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|Corrodent||Temp. °F||Conc. %||Recommended Material|
|Alum (Potassium or Sodium)||300||All||Hast. C|
|Aluminum Chloride||212||All||Hast. B|
|Aluminum Sulfate||212||All||316 SS|
|Ammonia, Dry||212||All||304 SS, 316 SS|
|Ammonium Hydroxide||212||All||304 SS, 316 SS|
|(Ammonia, Aqua)||212||All||304 SS, 316 SS|
|Ammonium Nitrate||300||All||304 SS|
|Ammonium Sulfate||212||All||316 SS|
|Amyl Acetate||300||All||304 SS|
|Atmosphere, (Industrial and Marine)||250||304 SS|
|Barium Compounds||See Calcium|
|Benzoic Acid||212||All||316 SS|
|Bordeaux Mixture||200||304 SS|
|Boric Acid||400||All||316 SS|
|Butyl Alcohol||See Alcohols|
|Butyric Acid||212||Hast. C|
|Calcium Bisulphite||75||All||Hast. C|
|Calcium Chloride||212||All||Hast. C|
|Calcium Hydroxide||300||20%||Hast. C|
|Calcium Hypochlorite||See Bleaching Powder|
|Carbolic Acid||See Pheno|
|Carbon Dioxide, Dry||800||All||Brass|
|Carbon Disulfide||200||304 SS|
|Carbonated Beverages||212||304 SS|
|Carbonated Water||212||All||304 SS|
|Chromic Acid||300||All||Hast. C|
|Citric Acid||212||All||Hast. C|
|Copper (10) Chloride||212||All||Hast. C|
|Copper (10) Nitrate||300||All||316 SS|
|Copper (10) Sulfate||300||All||316 SS|
|Copper Plating Solution (Acid)||75||304 SS|
|Copper Plating Solution (Cyanide)||180||304 SS|
|Corn Oil||200||304 SS|
|Ethyl Acetate||See Lacquer Thinner|
|Ethyl Chloride, Dry||500||Steel|
|Ethylene Glycol (Uninhibited)||212||All||304 SS|
|Fatty Acids||500||All||316 SS|
|Ferric Chloride||75||All||Hast. C|
|Ferric Sulfate||300||All||304 SS|
|Formic Acid||300||All||316 SS|
|Fluorine, Anhydrous||100||304 SS|
|Glue ph 6-8||300||All||304 SS|
|Hydrobromic Acid||212||All||Hast. C|
|Hydrochloric Acid (37 38%)||225||All||Hast. B|
|Hydrocyanic Acid||212||All||304 SS|
|Hydrogen Chloride, Dry||500||304 SS|
|Hydrogen Fluoride, Dry||175||Steel|
|Hydrogen Peroxide||125||10-100%||304 SS|
|Lacquers & Thinners||300||All||304 SS|
|Lactic Acid||300||All||316 SS|
|Magnesium Hydroxide (or Oxide)||75||All||304 SS|
|Magnesium Sulfate||212||40%||304 SS|
|Mercuric Chloride||75||10%||Hast. C|
|Methyl Chloride, Dry||75||Steel|
|Methylene Chloride||212||All||304 SS|
|Milk, fresh or sour||180||304 SS|
|Natural Gas||70||304 SS|
|Nitric Acid||75||All||304 SS|
|Nitric Acid||110||All||316 SS|
|Oleic Acid||See Fatty Acids|
|Palmitic Acid||See Fatty Acids|
|Phosphoric Acid||212||All||316 SS|
|Photographic Bleaching||100||All||304 SS|
|Potassium Compounds||See Sodium Compounds|
|Salt or Brine||See Sodium Chloride|
|Soap & Detergents||212||All||304 SS|
|Sodium Bicarbonate||212||20%||316 SS|
|Sodium Bisulfate||212||20%||304 SS|
|Sodium Bisulfite||212||20%||304 SS|
|Sodium Carbinate||212||40%||316 SS|
|Sodium Chromate||212||All||316 SS|
|Sodium Cyanide||212||All||304 SS|
|Sodium Hydroxide||212||30%||316 SS|
|Sodium Hypochlorite||75||10%||Hast. C|
|Sodium Nitrate||212||40%||304 SS|
|Sodium Nitrite||75||20%||316 SS|
|Sodium Sulfate||212||30%||316 SS|
|Sodium Sulfide||212||10%||316 SS|
|Sodium Sulfite||212||30%||304 SS|
|Sodium Thiosulfate||212||All||304 SS|
|Stearic Acid||See Fatty Acids|
|Sugar Solutions||See Glucose|
|Sulfur Chloride||75||Dry||316 SS|
|Sulfur Dioxide||500||Dry||316 SS|
|Sulfur Trioxide||500||Dry||316 SS|
|Sulfuric Acid||212||10%||316 SS|
|Sulfuric Acid||212||10-90%||Hast. B|
|Sulfuric Acid||212||90-100%||Hast. B|
|Sulfuric Acid, Fuming||175||Carp. 20|
|Sulfurous Acid||75||20%||316 SS|
|Titanium Tetrachloride||75||All||316 SS|
