hot junction. The temperature of the hot junction is customarily shown as Th on the diagram. The EMF generated at the hot junction is measured at the open ends of the thermocouple, which is known as the reference junction.

Fgr. 1

In order to make a thermocouple conform to some precisely defined EMF-temperature characteristic, it’s necessary that all metals used are refined to a high degree of pureness and all alloys are manufactured to an exact specification. This makes the materials used expensive; consequently, thermocouples are typically only a few centimeters long. It’s clearly impractical to connect a voltage-measuring instrument at the open end of the thermocouple to measure its output in such close proximity to the environment whose temperature is being measured, and therefore extension leads up to several meters long are normally connected between the thermocouple and the measuring instrument. This modifies the equivalent circuit to that shown in Fgr. 3a. There are now three junctions in the system and consequently three voltage sources, E1, E2, and E3, with the point of measurement of the EMF (still called the reference junction) being moved to the open ends of the extension leads.

The measuring system is completed by connecting the extension leads to the voltage-measuring instrument. As the connection leads will normally be of different materials to those of the thermocouple extension leads, this introduces two further EMF-generating junctions, E4 and E5, into the system, as shown in Fgr. 3b.The net output EMF measured (Em) is then given by:

Em = E1 + E2 + E3 + E4 + E5

… and this can be re-expressed in terms of E1 as E1 = Em _ E2 _ E3 _ E4 _ E5

In order to apply Equation ( -1) to calculate the measured temperature at the hot junction, E1 has to be calculated from Equation ( -4). To do this, it’s necessary to calculate the values of E2, E3, E4, and E5.

It’s usual to choose materials for the extension lead wires such that the magnitudes of E2 and E3 are approximately zero, irrespective of the junction temperature. This avoids the difficulty that would otherwise arise in measuring the temperature of the junction between the thermocouple wires and the extension leads, and also in determining the EMF/temperature relationship for the thermocouple/extension lead combination.

A near-zero junction EMF is achieved most easily by choosing the extension leads to be of the same basic materials as the thermocouple, but where their cost per unit length is reduced greatly by manufacturing them to a lower specification. As an alternative to using lower specification materials of the same basic type as the thermocouple, copper compensating leads are also sometimes used with certain types of base metal thermocouples. In this case, the law of intermediate metals has to be applied to compensate for the EMF at the junction between the thermocouple and compensating leads.

Unfortunately, the use of extension leads of the same basic materials as the thermocouple but manufactured to a lower specification is still prohibitively expensive in the case of noble metal thermocouples. It’s necessary in this case to search for base metal extension leads that have a similar thermoelectric behavior to the noble metal thermocouple. In this form, the extension leads are usually known as compensating leads. A typical example of this is the use of nickel/copper-copper extension leads connected to a platinum/rhodium-platinum thermocouple. It should be noted that the approximately equivalent thermoelectric behavior of compensating leads is only valid for a limited range of temperatures that is considerably less than the measuring range of the thermocouple that they are connected to.

To analyze the effect of connecting extension leads to the voltage-measuring instrument, a thermoelectric law known as the law of intermediate metals can be used. This states that the EMF generated at the junction between two metals or alloys A and C is equal to the sum of the EMF generated at the junction between metals or alloys A and B and the EMF generated at the junction between metals or alloys B and C, where all junctions are at the same temperature.

This can be expressed more simply as:

eAC = eAB + eBC

Suppose we have an iron-constantan thermocouple connected by copper leads to a meter. We can express E4 and E5 in Fgr. 4 as:

E4 = eiron_copper ; E5 = ecopper_constantan:

The sum of E4 and E5 can be expressed as E4 +E5 = eiron_copper + ecopper_constantan

Applying Equation ( -5): eiron_copper + ecopper_constantan = eiron_constantan.

Thus, the effect of connecting the thermocouple extension wires to the copper leads to the meter is canceled out, and the actual EMF at the reference junction is equivalent to that arising from an iron-constantan connection at the reference junction temperature, which can be calculated according to Equation ( -1). Hence, the equivalent circuit in Fgr. 3b becomes simplified to that shown in Fgr. 4.The EMF Em measured by the voltage-measuring instrument is the sum of only two EMFs, consisting of the EMF generated at the hot junction temperature, E1, and the EMF generated at the reference junction temperature, Eref. The EMF generated at the hot junction can then be calculated as:

E1 = Em + Eref :

Eref can be calculated from Equation if the temperature of the reference junction is known. In practice, this is often achieved by immersing the reference junction in an ice bath to maintain it at a reference temperature of 0_ C. However, as discussed in the following section on thermocouple tables, it’s very important that the ice bath remains exactly at 0_ C if this is to be the reference temperature assumed, as otherwise significant measurement errors can arise. For this reason, refrigeration of the reference junction at a temperature of 0_ C is often preferred.

