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Introduction to Thermotech USA
Thermocouple verses RTD
Accuracy
TC Measurement Range
How a TC Works
Common TC’s
TC Junctions ( Hot)
RTD’s
RTD Construction
Sheath Metals: Materials and Limits
Element Fittings and Accessories
Thermotech USA was established in 1990 in Baton Rouge, Louisiana. We manufacture industrial grade heat sensors and associated products to go with the sensors. This includes, but is not limited to, thermocouples and RTD’s. The basic difference in these two categories of heat sensors are explained below.
Both thermocouples and RTD’s are used to measure temperature. The thermocouple produces a small voltage proportional to the temperature and the RTD has a change in resistance due to the change in temperature that the element senses. Thermocouples and RTD’s sheated in 316 stainless steel tubing will look very similar. Without access to view the wires, you cannot tell the difference. Back to Thermotech USA index
Thermocouples are less accurate than RTD's. Typical accuracy for thermocouples is +/- 4 F for standard and +/- 2 F for special limits accuracy. RTD's accuracy statements are listed a different way. RTD’s accuracy is based on deviation from the ideal value for a certain temperature. Standard deviation is 0.1%/degree C change from the base calibration of the RTD. Higher accuracy RTD’s are available up to 0.01% know as band 5 accuracy. Back to Thermotech USA index
<pre>Thermocouples can measure a higher temperature range than RTD’s. RTD’s limits are approximately 1300 F. Even the very common type J thermocouple will measure up to around 1600 F. Thermocouples using the K type will measure up to 2300 F maximum (1800 F continuous measurement). For very high temperatures the type R or S are used and can measure up to 3000 F (2300 F continuous measurement)
See Thermocouple Specifications Back to Thermotech USA index
The TC is a very simple device that is simply the junction of two dissimilar metals. The junction of the two metals produces an electric voltage. The voltage produced is a very small direct current voltage and is actually in the millivolts ( mvDC ) range. Any two dissimilar metal wires will produce this voltage, however, certain types of thermocouples are selected as standard because of the characteristics of the voltage output. The mvDC must be sufficient in magnitude to amplify so other instrumentation can utilize it for display and control of temperature. In addition to being a strong enough signal, it also should be somewhat linear over a range of temperature. This is the reason for the standard TC types as listed below:
Types: Industry accepted common standard thermocouples are: J, K, E, T, R and S. There are several more types of thermocouples, however these are the most common with J and K being by far the most common.
Thermocouple elements are made with three types of hot junctions. The hot junction is the joint of the dissimilar metals that is where the temperature measurement is desired. These junctions are: Grounded; Ungrounded and Exposed. 1) Grounded is most common and is faster in response to temperature changes than ungrounded. 2) Ungrounded is as implied not connected to the metal sheath and therefore electrically isolated from the process lines or vessel. 3) Exposed tip is the fastest of the three, however there is no protective sheath at the tip with this hot junction type. Back to Thermotech USA index
RTD’s are a passive device, unlike the thermocouple, and does not emitt a signal of any type. An RTD is a winding similar to an electrical coil with very fine wire. The most commonly used wire used is platinum with nickel and copper as other choices. RTD elements are wound to specific ohm value at a certain temperature. The most common value is 100 ohms at 0 C (32 F) or ice point. Thus a 100 ohm, platinum RTD is generally taken to mean a 100 ohm platinum element that is exactly 100 ohms ( + / - the allowable tolerance) at 0 C. The resistance of the element increases in ohm’s value with an increase in temperature. The exacting requirements for precision and the use of platinum are reasons for the RTD temperature sensor to be a greater relative cost than thermocouples.
RTD’s are usually sheathed in stainless steel tubing as are thermocouples. Our standard is 316 stainless steel and the most common size, as with thermocouples, is 1/4 inch outside diameter tubing. The wiring used in Thermotech USA’s RTD’s is 20 gauge silver plated stranded copper wire. The wire is covered with a Teflon jacket and cut to customer specified lengths or our 6 inche standard if not specified. There is no positive or negative wire on RTD’s as they are passive and only change resistance. There can be 2, 3 or 4 wires depending on customer requirements. The extra wires for 3 or 4 wire elements are for longer lead line resistance error compensation. RTD’s are superior to thermocouples for accuracy of temperature measurement. RTD’s by Thermotech USA are manufactured for two basic temperature ranges. The standard RTD measures up to 350 F. Above the standard range, the high temperature construction should be used and it has a maximum temperature capability of 1300 F. Back to Thermotech USA index
The following are the maximum continuous operating temperature: Material Temperature 304 SS 1650 F 316 SS 1700 F 310 SS 2100 F Inconel 2100 F
In almost all cases we recommend the spring loaded fitting fitting for both elements. The spring loading keeps the tip ( this is the hot junction and where the temperature measurement is made) of the element in positive contact with the bottom of the thermowell. by keeping this tip in positive contact, the fastest response to a change in temperature is possible. If no well is used then this does not apply. Back to Thermotech USA index Connector Heads:
Thermotech can supply a variety of connecting heads for both RTD’s and thermocouples with terminal blocks. They are available in either weatherproof (WP) or explosion proof (X). Materials for WP heads are: cast aluminum, cast iron, deldrin plastic or polypropylene plastic. These heads can accommodate from a single thermocouple ( 2 connector) to a triplex sensor (6 connector). Back to Thermotech USA index Nipple-Union:
Various nipples and union combinations can be used in either galvanized steel or stainless steel. Back to Thermotech USA index
Specials:
Thermotech USA can build most any type of special if a drawing can be supplied for construction. Please contact us for you heat sensing requirements.
Email: thermotechusa@msn.com
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