Tuesday, 29 October 2013
Monday, 28 October 2013
Temperature Sensor : Thermocouple Basics
Temperature Sensor : Thermocouple Basics
Let's start with T. J. Seebeck, who in 1821 discovered what is now termed the thermoelectric effect. He noted that when two lengths of dissimilar metal wires (such as iron and Constantan) are connected at both ends to form a complete electric circuit, an emf is developed when one junction of the two wires is at a different temperature than the other junction.
Basically, the developed emf (actually a small millivoltage) is dependent upon two conditions: (1) the difference in temperature between the hot junction and the cold junction. Note that any change in either junction temperature can affect the emf value and (2) the metallurgical composition of the two wires.
Although a "thermocouple" is often pictured as two wires joined at one end, with the other ends not connected, it is important to remember that it is not a true thermocouple unless the other end is also connected! It is well for the user to remember this axiom: "Where there is a hot junction there is always a cold or reference junction" (even though it may seem hidden inside an instrument 1,000 feet away from the hot junction).
Still in Seebeck's century, two other scientists delved deeper into how the emf is developed in a thermoelectric circuit. Attached to their names are two phenomena they observed--the Peltier effect (for Jean Peltier in 1834) and the Thompson effect (for Sir William Thompson a.k.a Lord Kelvin in 1851). Without getting into the theories involved, we can state that the Peltier effect is the emf resulting solely from the contact of the two dissimilar wires. Its magnitude varies with the temperature at the juncture. Similarly, the Thompson effect can be summarized as having to do with emf's produced by a temperature gradient along a metal conductor. Since there are two points of contact and two different metals or alloys in any thermocouple, there are two Peltier and two Thompson emfs. The net emf acting in the circuit is the result of all the above named effects.
Polarity of the net emf is determined by (a) the particular metal or alloy pair that is used (such as iron-constantan) and (b) the relationship of the temperatures at the two junctions. The value of the emf can be measured by a potentiometer, connected into the circuit at any point.
In summary, the net emf is a function primarily of the temperature difference between the two junctions and the kinds of materials used. If the temperature of the cold junction is maintained constant, or variations in that temperature are compensated for, then the net emf is a function of the hot junction temperature.
In most installations, it is not practical to maintain the cold junction at a constant temperature. The usual standard temperature for the junction (referred to as the "reference junction") is 32°F (0°C). This is the basis for published tables of emf versus temperature for the various types of thermocouples.
The Law of Intermediate Temperatures provides a means of relating the emf generated under ordinary conditions to what it should be for the standardized constant temperature (e.g., 32°F). Referring to Figure 4-1, which shows thermocouples 1 and 2 made of the same two dissimilar metals; this diagram will provide an example of how the law works. Thermocouple 1 has its cold junction at the standard reference temperature of 32°F and its hot junction at some arbitrary intermediate reference temperature (in this case, 300°F). It generates 2.68 mv. Thermocouple 2 has its cold junction at the intermediate reference point of 300°F and its hot junction at the temperature being measured (700°F). It generates 4.00 mv. The Law of Intermediate Temperatures states the sum of the emfs generated by thermocouples 1 and 2 will equal the emf that would be generated by a single thermocouple (3, shown dotted) with its cold junction at 32°F and its hot junction at 700°F, the measured temperature. That is, it would hypothetically read 6.68 mv and represent the "true" emf according to the thermocouple's emf vs. temperature calibration curve.
Based upon this law, the manufacturer of an infrared thermocouple need only provide some means of substituting for the function of thermocouple 1 to provide readings referenced to the standard 32°F cold junction. Many instruments accomplish this with a temperature-sensitive resistor which measures the variations in temperature at the cold junction (usually caused by ambient conditions) and automatically develops the proper voltage correction.
Sunday, 27 October 2013
INTERNATIONAL TEMPERATURE SCALE : Temperature sensor
The first internationally recognized temperature scale was the international temperature scale of 1927 ITS-27. Its purpose was to define procedures by which specified, high quality yet practical thermometry could be calibrated such that the values of temperature obtained from them would be concise and consistent instrument-to-instrument and sensor-to-sensor, while simultaneously approximating to the appropriate thermodynamic values within the limits of the technology available. This goal remains intact today.