Thermocouple Tables

Although the preceding discussion has suggested that the unknown temperature, T, can be evaluated from the calculated value of the EMF, E1, at the hot junction using Equation ( -1), this is very difficult to do in practice because Equation (1) is a high-order polynomial expression. An approximate translation between the value of E1 and temperature can be achieved by expressing Equation ( -1) in graphical form as in Fgr. 1. However, this is not usually of sufficient accuracy, and it’s normal practice to use tables of EMF and temperature values known as thermocouple tables. These include compensation for the effect of the EMF generated at the reference junction (Eref), which is assumed to be at 0_ C. Thus, the tables are only valid when the reference junction is exactly at this temperature. Compensation for the case where the reference junction temperature is not at zero is considered later in this section.

Tables for a range of standard thermocouples are given in Appendix 3. In these tables, a range of temperatures is given in the left-hand column, and the EMF output for each standard type of thermocouple is given in the columns to the right. In practice, any general EMF output measurement taken at random won’t be found exactly in the tables, and interpolation will be necessary between the values shown in the table.

Example -1

If the EMF output measured from a chromel-constantan thermocouple is 14.419 mV with the reference junction at 0_ C, the appropriate column in the tables shows that this corresponds to a hot junction temperature of 200_ C.

Example -2

If the measured output EMF for a chromel-constantan thermocouple (reference junction at 0_ C) was 10.65 mV, it’s necessary to carry out linear interpolation between the temperature of 160_ C corresponding to an EMF of 10.501 mV shown in the tables and the temperature of 170_ C corresponding to an EMF of 11.222 mV. This interpolation procedure gives an indicated hot junction temperature of 162_ C.

Nonzero Reference Junction Temperature

If the reference junction is immersed in an ice bath to maintain it at a temperature of 0_ C so that thermocouple tables can be applied directly, the ice in the bath must be in a state of just melting. This is the only state in which ice is exactly at 0_ C, and otherwise it will be either colder or hotter than this temperature. Thus, maintaining the reference junction at 0_ C is not a straightforward matter, particularly if the environmental temperature around the measurement system is relatively hot. In consequence, it’s common practice in many practical applications of thermocouples to maintain the reference junction at a nonzero temperature by putting it into a controlled environment maintained by an electrical heating element. In order to still be able to apply thermocouple tables, correction then has to be made for this nonzero reference junction temperature using a second thermoelectric law known as the law of intermediate temperatures. This states that:

E Th ,T0 + = E Th ,Tr + E Tr ,T0 ð +, + where E(Th , T0) is the EMF with junctions at temperatures Th and T0, E(Th , Tr) is the EMF with junctions at temperatures Th and Tr, E(Tr , T0) is the EMF with junctions at temperatures Tr and T0, Th is the hot-junction measured temperature, T0 is 0_ C, and Tr is the nonzero reference junction temperature that is somewhere between T0 and Th.

Example -3

Suppose that the reference junction of a chromel-constantan thermocouple is maintained at a temperature of 80_ C and the output EMF measured is 40.102 mV when the hot junction is immersed in a fluid.

The quantities given are Tr = 80_ C and E(Th , Tr ) = 40.102 mV From the tables, E(Tr , T0) = 4.983 mV.

Now applying Equation ( -6), E(Th , T0) = 40.102 + 4.983 = 45.085 mV

Again referring to the tables, this indicates a fluid temperature of 600_ C.

In using thermocouples, it’s essential that they are connected correctly. Large errors can result if they are connected incorrectly, for example, by interchanging the extension leads or by using incorrect extension leads. Such mistakes are particularly serious because they don’t prevent some sort of output being obtained, which may look sensible even though it’s incorrect, and so the mistake may go unnoticed for a long period of time.

The following examples illustrate the sorts of errors that may arise.