|Tannic Acid||75||40%||Hast. B|
|Trichloracetic Acid||75||All||Hast. B|
|Zinc Chloride||212||All||Hast. B|
|Zinc Sulfate||212||All||316 SS|
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|Designation||Nominal Composition||Maximum Temp
(cont. Serv. Air)
|304SS||18% Chromium||900 C||1371 C-1454 C||Offers excellent resistance to many corrosive agents encountered in domestic and industrial use.|
|310SS||25% Chromium||1148 C||1371 C-1454 C||Good resistance to oxidation at temperatures up to 1148 C. Good resistance to thermal fatigue and cyclic heating.|
|898 C||1371 C-1454 C||Good resistance to a wider range of chemicals than 304SS. Withstands sulphurous acid compounds.|
|321SS||Similar to 304SS but Steel stabilized by Titanium addition||871 C||1371 C-1426 C||Not sensitive to inter-granular corrosion when heated within the carbide precipitation range of 482 C -815 C. Similar in corrosion resistance to 304SS.|
|347SS||Similar to 304SS but contains Tantalum and is Steel stabilized by Colombium addition||871 C||1371 C-1426 C||Excellent equivalent to 304SS for 426 C - 815 C range. Superior to 321SS where service is both corrosive and at an elevated temperature.|
|Similar to 304SS and 316SS but with reduced carbon||871 C||1371 C-1454 C||Low carbon versions of 304SS and 316SS (maximum of 0.03% carbon). Because of low carbon content the effects of carbide precipitation are reduced.|
|954 C||1426 C||Superior grade with excellent resistance to corrosive conditions.|
|Inconel 600||76% Nickel
|1148 C||1354 C-1412 C||Excellent material for severely corrosive environments. Resistant to oxidation at temperatures up to 1175 C.|
|Inconel 601||60.5% Nickel
|1148 C||1301 C-1367 C||Similar to Inconel 600 however higher Chromium content gives superior resistance to oxidizing, carburizing and Sulphur containing environments.|
|Incoloy 800||32.5% Nickel
|1093 C||1357 C-1385 C||Resistant to oxidation and carburization at elevated temperatures. It resists stress - corrosion cracking, Sulphur attack, internal oxidation, scaling and corrosion in a wide variety of industrial atmospheres.|
|Monel 400||66% Nickel
|537 C||1343 C||Highly resistant to corrosion by chlorinated solvents, glass etching agents, Sulphuric and many other acids, and practically all alkalies generally free from stress-corrosion cracking. Good resistant to salt water corrosion.|
|Hastelloy B||61% Nickel
|1204 C||1260 C-1354 C||Good corrosion resistance to Hydrochloric, Sulphuric, Phosphoric, and Acetic acids. Excellent corrosion resistance to Hydrogen-Chloride gas.|
|Hastelloy C||54% Nickel
|1204 C||1260 C-1354 C||Good corrosion resistance to many chemical environments, including Ferric and Cupric Chlorides, contaminated mineral acids, wet Chlorine gas. Oxidation resistance to 990 C.|
|Hastelloy X||47% Nickel
|1204 C||1260 C-1354 C||Good high temperature strength and resistance to oxidation to 1204 C. Also good for reducing conditions.|
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RTD elements take either of two forms: wire wound (see Figure 1) or thin film. Wire-wound elements are made primarily by winding a very fine strand of platinum wire into a coil until there is enough material to equal 100 Ω of resistance. The coil is then inserted into a mandrel and powder is packed around it to prevent the sensor from shorting and to provide vibration resistance. This is a time-consuming method and all work is done manually under a microscope, but the result is a strain-free design.
