Thermocouples’ Mode of Operation
In simple terms, thermocouples are temperature sensors. They are created by joining two dissimilar metals on one end and terminating them on the other. The joined end and the terminated end are referred to as sensing junction and reference junction, respectively. At the reference junction, temperature is always maintained at a constant, this temperature, called the reference temperature, creates a difference in temperature with the sensing junction. When the temperature at the two junctions differs, a potential difference is created and current flows in the circuit.
Due to the binding energies of electrons to metal ions, a thermoelectric voltage is created. The amount of voltage will be determined by the temperature and the metal ions used in the thermocouple instruments. The production of thermal voltage is facilitated by presence of a closed circuit and the phenomenon is referred to as ‘Seebeck effect’.
Past the measuring junction, each thermocouple wire must be electrically separated from the other. The temperature at the reference junction is maintained at 32 degrees Fahrenheit; at this temperature, no EMF is generated. When the temperature at the measuring junction changes, the pair of metals used on thermocouples will have a unique EMF variation. This variation is known as thermocouples variation.
Seeing that thermocouples are differential devices and not absolute temperature measuring devices, the temperature at the reference junction must give an accurate absolute temperature reading. This process is referred to as the reference junction compensation.
Thermocouples are the standard, cost-effective ways of measuring varying temperatures in many industries, and they are preferred for their accuracy. These devices find their application in many gadgets and machines up to +2500 degrees Celsius; they are used in boilers, ovens, aircraft engines, and water heaters among others. Type K thermocouples are the most common in industries, but there are also other thermocouple types; industries choose based on their needs.
Thermocouple Instruments - Hotwatt, Inc.
Why Use Thermocouples?
Thermocouples have a wide range of applications and a horde of advantages. First off, they can measure a wide range of temperature. Depending on the type of wire used, thermocouples can measure temperatures as low as -200 degrees Celsius and as high as +2500 degrees Celsius. These devices are rugged; they are not affected by shock or vibrations and as such, they can be used in busy environments.
The low capacity and small size of thermocouples enable them to respond rapidly to small temperature changes especially where the sensing junction is exposed. In normal cases, they respond to temperature change in less than a second. Thermocouples do not need excitation power and as such, they are not prone to self-heating, which may destroy them; this way, they are intrinsically safe.
Issues with Thermocouples
The thermocouples’ voltage needs to be converted to a temperature reading and this takes substantial signal conditioning. Signal conditioning is the process of converting one type of electronic signal into a different type ready for the next step in signal interpretation. A while ago, conditioning needed a lot of investment and when not done right, inaccuracies may be seen on the final reading. Besides these metallurgical properties, the thermocouple accuracy affected is as good as the measurement of the reference junction temperature.
Seeing that thermocouples are created with two dissimilar metals, they are susceptible to corrosion. In some environments, continued corrosion leads to inaccuracies. Ergo, thermocouples need to be protected either by use of protecting tubes or insulating material, and maintained at all times. Thermocouples are able to pick micro-volt level changes in signal and as such, noise from magnetic and electrical fields can result to errors and inaccuracies.
Stray noise from magnetic fields is curbed by twisting the thermocouple wires together while noise from electrical fields is reduced by use of a shielding cable. Again, the measuring device needs to filter signals either by use of hardware or software to reject the frequency line 50 Hz/60Hz and related frequencies.
Thermocouples are classified based on the material used to make them. The combination of materials are approved and standardized by relevant authorities; ergo, their combinations and color coding must be followed for universal cohesion. The different types of thermocouples are designated with a combination of letters, E, J, K, T, S and R. Some of these are more common than others, such as E, J and K type thermocouple.
Type E thermocouples are created with a combination of Chromel and Constantan, both alloys of Nickel, Nickel-Chromium and Copper-Nickel. This type is able to measure temperatures between -200 degrees Celsius to +900 degrees Celsius. Its color code is purple.
J type thermocouple is created with a combination of iron as the positive and constantan as the negative. A type J thermocouple measures between 0 degrees to 750 degrees Celsius. The color code of J type thermocouple is black.
A K type thermocouple is made of Alumel and Chromel, Nickel-Aluminum and Nickel Chromium. A type K thermocouple is able to measure between 0 degrees and 1100 degrees Celsius. The color code for this type is green.
Type T thermocouples feature copper vs constantan used in type J thermocouple. They measure temperatures between -185 and 300 degrees Celsius. The color code is brown.
Type N thermocouples are created with Nicrosil and Nisil and can measure temperatures between 0 to 1200 degrees Celsius. They are pink in color.
R and S type thermocouples are almost similar; both are offered with a Platinum Rhodium and Platinum but at different concentrations, giving the capacity to measure between 0 and 1600 degrees Celsius and between 0 and 1550 degrees Celsius respectively. They are both color coded orange.
Type B thermocouples are made of platinum 30 percent rhodium and platinum 6 percent rhodium and measure between 0 and 1600 degrees Celsius. They are coded color grey.
A thermocouple wire is expensive because it is created to meet stringent quality as set by the industry. To this end, most thermocouple instruments are created with extension wires closer to the thermocouple meter or to the closest convenient point. These connection points need to be isothermal to each other. The extension wires are fabricated to a lesser quality.