Example 4:

This example is an exercise in the use of thermocouple tables, but it also serves to illustrate the large errors that can arise if thermocouples are used incorrectly. In a particular industrial situation, a chromel-alumel thermocouple with chromel-alumel extension wires is used to measure the temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible has inadvertently interchanged the extension wires from the thermocouple. The ends of the extension wires are held at a reference temperature of 0_ C and the output EMF measured is -1 mV.

If the junction between the thermocouple and extension wires is at a temperature of 40_ C, what temperature of fluid is indicated and what is the true fluid temperature?

Solution:

The initial step necessary in solving a problem of this type is to draw a diagrammatical representation of the system and to mark on this the EMF sources, temperatures, etc., as shown in Fgr. 5. The first part of the problem is solved very simply by looking up in thermocouple tables what temperature the EMF output of 12.1 mV indicates for a chromel-alumel thermocouple. This is 297.4_ C. Then, summing EMFs around the loop:

V = 12:1 = E1 + E2 + E3 or E1 = 12:1 _ E2 _ E3 E2 = E3 = Emf alumel_chromel

=_Emf chromel_alumel

=_1:611 mV

Hence E1 = 12.1 + 1.611 + 1.611 = 15.322 mV.

Interpolating from the thermocouple tables, this indicates that the true fluid temperature is 374.5_ C.

Example 5

This example also illustrates the large errors that can arise if thermocouples are used incorrectly. An iron-constantan thermocouple measuring the temperature of a fluid is connected by mistake with copper-constantan extension leads (such that the two constantan wires are connected together and the copper extension wire is connected to the iron thermocouple wire). If the fluid temperature was actually 200_ C and the junction between the thermocouple and extension wires was at 50_ C, what EMF would be measured at the open ends of the extension wires if the reference junction is maintained at 0_ C?

What fluid temperature would be deduced from this (assuming that the connection mistake was not known)?

Solution:

Again, the initial step necessary is to draw a diagram showing the junctions, temperatures, and EMFs, as shown in Fgr. 6. The various quantities can then be calculated:

E2 = Emf iron_copper +50 By the law of intermediate metals:

Emf(iron_copper)50

= Emf(iron_constantan)50

_ Emf(copper_constantan)50

= 2.585 _ 2.035 [from thermocouple tables] = 0.55 mV

E1 = Emf(iron_constantan)200 = 10.777 [from thermocouple tables] V = E1 _ E2 = 10.777 _ 0.55 = 10.227

Using tables and interpolating, 10.227 mV indicates a temperature of 10:227 _ 10:222

Thermocouple Types

The five standard base metal thermocouples are chromel_constantan (type E), iron_constantan (type J), chromel_alumel (type K), nicrosil_nisil (type N), and copper_constantan (type T).

These are all relatively inexpensive to manufacture but become inaccurate with age and have a short life. In many applications, performance is also affected through contamination by the working environment. To overcome this, the thermocouple can be enclosed in a protective sheath, but this has the adverse effect of introducing a significant time constant, making the thermcouple slow to respond to temperature changes. Therefore, as far as possible, thermocouples are used without protection.

Chromel_constantan thermocouples (type E) give the highest measurement sensitivity of 68 mV/ _ C, with an inaccuracy of _0.5% and a useful measuring range of _200_ C up to 900_ C.

Unfortunately, while they can operate satisfactorily in oxidizing environments when unprotected, their performance and life are seriously affected by reducing atmospheres.

Iron_constantan thermocouples (type J) have a sensitivity of 55 mV/ _ C and are the preferred type for general-purpose measurements in the temperature range of _40 to +750_ C, where the typical measurement inaccuracy is _0.75%. Their performance is little affected by either oxidizing or reducing atmospheres.

Copper_constantan thermocouples (type T) have a measurement sensitivity of 43 mV/_ C and find their main application in measuring subzero temperatures down to _200_ C, with an inaccuracy of _0.75%. They can also be used in both oxidizing and reducing atmospheres to measure temperatures up to 350_ C.

Chromel_alumel thermocouples (type K) are widely used, general-purpose devices with a measurement sensitivity of 41 mV/_ C. Their output characteristic is particularly linear over the temperature range between 700 and 1200_ C and this is therefore their main application, although their full measurement range is _200 to +1300_ C. Like chromel_constantan devices, they are suitable for oxidizing atmospheres but not for reducing ones unless protected by a sheath. Their measurement inaccuracy is _0.75%.