|Figure 1. The wire-wound element is built by winding a small-diameter platinum sensing wire around a nonconducting mandrel.||Figure 2. The thin film sensing element is made by depositing a thin layer of platinum in a resistance pattern on a ceramic substrate. A layer of glass or epoxy is applied for moisture protection.|
When discussing RTDs, several specifications must be considered:
- Wiring configuration (two-, three-, or four-wire)
- Response time
Serious lead wire resistance errors can occur when using a two-wire RTD especially in a 100 Ω sensor. In a two-wire circuit, a current is passed through the sensor. As the temperature of the sensor increases, the resistance increases. This increase in resistance will be detected by an increase in the voltage ( V = I � R). The actual resistance causing the voltage increase is the total resistance of the sensor and the resistance introduced by the lead wires. As long as the lead wire resistance remains constant, it will not affect the temperature measurement. The wire resistance will change with temperature, however, so as the ambient conditions change, the wire resistance will also change, introducing errors. If the wire is very long, this source of error could be significant. Two-wire RTDs are typically used only with very short lead wires, or with a 1000 Ω element.
In a three-wire RTD there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current, while L2 acts only as a potential lead. Ideally, the resistances of L1 and L3 are perfectly matched and therefore cancelled. The resistance in R3 is equal to the resistance of the sensor, Rt , at a given temperature (usually the midpoint of the temperature range). At this point, no current passes through the centre lead. As the temperature of the sensor increases, the resistance of the sensor increases, causing the resistance to be out of balance. Current then flows in the centre lead and will indicate an offset temperature.
The optimum form of connection for RTDs is a four-wire circuit It removes the error caused by mismatched resistance of the lead wires. A constant current is passed through L1 and L4; L2 and L3 measure the voltage drop across the RTD. With a constant current, the voltage is strictly a function of the resistance and a true measurement is achieved. This design is slightly more expensive than two- or three-wire configurations, but is the best choice when a high degree of accuracy is required.
Self-Heating. To measure resistance, it is necessary to pass a current through the RTD. The resultant voltage drop across the resistor heats the device in an effect known as the I 2 R, or Joule heating. The sensor's indicated temperature is therefore slightly higher than the actual temperature. The amount of self-heating also depends heavily on the medium in which the RTD is immersed. An RTD can self-heat up to 100 3 higher in still air than in moving water.
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When accuracy is not critical, a two-wire RTD is the least expensive; offering. Using lead wires to place any distance between a two wire RTD and a receiving device will further compromise its accuracy. The potential for poor accuracy from a two-wire RTD stems from its inability to compensate for lead length, resistance that changes the ohm value of the original signal. A two-wire RTD should be used only in applications where the receiving device connects directly to the sensor
Three Wire RTD
Three-wire RTD's compensate for resistance resulting from length differences by adding a third lead to the RTD. To accomplish this requires that the wires match exactly. Any difference in resistance between the lead wires will cause an imbalance, which will compromise the accuracy of the RTD. Lead length variance, work hardening or corrosion, and manufacturing irregularities are errors to avoid. Quality manufacturing is critical to insure balance of all three leads.
