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Helping thermocouples do the job...
If it's true that temperature is the most widely measured process parameter, then it is also probably true that thermocouples (TC's) are the most widely used process sensor. The following article discusses some problems often met while using thermocouples and provides some practical solutions.
"Green Rot" attack
Type "K" thermocouples are widely used for temperature measurement and control up to about 2000 Deg F. They operate very well in oxidizing atmospheres. However, if a reducing gas (such as hydrogen) is present, a reducing atmosphere can come in contact with the wires. Under these conditions, with only a very small amount of oxygen present, the chromium in the chromel alloy oxidizes. This reduces the emf output and the thermocouple reads low. This phenomenon is known as "green rot," due to the color of the affected alloy. Although not always distinctively green, the chromel wire will develop a mottled silvery skin and become magnetic. An easy way to check for this problem is to see if the two wires are magnetic. (Normally, chromel is non-magnetic.)
Hydrogen in the atmosphere is the usual cause of green rot. At high temperatures, it can diffuse through solid metals or an intact metal thermowell. Even the sheath of a magnesium oxide insulated thermocouple will not keep the hydrogen out.
To overcome this problem, a "purged" thermowell is used. Here, a flow of air is brought down through a small tube inside the thermowell to sweep out any hydrogen which has entered the well. (See Figure 1) The small air flow becomes heated on its way down the tube, so it doesn't chill the sensing junction.
High Temperature Measurements
At temperatures higher than about 2200 Deg F, all the normally used thermocouple alloys are to close to their melting points to be used. The only available metals with significantly higher melting points are molybdenum and tantalum (4730 Deg F and 5425 Deg F respectively). However, both oxidize very badly in air (or an oxidizing atmosphere) at fairly low temperatures. Applying a protective, disilicized coating to molybdenum will enable it to be used up to about 3000 Deg F, however, the range of available sizes is limited and these thermocouples are expensive.
Another solution is to use closed-end ceramic protection tubes to contain the thermocouples. This allows the use of platinum/platinum-rhodium or tungsten/tungsten-rhenium thermocouples. The platinum types are good in oxidizing or inert atmospheres, but are not recommended for vacuum or reducing conditions. On the other hand, tungsten/rhenium thermocouples are not suitable for oxidizing conditions, but can be used in a clean neutral or reducing atmosphere such as dry hydrogen. Platinum type thermocouples with 10% or 13% rhodium (type S and R) are recommended for use up to 2550 Deg F. Above this, type "B" (platinum-30% rhodium/platinum-6% rhodium) should be used. This is satisfactory up to 3100 Deg F. Tungsten 5% rhenium/tungsten 26% rhenium can be used up to 5000 Deg F.
High purity alumina is the most suitable protection tube for platinum type thermocouples. It is preferred to Mullite (which contains silica) because of the danger of silicon being reduced and contaminating the platinum. High purity alumina isolators should also be used.
Alumina protection tubes are gas-tight and can be used up to the limit of the platinum type thermocouples. However, they are liable to crack if subjected to thermal shock. This problem can be solved by installing the alumina tube inside an outer protection tube of silicon carbide (see Fig. 2).
Multicouples and "swamping boxes"
To measure and control the temperature in a large reaction vessel, you often need to insert a large thermowell made from pipe in which a number of thermocouples are installed at different depths. The thermocouples may be spring-loaded against the inner wall of the thermowell, as shown in Fig. 3. Another arrangement uses guide tubes welded to plugs, which in turn are welded into the thermowell at the desired locations (see Fig 4). The thermocouples are then inserted into these guide tubes and bottomed in the plugs. With both arrangements, it's possible to withdraw and change thermocouples without removing the thermowell.
In some applications, individual readings must be monitored. In others, however, an average of the signals is needed. This is obtained by connecting the extension wires in parallel, provided the resistance of the different circuits is similar. If there are large differences, errors will result. Owing to the different lengths of the thermocouples in a large vessel, there will, in fact, be significant differences. However, if a 500 ohm resistor is included in each circuit, the percentage difference in resistance between the circuits becomes negligible. Figure 5 shows a typical "swamping box" assembly with the 500 ohm resistor in series with each thermocouple.
For some applications, thermowell resistance to environments can be greatly improved with surface applied coatings. Thus, a thermowell exposed to abrasion may be provided with a hard surface by applying one of the "Stellite" or "Colmonoy" alloy coatings to the outside. While hardness levels tend to fall off with increasing temperature, some of these coatings remain quite hard (e.g. Rockwell C45) up to at least 1000 Deg F.
Coatings can also increase the resistance to attack by sulfur at high temperatures. Using a process known as "Alonizing" (Alon Processing Inc.) a diffused aluminum coating can be applied to high temperature alloys such as Inconel 600 or Incoloy 800.
Thermowells are often used to protect thermocouples when monitoring the temperature of highly corrosive chemical solutions and agents. Solid machined and drilled tantalum thermowells are good for these applications. But to save money, a thin (0.013 - 0.015 inch thick) tantalum sleeve is often used. This is fitted over a thermowell made of type 316 stainless steel (see Fig. 6).
Content for this article was provided compliments of Thermo Electric
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