Nicrosil_nisil thermocouples (type N) were developed with the specific intention of improving on the lifetime and stability of chromel-alumel thermocouples. They therefore have similar thermoelectric characteristics to the latter but their long-term stability and life are at least three times better. This allows them to be used in temperatures up to 1300_ C. Their measurement sensitivity is 39 mV/ _ C and they have a typical measurement uncertainty of _0.75%. A detailed comparison between type K and N devices.

Nickel/molybdenum-nickel-cobalt thermocouples (type M) have one wire made from a nickel- molybdenum alloy with 18% molybdenum and the other wire made from a nickel-cobalt alloy with 0.8% cobalt. They can measure at temperatures up to 1400_ C, which is higher than other types of base metal thermocouples. Unfortunately, they are damaged in both oxidizing and reducing atmospheres. This means that they are rarely used except for special applications such as temperature measurement in vacuum furnaces.

Noble metal thermocouples are expensive, but they enjoy high stability and long life even when used at high temperatures, although they cannot be used in reducing atmospheres.

Unfortunately, their measurement sensitivity is relatively low. Because of this, their use is mainly restricted to measuring high temperatures unless the operating environment is particularly aggressive in low-temperature applications. Various combinations of the metals platinum and tungsten and the metal alloys of platinum-rhodium, tungsten-rhenium, and gold-iron are used.

Platinum thermocouples (type B) have one wire made from a platinum-rhodium alloy with 30% rhodium and the other wire made from a platinum-rhodium alloy with 6% rhodium.

Their quoted measuring range is +50 to +1800_ C, with a measurement sensitivity of 10 mV/ _ C.

Platinum thermocouples (type R) have one wire made from pure platinum and the other wire made from a platinum-rhodium alloy with 13% rhodium. Their quoted measuring range is 0 to +1700_ C, with a measurement sensitivity of 10 mV/_ C and quoted inaccuracy of _0.5%.

Platinum thermocouples (type S) have one wire made from pure platinum and the other wire made from a platinum-rhodium alloy with 10% rhodium. They have similar characteristics to type R devices, with a quoted measuring range of 0 to +1750_ C, measurement sensitivity of 10 mV/ _ C, and inaccuracy of _0.5%.

Tungsten thermocouples (type C) have one wire made from pure tungsten and the other wire made from a tungsten/rhenium alloy. Their measurement sensitivity of 20 mV/_ C is double that of platinum thermocouples, and they can also operate at temperatures up to 2300_ C. Unfortunately, they are damaged in both oxidizing and reducing atmospheres.

Therefore, their main application is temperature measurement in vacuum furnaces.

Chromel-gold/iron thermocouples have one wire made from chromel and the other wire made from a gold/iron alloy which is, in fact, almost pure gold but with a very small iron content (typically 0.15%). These are rare, special-purpose thermocouples with a typical measurement sensitivity of 15 mV/_ K designed specifically for cryogenic (very low temperature) applications.

The lowest temperature measureable is 1.2_ K. Several versions are available, which differ according to the iron content and consequent differences in the measurement range and sensitivity. Because of this variation in iron content, and also because of their rarity, these don’t have an international type letter.

Thermocouple Protection

Thermocouples are delicate devices that must be treated carefully if their specified operating characteristics are to be maintained. One major source of error is induced strain in the hot junction. This reduces the EMF output, and precautions are normally taken to minimize induced strain by mounting the thermocouple horizontally rather than vertically. It’s usual to cover most of the thermocouple wire with thermal insulation, which also provides mechanical protection, although the tip is left exposed if possible to maximize the speed of response to changes in the measured temperature. However, thermocouples are prone to contamination in some operating environments. This means that their EMF/temperature characteristic varies from that published in standard tables. Contamination also makes them brittle and shortens their life.

Table -1 Common Sheath Materials for Thermocouples

Material—Maximum--Operating Temperature (ºC)

Where they are prone to contamination, thermocouples have to be protected by enclosing them entirely in an insulated sheath. Some common sheath materials and their maximum operating temperatures are shown in Table -1. While the thermocouple is a device that has a naturally first-order type of step response characteristic, the time constant is usually so small as to be negligible when the thermocouple is used unprotected. However, when enclosed in a sheath, the time constant of the combination of thermocouple and sheath is significant. The size of the thermocouple and hence the diameter required for the sheath have a large effect on the importance of this. The time constant of a thermocouple in a 1-mm-diameter sheath is only 0.15 s and this has little practical effect in most measurement situations, whereas a larger sheath of 6 mm diameter gives a time constant of 3.9 s that cannot be ignored so easily.