Four Wire RTD
Errors caused by resistance imbalance between leads are cancelled out in a four-wire RTD circuit. Four-wire RTD's are used where superior accuracy is critical or if the sensor is installed far from the receiving device. In a four-wire RTD one pair of wires carries the current through the RTD the other pair senses the voltage across the RTD. 2- and three-wire RTD's require heavier lead wire because thicker wire, by creating less resistance to the measured signal, reduces measurement distortion. Therefore lighter gauge wire, less expensive, may be used in four-wire RTD applications. RTD's are limited to temperatures of 1200 � F and because of the construction of the sensing element, RTD's do not do well in high-vibration and severe mechanical shock environments. When selecting a temperature sensor for an application you should consult your temperature sensor manufacturer for recommendations.
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Tolerance/Accuracy is calculated as:
change in t=+/- (0.3+0.005|t|)
change in t=+/- (0.15+0.002|t|)
1/3 Class B
change in t=+/- 1/3 x (0.3+0.005|t|)
1/5 Class B
change in t=+/- 1/5 x (0.3+0.005|t|)
|1/10 Class B||change in t=+/- 1/10 x (0.3+0.005|t|)|
|t| = absolute temperature in �C. Where elements have a resistance of n x 100 Ohms then the basic values and tolerances also have to be multiplied by n
These three terms are often confused, but it is important to understand the difference.
Accuracy. IEC standard 751 sets two tolerance classes for the accuracy of RTDs: Class A and Class B:
Class A: Δt = �(0.15 + 0.002 � | t | )
Class B: Δt = �(0.30 + 0.005 � | t | )
| t | = absolute value of temperature in �C
Class A applies to temperatures from �200�C to 650�C, and only for RTDs with three- or four-wire configurations. Class B covers the entire range from �200�C to 850�C.
This is the sensor's ability to maintain a consistent output when a constant input is applied.
Physical or chemical changes can cause calibration drift. The material that the platinum is adhered to,
whether wound on a mandrel or on a substrate, can expand and contract, straining the wire.
Drift rates conservatively specified by manufacturers are typically 0.05�C/yr
Repeatability is the sensor's ability to give the same output or reading under repeated identical conditions. Absolute accuracy is not necessary in most applications. The focus should be on the stability and repeatability of the sensor. If an RTD in a 100.00�C bath consistently reads 100.06�C, the electronics can easily compensate for this error. The stability of RTDs is exceptional, with most experiencing drift rates of 0.05�C over a five-year period.
Response time varies according to the application. It is the sensor's ability to react to a change in temperature, and depends on the sensor's thermal mass and proximity to the material being tested. For instance, an RTD sensor in a thermowell will react more slowly than the same sensor immersed directly into a process. RTD specifications will list the sensor's time constant, which is the time it takes for an RTD to respond to a step change in temperature and come to 63% of its final equilibrium value. Response times are calculated in water flowing at 0.2 m/s and in air flowing at 1 m/s. This gives a useful comparison of RTD sensor configurations.
|Lead wires have resistance that is a function of the material used, wire size, and lead length. This resistance can add to the measured RTD resistance, and improper wire compensation can result in significant errors. The common configurations of RTDs are two (A), three (B), or four wires (C)|
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Resistance temperature detectors Calculations
Resistance temperature detectors (RTDs) operate on the inherent propensity of metals to exhibit a change in electrical resistance as a result of a change in temperature. We are all aware that metals are conductive materials. It is actually the inverse of a metal's conductivity, or its resistivity, that brought about the development of RTDs. Each metal has a specific and unique resistivity that can be determined experimentally. This resistance, R, is directly proportional to a metal wire's length, L, and inversely proportional to the cross-sectional area, A:
R =ρL/A (1)
ρ = the constant of proportionality, or the resistivity of the material
Principle of Operation
RTDs are manufactured from metals whose resistance increases with temperature. Within a limited temperature range, this resistivity increases linearly with temperature:
ρt = ρ0 [1 + a (t-t 0 )] (2)
ρt = resistivity at temperature, t
ρ0 = resistivity at a standard temperature, t 0
a = temperature coefficient of resistance (°C -1 )
Combining Equations 1 and 2, setting t 0 to 0°C, and rearranging to the standard linear y = mx + b form, it is clear that resistance vs. temperature is linear with a slope equal to a:
R/R0 = αt + 1 (3)
In theory, any metal could be used to measure temperature. The metal selected should have a high melting point and an ability to withstand the effects of corrosion. Platinum has therefore become the metal of choice for RTDs. Its desirable characteristics include chemical stability, availability in a pure form, and electrical properties that are highly reproducible.
