While thermocouples are reliable temperature measurement devices, they do drift with time. Maximum exposure temperature, cyclic measurements, and frequency of the cycles affect the metallurgy with a resultant drift, usually downward. You can learn more about industrial temperature measurement using versatile and low-maintenance thermocouples from this WIKA reference page.
Our website uses cookies. By continuing to use it, you agree to their use. Further information Agree. The following are the six most common causes of thermocouple measuring errors, followed by ways to rectify them: 1. Selecting the Wrong Type of Thermocouple on the Transmitter You can run into problems if you choose the wrong type of thermocouple when inputting the settings into the transmitter during installation.
Problems Related to the Thermocouple Extension Wire If you accidentally reverse the polarity of the thermocouple lead wires, the measured temperature will be incorrect by the difference in temperature of the two ends of the leads. Solution: Doublecheck the connection and, if necessary, swap the thermocouple lead wires. Inherent Variations in Alloys No two batches of wires are exactly alike. The temperature range of the various types of sheaths can be seen in the chart below.
Irradiated PVC has moderate cost, heat resistance, good electrical and physical properties, excellent abrasion protection, and is solder resistant.
Teflon has high cost, a high temperature rating, excellent chemical resistance, and electrical properties but has poor cut through resistance. Tefzel has high cost, good electrical and physical properties, and excellent chemical resistance. Kapton has excellent physical, electrical, and mechanical properties over a wide variety of temperature ranges and is used in applications where there is extreme heat and vibration.
It maintains its mechanical properties in the harshest of conditions. Polyethylene has low cost, excellent electrical properties, high flammability, and is stiffer than vinyl. Irradiated Polyethylene has moderate cost, excellent electrical, physical, and chemical properties, id flame retardant, and is resistant to environmental stress cracking. Polysulfone has low cost, good electrical properties, excellent physical properties, and good handling properties with poor chemical resistance.
Fiberglass excellent for high temperature applications and suitable for use with ambient temperatures where there is a possibility of hot spots. Ceramic is used in commercial ovens and furnaces and can monitor ambient temperatures of fireboxes, kilns, and grills.
Conductor Jacket can be put over the primary insulation and is necessary when additional mechanical protection is needed. A jacket for vinyl Insulation Is nylon with polyethylene used for vinyl or nylon insulation. A conductor jacket acts as a mechanical barrier and prevents shorting. Thermocouples come in different types for a variety of applications and use a system of letters to identify each type.
There is a wide range of thermocouple types with their own characteristics and temperature ranges. The difference between each type is determined by their durability, temperature range, resistance, and applications.
Types E, T, N, J, and K are the most common of the thermocouples and have base metals of iron, copper, nickel, and other such metals. Below is a description of each. Type E thermocouples have chromel, a nickel and chromium alloy, as a positive leg and constantan as the negative leg. Type E can be used in sub-zero temperatures and have colorings of red or purple. Type J thermocouples have iron for the positive leg and constantan as the negative one.
They are used in oxidizing, vacuum, inert, and reducing atmospheres. Type J thermocouples have to be closely monitored since the iron leg can rust.
Type K thermocouples have chromel for the positive leg and alumel for the negative. Alumel is an alloy made of mostly nickel with low percentages of aluminum, silicon, and manganese. Their color coding is red or yellow. The type N thermocouple is nicrosil, a nickel chromium alloy, as the positive leg and nisil, a nickel, silicon, and magnesium alloy, as the negative leg.
Noble metal thermocouples, or platinum thermocouples, are types B, R, S, and P with precious metal elements. They are accurate at very high temperatures and have a long lifespan of use. The Type B thermocouple is used in extremely high temperature applications and has the highest temperature limit of all of the thermocouples with exceptional accuracy and stability.
The Type R is used for high temperature applications and has a higher percentage of Rhodium than the Type S, which makes it more expensive. Type R has the same performance as Type S and can be used for low temperature applications because of its stability and high accuracy. The Type S is used for very high temperature applications in the BioTech and Pharmaceutical industries.
It is used for low temperature applications due to its accuracy and stability. A Type K extension wire is used to connect a Type P thermocouple to the measuring instrument. Thermocouples are widely used temperature sensors because of their wide temperature capabilities, ruggedness, and low cost. They are found in home appliances, industrial processes, electric power generation, furnace monitoring and control, food and beverage processing, automotive sensors, aircraft engines, rockets, and spacecraft.
Their small size and fast response, as well as their ability to endure shocks and vibrations, makes them perfect for temperature control and measurement. Thermocouples are perfect for the food industry because they supply accurate readings in a few seconds. Food products can be checked in any phase of production.
Food production thermocouples are a two piece unit with a handheld readout unit and detachable probe. In the tip of the probe are two wires connected to each other. Flat headed probes measure surface temperatures, needle probes take internal measurements and the air temp of ovens. Extruders require high temperature and pressure. The sensor tip has to be positioned in the molten plastic under high pressure conditions. The thermocouple measures the temperature and is directly installed into the process.
These units have high degree of accuracy, with a rapid response time, and can have a type K thermocouple probe. For very high temperatures, other exotic sheath materials are also available. View our line of high temperature exotic thermocouple probes.
The tip of the thermocouple probe is available in three different styles. Grounded, ungrounded and exposed. With a grounded tip the thermocouple is in contact with the sheath wall. A grounded junction provides a fast response time but it is most susceptible to electrical ground loops. In ungrounded junctions, the thermocouple is separated from the sheath wall by a layer of insulation. The tip of the thermocouple protrudes outside the sheath wall with an exposed junction.
Exposed junction thermocouples are best suited for air measurement. Surface Probe Measuring the temperature of a solid surface is difficult for most types of temperature sensors. In order to assure an accurate measurement, the entire measurement area of the sensor must be in contact with the surface. This is difficult when working with a rigid sensor and a rigid surface. Since thermocouples are made of pliable metals, the junction can be formed flat and thin to provide maximum contact with a rigid solid surface.
These thermocouples are an excellent choice for surface measurement. The thermocouple can even be built in a mechanism which rotates, making it suitable for measuring the temperature of a moving surface. Wireless Thermocouples These wireless transmitters measure different sensor inputs, including but not limited to pH, RTD, relative humidity. The data transmission is performed via wireless to a computer or a network. Which type of infrared sensing technology will work better for my application—an infrared thermometer or a thermal imaging camera?
Digital Panel Meter Applications. Also, a thermistor is more quickly, and irreversibly, damaged at temperatures above its stated limit. Speed A fast response is important in many applications. The speed of a temperature sensor is directly affected by its mass or size. The larger the sensor, or probe assembly, the slower the reading. A thermistor is made of a bead of carbon with two wires attached.
It is then coated with epoxy or glass. However, a thermocouple can still be made smaller than even the tiniest thermistors. Afterall, a thermocouple is made of two wires whereas a thermistor adds the bit of carbon and a coating. Using fine gauge thermocouple wire, the sensing bead or weld can also fit inside very narrow diameter tubes, such as hypodermic needles.
Probe Design Because a thermocouple is made from the junction of two different metals, the possibilities for various mechanical designs are greater than for other sensors. Very fine gauge wire can be used for tiny sensor assemblies. Flat wire is often used for surface thermocouples. Heavy gauge wire can be used for very high-temperature probes or for extremely rugged assemblies.
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