Thermocouple Manufacture

Thermocouples are manufactured by connecting together two wires of different materials, where each material is produced so as to conform precisely with some defined composition specification. This ensures that its thermoelectric behavior accurately follows that for which standard thermocouple tables apply. The connection between the two wires is affected by welding, soldering, or, in some cases, just by twisting the wire ends together. Welding is the most common technique used generally, with silver soldering being reserved for copper-constantan devices.

The diameter of wire used to construct thermocouples is usually in the range between 0.4 and 2 mm. Larger diameters are used where ruggedness and long life are required, although these advantages are gained at the expense of increasing the measurement time constant. In the case of noble metal thermocouples, the use of large diameter wire incurs a substantial cost penalty. Some special applications have a requirement for a very fast response time in the measurement of temperature, and in such cases wire diameters as small as 0.1 mm can be used.

Thermopile

The thermopile is the name given to a temperature-measuring device that consists of several thermocouples connected together in series, such that all the reference junctions are at the same cold temperature and all the hot junctions are exposed to the temperature being measured, as shown in Fgr. 7. The effect of connecting n thermocouples together in series is to increase the measurement sensitivity by a factor of n. A typical thermopile manufactured by connecting together 25 chromel-constantan thermocouples gives a measurement resolution of 0.001_ C.

Digital Thermometer

Thermocouples are also used in digital thermometers, of which both simple and intelligent versions exist (for a description of the latter, see Section -12). A simple digital thermometer is a combination of a thermocouple, a battery-powered, dual-slope digital voltmeter to measure the thermocouple output, and an electronic display. This provides a low noise, digital output that can resolve temperature differences as small as 0.1_ C. The accuracy achieved is dependent on the accuracy of the thermocouple element, but reduction of measurement inaccuracy to _0.5% is achievable.

Continuous Thermocouple

The continuous thermocouple is one of a class of devices that detect and respond to heat.

Other devices in this class include the line-type heat detector and heat-sensitive cable. The basic construction of all these devices consists of two or more strands of wire separated by insulation within a long thin cable. While they sense temperature, they don’t in fact provide an output measurement of temperature. Their function is to respond to abnormal temperature rises and thus prevent fires, equipment damage, etc.

Fgr. 7

The advantages of continuous thermocouples become more apparent if problems with other types of heat detectors are considered. Insulation in the line-type heat detector and heat-sensitive cable consists of plastic or ceramic material with a negative temperature coefficient (i.e., the resistance falls as the temperature rises).An alarm signal can be generated when the measured resistance falls below a certain level. Alternatively, in some versions, the insulation is allowed to break down completely, in which case the device acts as a switch. The major limitation of these devices is that the temperature change has to be relatively large, typically 50_200_ C above ambient temperature, before the device responds. Also, it’s not generally possible for such devices to give an output that indicates that an alarm condition is developing before it actually happens, and thus allow preventative action.

Furthermore, after the device has generated an alarm it usually has to be replaced. This is particularly irksome because there is a large variation in the characteristics of detectors coming from different batches and so replacement of the device requires extensive onsite recalibration of the system.

In contrast, the continuous thermocouple suffers from very few of these problems. It differs from other types of heat detectors in that the two strands of wire inside it are a pair of thermocouple materials {separated by a special, patented mineral insulation and contained within a stainless-steel protective sheath. If any part of the cable is subjected to heat, the resistance of the insulation at that point is reduced and a "hot junction" is created between the two wires of dissimilar metals. An EMF is generated at this hot junction according to normal thermoelectric principles.

The continuous thermocouple can detect temperature rises as small as 1_ C above normal. Unlike other types of heat detectors, it can also monitor abnormal rates of temperature rise and provide a warning of alarm conditions developing before they actually happen. Replacement is only necessary if a great degree of insulation breakdown has been caused by a substantial hot spot at some point along the detector's length. Even then, the use of thermocouple materials of standard characteristics in the detector means that recalibration is not needed if it’s replaced. Because calibration is not affected either by cable length, a replacement cable may be of a different length to the one it’s replacing. One further advantage of continuous thermocouples over earlier forms of heat detectors is that no power supply is needed, thus significantly reducing installation costs.

Cont...

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Updated: Tuesday, 2014-03-25 17:43 PST