Platinum RTDs are made of either IEC/DIN-grade platinum or reference-grade platinum. The difference lies in the purity of the platinum. The IEC/DIN standard is pure platinum that is intentionally contaminated with other platinum group metals. The reference-grade platinum is made from 99.99% pure platinum. Both probes will read 100 Ω at 0°C, but at 100°C the DIN grade platinum RTD will read 138.5 Ω and the reference grade will read 139.02 Ω. International committees have been established to develop standard curves for RTDs. The committees have defined a mean temperature coefficient to be between 0°C and 100°C. Solving Equation (3) for a:
α = (R 100 � R 0 ) / R 0 t (4)
IEC/DIN grade platinum: a = 0.00385 Ω/Ω/°C
reference grade platinum: a = 0.003926 Ω/Ω/°C (max.)
The relationship between resistance and temperature can be approximated by the Callendar-Van Dusen equation:
T = temperature (°C)
R = resistance at temperature T
R0 = resistance at the ice point
α = constant (gives the linear approximation to the R vs. T curve)
β= constant (b = 0 when T is >0°C)
The actual values for the coefficients,α, δ, and β are determined by testing the RTD at four temperatures and solving the equations. The Callendar-Van Dusen equation can be simplified to:
Rt = R0 [1 + At + Bt2 + C(t -100°C)]t 3 (6)
In the positive quadrant, temperatures over 0°C, the behaviour of a PRT may be described by a quadratic equation in the form:
Rt = R0 (1 + At + Bt2 ) (7)
As written, the above implies that valid equations may be generated from empirical data taken using 0°C plus two arbitrarily selected positive temperatures. For a single PRT, the constants A and B could be slightly different, depending on the temperatures selected.
Callendar resolved the issue by defining two additional fixed points:
- The boiling point of water, 100°C
- The triple point of zinc, 419.58°
The coefficients A, B, and C depend on the wire material (i.e., platinum) and its purity. International standard IEC 751 describes the specifications that permit universal interchange ability among platinum RTDs.
The coefficients for platinum RTDs according to the IEC 751-2 (ITS90) Standard are:
A = 3.9083 x 10-3 C-1
B = -5.775 x 10-7 C-2
C = -4.183 x 10-12 C-3
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Temperature calibration provides a means of quantifying uncertainties in temperature measurement in order to optimise sensor and/or system accuracies. Uncertainties result from various factors including.
a) Sensor tolerances which are usually specified according to published standards and manufacturers specifications.
b) Instrumentation (measurement) inaccuracies, again specified in manufacturers specifications.
c) Drift in the characteristics of the sensor due to temperature cycling and ageing.
d) Possible thermal effects resulting from the installation, for example thermal voltages created at interconnection junctions.
A combination of such factors will constitute overall system uncertainty. Calibration procedures can be applied to sensors and instruments separately or in combination. Calibration can be performed to approved recognised standards (National and International) or may simply constitute checking procedures on an 'in-house' basis. Temperature calibration has many facets, it can be carried out thermally in the case of probes or electrically (simulated) in the case of instruments and it can be performed directly with certified equipment or indirectly with traceable standards.
Thermal (temperature) calibration is achieved by elevating (or depressing) the temperature sensor to a known, controlled temperature and measuring the corresponding change in its associated electrical parameter (voltage or resistance).
The accurately measured parameter is compared with that of a certified reference probe; the absolute difference represents a calibration error.
This is a comparison process.
If the sensor is connected to a measuring instrument, the sensor and instrument combination can be effectively calibrated by this technique. Absolute temperatures are provided by fixed point apparatus and comparison measurements are not used in that case. Electrical calibration is used for measuring and control instruments which are scaled for temperature or other parameters.
