TECHNICAL
Some technical aspects of thermocouples that may be of interest to you. If you have any questions about determining the right application for you, just ask us!
Thermocouples
Fundamental laws explaining how thermocouples work are:
- Law of Homogeneous Materials - (originally known as the Law of Homogeneous Metals) A homogeneous wire is one that is physically and chemically the same throughout. This law states that a thermocouple circuit that is made with a homogeneous wire cannot generate an electromagnetic field (EMF), even if it is at different temperatures and thicknesses throughout. In other words, a thermocouple must be made from at least two different materials in order to generate a voltage. A change in the area of the cross section of a wire, or a change in the temperature in different places in the wire, will not produce a voltage.
- Law of Intermediate Metals - (originally known as the Law of Intermediate Metals) The sum of all of the EMF's in a thermocouple circuit using two or more different metals is zero if the circuit is at the same temperature. This law is interpreted to mean that the addition of different metals to a circuit will not affect the voltage the circuit creates. The added junctions are to be at the same temperature as the junctions in the circuit. For example, a third metal such as copper leads may be added to help take a measurement. This is why thermocouples may be used with digital multimeters or other electrical components and also why solder may be used to join metals to form thermocouples.
- Law of Successive or Intermediate Temperatures - A thermocouple made from two different metals produces an EMF, E1, when the metals are at different temperatures, T1 and T2, respectively. Suppose one of the metals has a temperature change to T3, but the other remains at T2. Then the EMF created when the thermocouple is at temperatures T1 and T3 will be the summation of the first and second, so that Enew = E1 + E2 This law allows a thermocouple that is calibrated with a reference temperature to be used with another reference temperature. It also allows extra wires with the same thermoelectric characteristics to be added to the circuit without affecting its total emf.
These three fundamental laws can be combined and stated as follows:
The algebraic sum of thermoelectric EMF's producted in any given circuit containing any number of dissimilar homogeneous metals is a function only of the temperature of the junction. If all but one of the junctions in such a circuit are maintained at some reference temperature, the EMF generated depends only on the temperature of that one junction and can be used as a measure of temperature.
Definitions
- A thermocouple is a temperature sensor formed by joining two dissimilar metals and applying a temperature differential between the measuring junction and the reference junction.
- A resistance temperature detector (RTD) is a circuit element whose resistance increases with increasing temperature in a predictable manner.
- A thermistor is a contraction for thermally sensitive resistor. A thermistor is a semiconductor circuit element which typically exhibits a high negative coefficient of resistance.
Junctions
The commonly used thermocouple junctions are listed below, with their response times (with 3/16" diameter sheaths), but other special junction types can be engineered to meet your application. As a general rule, it can be stated that the greater the mass of the junction, the greater the response time, and the longer the service life of the thermocouple.
RTDs versus Thermocouples
Selection of the proper temperature sensor for a given application is not simple; the many variables involved means that the specifier must carefully define process parameters and control or monitoring requirements. The wide variety of temperature sensors available, both thermocouple and RTD, allows selection of the best sensor for measurement optimization. The table is a brief comparison of standard ISA J and K calibration thermocouples with a standard DIN-tolerance 1 00-ohm at 0°C platinum RTD. As can be seen, both offer advantages in certain areas. All values listed are typical, representative of the two sensor types.
As shown, RTD's are generally more accurate; however, the differences tend to disappear above 500°C. Thermocouples have a much faster response time, on the order of three times faster, than RTD's. Thermocouples are tip sensitive wherever RTD's are "stem sensitive," making thermocouples more suitable for point sensing. Thermocouples generally have a higher upper temperature limit (although this is calibration dependent), making them the sensor of choice in elevated temperature applications. It is obvious that each type has advantages for specific requirements of accuracy, range, and specific application factors.
Upper Use Temperatures for Thermocouples
Gauge of Alloy Does Affect Thermocouple Operating Range. There's common fallacy among users of thermocouple material that thermocouple alloy, regardless of gauge, is capable of operating through out the entire thermocouple range. Unfortunately, this is not the case.
For example, it may be assumed that if Type K covers a temperature range from 32°F to 2300°F, then 28 ga. Type K alloy should be suitable for use at 2282°F. In actuality, this is not true.
As the size of the thermocouple conductor decreases, the recommended upper operating temperature also decreases. This is because thermocouple wire oxidizes rapidly and contaminates easily, especially at high temperatures.
The smaller the thermocouple conductor, the less wire (or mass) there is to resist these environmental influences. This result in serious decalibration of the thermocouple, which effectively renders the sensor useless.
