Temperature Sensors Workshop

Tempsens Instruments Pvt Ltd one of the largest manufacturer of temperature sensors in India with a long experience of more than 35 years in the field of measurement organized workshop on "Temperature Sensors". Here is the Glimpse of the workshop

Interview of Sh. V.P. Rathi published in Industrial Automation of Nov-2013 edition.

for interview details

Tempsens Instruments Pvt Ltd

What is Green rot effect?

Type K thermocouples is the hysteresis effect that occurs when a Type K thermocouple is cycled up and down in temperatures above and below 800°C, and when Oxygen is deficient. The re-ordering of the crystalline structure changes with each cycle. After the first pass above this temperature, the Type K temperature indication will probably be accurate. However, with each additional cycle after this one, the error will increase more and more. The Type K thermocouple also experiences a cumulative drift after a period of time at temperatures above 900degC. Finally, this thermocouple experiences a physical defect called “green rot” which is caused due to preferential oxidation of the chromel leg.

Platinum in Glass industry

Use of platinum in glass industries today is very vital as platinum is only metal which is un

wetted to glass, so to improve the life of the assemblies which are in direct contact with

molten glass platinum is essentially used.

In this paper we will discuss about an alloy of platinum (called hardened platinum) which is

specially developed to be used in molten glass temperature measurement with thermocouples.

Platinum Thimbles for Thermocouples

Thimbles made of special platinum alloy containing 10% Rhodium with

small% of Zirconium oxide or yttrium oxide (as grain stabilizer), are

developed specially for the application in glass industries.

Theses materials offer significantly improved performance over

conventional platinum and its alloys.

It is a new class of materials and has optimized it for special

applications:

 High strength with good ductility in the high temperature range (up to 1700°C).

 Excellent weld ability while the strength is maintained.

 Exceptional corrosion resistance and a more stable microstructure over longer service

times.

 Less re-crystallization.

These characteristics allow for longer service lives for

the individual components and permit the precious

metals to be used more economically, for instance

through reduced wall thicknesses. The higher

strength of the material also has a stabilizing effect

on the equipment manufactured from it. Strengthening components of e. g. molybdenum,

ceramics or refractory metals thus become effectively redundant. The inclusion of finely

distributed zirconia as a dispersion impedes grain growth to a temperature just below the

melting point. Due to the modified, finer microstructure, it is considerably less sensitive to

corrosion processes along the grain boundaries than comparable materials. This ensures better

corrosion resistance

Thermowells

Thermowells provide protection to temperature probes against unfavorable operating condition such as corrosive media, physical impact and higher-pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to "open-up" the process.

Thermowell take many different forms and utilizes a variety of material (usually stainless steel); there is a wide variety of thread or flange fitting depending on the requirement of the installation. They can either be drilled from solid material for the highest pressure integrity or they can take the form of a thermo pocket fabricated from tubing and hexagonal bushes or flanges; the later construction allows longer immersion length.

Thermowells transfer heat to the installed sensor and due to that thermal inertia is introduced.

Any temperature change in the process took longer time to affect the sensor if thermowells are present; thus the response time is increased. This factor must be considered when specifying a thermowell; except when thermal equilibrium exists, a temperature measurement will probably be inaccurate to some extent.

Optimum bore is an important parameter since physical contact between the inner wall of the thermowell and the probe is essential for thermal coupling. In the case of a thermocouple, which is tip sensing, it is important to ensure that the probe is fully sheathed (in contact with the tip of the thermowell). For Pt 100 sensor, which is stem sensing, the difference between the probe outside diameter and bore must be kept to an absolute 

Ultra High Temperature Thermocouples

High-Temperature Thermocouples

Temperatures up to 2000°C and above 

Tempsens offers special Ultra high temperature thermocouples for high temperatures upto 2300 °C for highly corrosive and/or reducing atmospheres. These thermocouples are offered in either Pl atinum/Rhodium (t ypes R, S , or B)  Tungsten/Rhenium (types C or D) elements, with a variety of insulations and sheath materials. Depending upon the sheath material selected, these thermocouples are used in inert, oxidizing, reducing or vacuum conditions. The maximum temperature is based on the lowest max. temperature of the element, insulation and sheath material. The use of high-temperature sheath materials in an oxidizing atmosphere is possible only to a limited (low) temperature. Excluded from this are sheath materials of platinum alloys. The table on page 4 of this product information serves as an indication. To a large extent these thermocouples are used in Temperatures up to 2000°C and above aeronautics, research laboratories and in industry. Ultra High Temperature Thermocouples Ÿ Custom - tailored designs available for many applications Ÿ Suitable for oxidizing, reducing, Ÿ Neutral atmospheres and vacuum Ÿ Pressure-/vacuum-proof bushings available in many forms Ÿ Transition elements variable within a wide range Ÿ Profile thermocouples available on request Ÿ Calibration at high temperature in Inert or vacuum atmosphereSpecial Advantages :In the case of long thermocouples it can be advantageous for cost reasons to have a transition to a different material – e.g. Inconel or stainless steel. The insertion length of the part exposed to the high temperature can be dimensioned as requested. also compensating cables can be used after the transition sleeve.For special applications it is possible on request to mount several thermocouples into a common protection tube. The position of the measuring points for profile thermocouples can be chosen within a wide range.These thermocouples are supplied with tracable calibration and material certificates, in case required each of the thermocouple can be calibrated in specific inert or vacuum atmosphere.