An electrical signal, precisely generated to match that produced by the appropriate sensor at various temperatures is applied to the instrument which is then calibrated accordingly. The sensor is effectively simulated by this means which offers a very convenient method of checking or calibration. A wide range of calibration 'simulators' is available for this purpose; in many cases, the operator simply sets the desired temperature and the equivalent electrical signal is generated automatically without the need for computation.
However this approach is not applicable to sensor calibration for which various thermal techniques are used.
THERMAL TEMPERATURE CALIBRATION.
Essentially the test probe reading is compared with that of a certified reference probe whilst both are held at a common, stable temperature. Alternatively, if a fixed point cell is used, there is no comparison with a certified thermometer; fixed point cells provide a highly accurate, known reference temperature, that of their phase conversion. Equipment required for a Calibration System.
The equipment required to achieve thermal calibration of temperature probes is dependent on the desired accuracy and also ease of use. The greater the required accuracy, the more demanding the procedure becomes and of course, the greater the cost. The required equipment generally falls into one of three groups.
1) General purpose system for testing industrial plant temperature sensors will usually provide accuracies between 1.0C and 0.1C using comparison techniques.
2) A secondary standards system for high quality comparison and fixed point measurements will provide accuracies generally between 0.1C and 0.01C.
3) A primary standards system uses the most advanced and precise equipment to provide accuracies greater than 0.001C.
A typical general purpose system comprises.
* A thermal reference (stable temperature source).
* A certified PT100 reference probe complete with its certificate.
* A precision electronic digital thermometer, bridge or DVM (digital voltmeter).
A convenient form of thermal reference is the dry block calibrator. Such units are available with various ranges spanning from -50C to +1200C and have wells to accept various test and reference probe diameters. Alternative temperature sources for comparison techniques include precisely controlled ovens and furnaces and stirred liquid baths.
Dry Block Calibrators.
Dry block calibrators provide the most convenient, portable facilities for checking industrial probes and they usually achieve reasonably rapid heating and cooling. The units consist of a specially designed heated block within which is located an insert having wells for the probes. The block temperature is controlled electronically to the desired temperature. The whole assembly is housed in a free-standing case. Although the block temperature is accurately controlled, any indication provided should be used for guidance only. As with any comparison technique, a certified sensor and indicator should be used to measure the block temperature and used as a reference for the test probe. Two types of unit are available; portable units which can be taken on to plant for on-site calibration and laboratory units to which industrial sensors are brought as required.
Alternative 'temperature' sources.
Many laboratory furnaces and ovens are available which are specially designed for temperature calibrations. Precisely controlled, they feature isothermal or defined thermal gradient environments for probes. Stirred liquid baths provide superior thermal environments for probe immersion since no air gaps exist between the probe and medium. Thermal coupling is therefore much better than the alternatives described and stirring results in very even heat distribution throughout the liquid Alcohols are used for temperatures below 0C, water from 0C to 80C and oils for up to 300C. Various molten salts and sand baths are used for temperatures in excess of 300C.
A Reference Standard Platinum Resistance Thermometer is a specially constructed assembly using a close tolerance Pt100 sensing resistor or a specially wound platinum element with a choice of Ro values. Construction is such as to eliminate the possibility of element contamination and various techniques are utilised to this end such as special sheath materials, gas filling and special coil suspension. Precision Temperature indicators are available in a wide variety of configurations and with alternative accuracy and resolution specifications.
By definition, such instruments must be highly accurate and very stable. Normally, the performance of the measuring instrument will be superior to that of the reference sensor to avoid compromising the system performance.
As with any measuring system, such factors must be considered when specifying system components.
Fixed points are the most accurate devices available for defining a temperature scale. Fixed point devices utilise totally pure materials enclosed in a sealed, inert environment; they are usually fragile and need to be handled with care. They work in conjunction with apparatus which surrounds them and provides the operational conditions required for melting and freezing to obtain the reference plateaux. The housings incorporate isothermal blocks with wells into which the probes are placed. Since fixed point temperatures are defined by physical laws, comparison of the test probe to a reference probe is not required.
Electrical Calibration - Simulators and Sources.