ITS-27 extended from just below the boiling point of oxygen, -200°C, to beyond the freezing point of gold, 1065°C. interpolation formulae were specified for platinum resistance thermometer calibrated at 0°C & at the boiling points of oxygen, water and sulphur (445°C). Above 660°C, the Pt-10% Rh vs. Pt thermocouple was specified for measurement. Above the gold point optical pyrometry was employed and the values of the fixed points were based on the best available gas thermometry data of the day.
ITS-27 was revised somewhat in 1948, and then more substantially in1968-with the adoption of the international practical temperature scale, IPTS-68. 1975 saw realignment with thermodynamic temperature through some numerical changes, and 1976 witnessed the introduction of the provisional 0.5 to 30Ktemperature scale EPT-76. The current scale, ITS-90, was adopted by the International Committee of Weights and Measures at its meeting in 1989, in th accordance with the request embodied in Resolution 7 of the 18 General Conference of Weights and Measures of 1987. This scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0.5 K to 30 K Temperature Scale.
Wednesday, 23 October 2013
REVIEW OF RTD AND THERMOCOUPLE BASICS ( Temperature Sensor )
RTD'S contain a sensing element which is an electrical resistor that changes resistance with temperature. This change in resistance is well understood and is repeatable. The sensing element in an RTD usually contains either a coil of wire, or a grid of conductive film which has a conductor pattern cut into it. Extension wires are attached to the sensing element so it's electrical resistance can be measured from some distance away. The sensing element is then packaged so it can be placed into a position in the process where it will reach the same temperature that exists in the process.
Thermocouples, on the other hand, contain two electrical conductors made of different materials which are connected at one end. The end of the conductors which will be exposed to the process temperature is called the measurement junction. The point at which the thermocouple conductors end (usually where the conductors connect to the measurement device) is called the reference junction When the measurement and reference junctions of a thermocouple are at different temperatures, a millivolt potential is formed within the conductors.
Knowing the type of thermocouple used, the magnitude of the millivolt potential within the thermocouple, and the temperature of the reference junction allows the user to determine the temperature at the measurement junction.
The millivolt potential that is created in the thermocouple conductors differs depending on the materials used. Some materials make better thermocouples than other because the millivolt potentials created by these materials are more repeatable and well established. These thermocouples have been given specific type designations such as Type E, J, K, N, T, B, R and S.
Tuesday, 22 October 2013
Strengths & Weaknesses of Temperature sensor
Each type of temperature sensor has particular strengths and weaknesses.
RTD Strengths:
RTD's are commonly used in applications where repeatability and accuracy are important considerations. Properly constructed Platinum RTD's have very repeatable resistance vs. temperature characteristics over time. If a process will be run at a specific temperature, the specific resistance of the RTD at that temperature can be determined in the laboratory and it will not vary significantly over time. RTD's also allow for easier interchangeability since their original variation is much lower than that of thermocouples. For example, a Type K thermocouple used at 400°C has a standard limit of error of ±4°C. A 100-Ohm DIN, Grade B platinum RTD has an interchangeability of ±2.2°C at this same temperature. RTD's also can be used with standard instrumentation cable for connection to display or control equipment where thermocouples must have matching thermocouple wire to obtain an accurate measurement.
RTD Weaknesses:
In the same configuration, you can expect to pay from 2 to 4 times more for an RTD than for a base metal thermocouple. RTD's are more expensive than thermocouples because there is more construction required to make the RTD including manufacture of the sensing element, the hooking up of extension wires and assembly of the sensor. RTD's do not do as well as thermocouples in high vibration and mechanical shock environments due to the construction of the sensing element. RTD's are also limited in temperature to approximately 650°C where thermocouples can be used as high as 1700°C.
Thermocouple Strengths:
Thermocouples can be used to temperatures as high as 1700°C, generally cost less than RTD's and they can be made smaller in size (down to approximately .020'' dia) to allow for faster response to temperature. Thermocouples are also more durable than RTD's and can therefore be used in high vibration and shock applications.
Thermocouple Weaknesses:
Thermocouples are less stable than RTD's when exposed to moderate or high temperature conditions. In critical applications, thermocouples should be removed and tested under controlled conditions in order to verify performance. Thermocouple extension wire must be used in hooking up thermocouple sensors to thermocouple instrument or control equipment. Use of instrumentation wire (plated copper) will result in errors when ambient temperatures change.
Monday, 21 October 2013
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