Temperature Sensor : Thermocouple Basics

Temperature Sensor : Thermocouple Basics

Let's start with T. J. Seebeck, who in 1821 discovered what is now termed the thermoelectric effect. He noted that when two lengths of dissimilar metal wires (such as iron and Constantan) are connected at both ends to form a complete electric circuit, an emf is developed when one junction of the two wires is at a different temperature than the other junction. 

  Basically, the developed emf (actually a small millivoltage) is dependent upon two conditions: (1) the difference in temperature between the hot junction and the cold junction. Note that any change in either junction temperature can affect the emf value and (2) the metallurgical composition of the two wires. 

  Although a "thermocouple" is often pictured as two wires joined at one end, with the other ends not connected, it is important to remember that it is not a true thermocouple unless the other end is also connected! It is well for the user to remember this axiom: "Where there is a hot junction there is always a cold or reference junction" (even though it may seem hidden inside an instrument 1,000 feet away from the hot junction). 

  Still in Seebeck's century, two other scientists delved deeper into how the emf is developed in a thermoelectric circuit. Attached to their names are two phenomena they observed--the Peltier effect (for Jean Peltier in 1834) and the Thompson effect (for Sir William Thompson a.k.a Lord Kelvin in 1851). Without getting into the theories involved, we can state that the Peltier effect is the emf resulting solely from the contact of the two dissimilar wires. Its magnitude varies with the temperature at the juncture. Similarly, the Thompson effect can be summarized as having to do with emf's produced by a temperature gradient along a metal conductor. Since there are two points of contact and two different metals or alloys in any thermocouple, there are two Peltier and two Thompson emfs. The net emf acting in the circuit is the result of all the above named effects. 

  Polarity of the net emf is determined by (a) the particular metal or alloy pair that is used (such as iron-constantan) and (b) the relationship of the temperatures at the two junctions. The value of the emf can be measured by a potentiometer, connected into the circuit at any point. 

  In summary, the net emf is a function primarily of the temperature difference between the two junctions and the kinds of materials used. If the temperature of the cold junction is maintained constant, or variations in that temperature are compensated for, then the net emf is a function of the hot junction temperature. 

  In most installations, it is not practical to maintain the cold junction at a constant temperature. The usual standard temperature for the junction (referred to as the "reference junction") is 32°F (0°C). This is the basis for published tables of emf versus temperature for the various types of thermocouples. 

  The Law of Intermediate Temperatures provides a means of relating the emf generated under ordinary conditions to what it should be for the standardized constant temperature (e.g., 32°F). Referring to Figure 4-1, which shows thermocouples 1 and 2 made of the same two dissimilar metals; this diagram will provide an example of how the law works. Thermocouple 1 has its cold junction at the standard reference temperature of 32°F and its hot junction at some arbitrary intermediate reference temperature (in this case, 300°F). It generates 2.68 mv. Thermocouple 2 has its cold junction at the intermediate reference point of 300°F and its hot junction at the temperature being measured (700°F). It generates 4.00 mv. The Law of Intermediate Temperatures states the sum of the emfs generated by thermocouples 1 and 2 will equal the emf that would be generated by a single thermocouple (3, shown dotted) with its cold junction at 32°F and its hot junction at 700°F, the measured temperature. That is, it would hypothetically read 6.68 mv and represent the "true" emf according to the thermocouple's emf vs. temperature calibration curve. 

  Based upon this law, the manufacturer of an infrared thermocouple need only provide some means of substituting for the function of thermocouple 1 to provide readings referenced to the standard 32°F cold junction. Many instruments accomplish this with a temperature-sensitive resistor which measures the variations in temperature at the cold junction (usually caused by ambient conditions) and automatically develops the proper voltage correction.