Indicators and controllers are calibrated by injecting signals which simulate thermocouples, resistance thermometers or thermistors. A simulator provides a very quick and convenient method for calibrating an instrument at many points. Very sophisticated and highly accurate laboratory instruments are available; conversely, compact and convenient portable units are available to permit on-site checking and calibration with a good level of accuracy.
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The term "Thermistor" is used to describe a range of electronic components whose principle characteristic is that their electrical resistance changes in response to changes in their temperature. The word "Thermistor" derives from the description "thermally sensitive resistor". Thermistors are further classified as "Positive Temperature Coefficient" devices (PTC devices) or "Negative Temperature Coefficient" devices (NTC devices). PTC devices are devices whose resistance increases as their temperature increases. NTC devices are devices whose resistance decreases as their temperature increases.
NTC thermistors are manufactured from proprietary formulations of ceramic materials based on transition metal oxides. A discrete thermistor such as a chip, disc or rod is a fundamental electrical component.
Alpha () (Temperature Coefficient):
Alpha, a material characteristic, is defined as the percentage resistance change per degree Centigrade. Alpha is also referred to as the temperature coefficient. For Negative Temperature Coefficient (NTC) Thermistors, typical values of alpha are in the range -3%/°C to -6%/°C. The temperature coefficient is a basic concept in thermistor calculations. Because the resistance of NTC thermistors is a nonlinear function of temperature, the alpha value of a particular thermistor material is also nonlinear across the relevant temperature range.
Where RT is the resistance of the component at the relevant temperature T (°C), dR/dt is the gradient of the Resistance vs Temperature curve at that temperature point, and alpha is expressed in units of "percentage change per degree Centigrade". (Note: In some texts the "100" term is omitted from the equation, but it is understood or implied in the units in which alpha values are specified.)
Thermal Time Constant (T.C.):
When a thermistor is being used to monitor the temperature of it�s environment then the accuracy of measurement of the resistance of the thermistor is critical. While the power dissipated in the thermistor is an important factor in this measurement as discussed in the previous section, the thermal characteristics of the system and the thermistor are important also. This is especially relevant in systems where the temperature is changing with time. The dynamic thermal response of the thermistor must be considered in these situations. To quantify this dynamic response, the concept of a Thermal Time Constant (T.C.) is used in the thermistor industry and it is defined as follows: The Thermal Time Constant for a thermistor is the time required for a thermistor to change its body temperature by 63.2% of a specific temperature span when the measurements are made under zero-power conditions in thermally stable environments.
This concept is illustrated in the example below:
Example: A thermistor is placed in an oil bath at 25°C and allowed to reach equilibrium temperature. The thermistor is then rapidly moved to an oil bath at 75°C. The T.C. is the time required for the thermistor to reach 56.6°C (63.2% of the temperature span).
The dominant factors that affect the T.C. of a thermistor are:
- The mass and the thermal mass of the thermistor itself
- Custom assemblies and thermal coupling agents that couple the thermistor to the medium being monitored.
- Mounting configurations such as a probe assembly or surface mounting.
- Thermal conductivity of the materials used to assemble the thermistor in probe housings.
- The environment that the thermistor will be exposed to and the heat transfer characteristics of that environment. Typically, gases are less dense than liquids so that thermistors have greater time constants when monitoring temperature in a gaseous medium than in a liquid one.
The definition of Thermal Time Constant arises from the exponential nature of the rate of transfer of heat between the thermistor and the medium that it is monitoring. It is similar in principle to the definition of time constants in describing the responses of systems where physical effects have an exponential response with respect to time.
Graph # 8 illustrates determination of T.C. for the thermistor of the previous example using a strip chart recorder. When the thermistor is transferred from a 25°C oil bath to a 75°C oil bath it�s resistance will change and the voltage drop across it can be measured using the chart recorder. By measuring the graph and the speed of the chart recorder the T.C. for the device in a stable oil bath environment can be determined.
Time Constant recording of a thermistor element using a strip chart recorder.
The value of resistance of a thermistor that is measured in a physical system depends on the power dissipated in the thermistor due to the measurement method and also on the thermal characteristics of a dynamic temperature system. It is important to consider both effects in implementing thermistor sensing systems.