As shown above, all wires are color coded as per international standards to help distinguish between different types of thermocouples.
Reference Junction Compensation
For accurate absolute temperature readings, the reference junction temperature must be known. In conventional thermocouples, the reference junction is dipped in an ice bath, 0 degrees Celsius. This method characterizes all types of thermocouples.
Keeping the junction in an ice bath does not work for most applications. To this end, most systems apply reference junction compensation where a temperature sensitive device such as an IC, diode, resistance temperature detector (RTD) thermocouple, or a thermistor is used. The reference junction is allowed to have temperature variations. The heat sensitive device measures the temperature at this junction; the measurements are used to compensate for the temperature variations at the junction. The final thermocouple accuracy depends on the accuracy of the reference junction temperature reading.
The thermocouple sensor used does not affect accuracy much as long as the reading is taken accurately. Some of the thermocouple reader types used in the market include:
Thermistors come in very small packages and they offer fast response. However, these sensors require linearization before use and their accuracy worsens as the temperature range increases. Thermistors need excitation current, which may result to self-heating. Compared to other thermocouple reader types, the overall accuracy of thermistors is poor.
An RTD thermocouple is loved due to its high accuracy and stability. They also do not require linearization. However, they are relatively larger and they cost more than other any other thermocouple sensor.
Remote Thermal Diodes
When used near thermocouples connectors, diodes are excellent temperature sensors. Diodes are attached to a conditioning chip, which converts their voltage to a digital or an analog output. The accuracy of diodes sway ±1 degree Celsius.
Integrated Temperature Sensors
Here, a standalone IC is connected close to the reference junction. This IC combines signal conditioning with reference junction compensation for accurate readings. Like diodes, ICs give readings that are ±1 degree Celsius accurate.
Thermocouple Instruments Junctions
There are three main junctions used for industrial application thermocouples. They include:
True to its name, this junction is in touch with the environment. The exposure to environment ups the sensitivity of the junction and consequently reduces the response time. However, it has a short life span; as such, it is seldom used.
This type of junction is created for rapid responses. It has good electro-magnetic shielding properties making it ideal in a high number of applications. It is common in electrical equipment though it has also been used in other industries.
This is created with more response time than the ungrounded junction. Its shielding properties are great. Given its speed of response, the junction is commonly used in oil and gas industries for control applications.
For protection, thermocouples may be mounted inside Thermowells. This is normally the case when the thermocouples are used to measure the temperature of a flowing fluid. When this happens a thermocouple probe is sheathed and covered in a tube that is corrosion resistant. The sheath is created a quarter inch in diameter and the thermocouple probe is fitted with a spring to ensure that it stays in contact with the Thermowell bottom at all times.
Inside the thermocouple probe, electrical connections are done by a screw cover head. A ceramic insulation material is used to separate wires and sheath. There are different variations of probes as expounded below.
True to its name, this type of a probe is clamped on to the fluid carrying pipe clamping thermocouples' measuring junction to the pipe. This method comes in handy where a temperature reading is needed and earlier provisions were not made. Again, it can be used to troubleshoot a process acting as a thermocouple thermometer. However, the measurements may not be accurate unless the surrounding is well insulated from the outer environment.
Clamp-on thermocouples can also be welded on the surface of the items whose temperature is to be measured for instance a furnace tube or clamped on the outer surface for instance on reactor walls. There are different variations of clamp-on thermocouples, which should be selected based on the needs of the buyer.
Duplex thermocouples are in doubles. They are created like conventional thermocouples but instead of a single wire on each type, they feature two wires. This means they have two chromels and two alumel wires at the measuring junction in a K type thermocouple. These two wires then branch out to different circuits offering two distinct readings. When both circuits are created similar, the two readings are the same. These types of thermocouples are ideal for comparing one instrument against another especially when a thermocouple meter, such as a test voltmeter is used; this determines whether a remote reading comes out wrong because of problems in the circuit or in the instrument.
The cost of producing duplex thermocouples is less since they can be created in one assembly. However, when one circuit develops a fault, the entire system has to be replaced. Granted, duplexes are less reliable compared to two individual thermocouples.
The circuits of thermocouples can be grounded or ungrounded. For safety reasons, a grounded thermocouple is preferred. Not only that, grounding reduces electrical noise and enhances the response time of thermocouples. Ungrounded circuits are ideal where faults may develop as a result of grounding or where lightning may strike as a result of grounding; this is mostly in farm areas.
Grounding is made on the negative side of the instrument. For better results, the grounding should be done at the source rather than at secondary instruments; by so doing, common noise is curbed. As a rule of thumb, grounding should be done at only one place.
Thermocouples can be classified as:
- Intentionally ungrounded
- Intentionally grounded
- Unintentionally grounded
Unintentionally grounded thermocouples are used where there is no contact between the measuring junction and the thermocouples or where there is a bad contact. They are also used when there is formation of high resistance film of chemicals at the measuring junction.
Whether it’s a thermocouple thermometer or any other thermocouple type, it must be shielded from noise. This is mostly done by use of covering the wires with a grounded metal shield.