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