The word pyrometer is derived from the Greek word “Pyro” which means fire and “meter” means measurement so it means measurement of fire. Pyrometer is a device used to measure the temperature of molten metal. The thermocouple pyrometer that utilizes interchangeable thermocouple tips on immersion temperature measurement applications.
Principle of Operation
It works on the principle of Seebeck effect. The See beck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances or in other words The Seebeck effect is the conversion of temperature differences directly into electricity.
When heat is applied to one of the two conductors or semiconductors, heated electrons flow toward the cooler one. If the pair is connected through an electrical circuit, direct current (DC) flows through that circuit. The voltages produced by See beck effect is small, usually only a few micro volt's per Kelvin of temperature difference at the junction.
If the temperature difference is large enough, some See beck-effect devices can produce a few mill volts. The Seebeck effect is commonly used in a device called a thermocouple (because it is made from a coupling or junction of materials, usually metals) to measure a temperature difference directly or to measure an absolute temperature by setting one end to a known temperature.
The field of molten metal temperature is unique in terms of the speed of measurement. An expandable thermocouple is dipped for barely 3-4 seconds and then withdrawn. Since the temperature signal is available for a very short period, the sensed temperature value has to be stored in memory so that it can be displayed until required.
Since the immerse period is very brief, the status of measurement cannot be properly determined by the operator. Thus an audio-visual alarm system compensates with the speed and situation of measurement has to be part of the system so as to tell the operator the status as measurement i.e. when to dip and when to withdraw the lance.
Molten melt temperature measurement systems is a high versatile equipment and works on the principle of plateau/averaging and digital filtering technique. The mv output of thermocouple is suitable amplifier by the input amplifier circuit. The necessary offset for the cold junction compensation is provided by the CJC circuit. The amplifier output is led to the ADC. A CPU monitors various output of the ADC and controls timers operations. The timer ADC and CPU together liberalize the mv/temperature curve and convert the mv input to an equipment temperature output in a digital form.
The CPU controls the display unit. The temperature equivalent of mv generated by thermocouple is displayed on the seven segment display. Once the measurement is over, the bath temperature is frozen on the display unit a fresh thermocouple is fixed or the system is reset or switch off.
When the thermocouple is fixed and is healthy the green led on the front panel and the green lamp will glow. The instruments will display the room temperature. After the thermocouple is dipped into the molten metal and the temperature crosses software controlled preset value (usually 2000 degree Celsius) the amber led on the instrument and the amber lamp will glow which indicates that the measurement is in progress. A steady rise in temperature is recorded on the display.
During measurement if plateau is detected within 1.2 seconds further measurement is aborted and by digital filtering technique the average of the reading within the 5 degree Celsius are taken. When the plateau is detected the CPU stores the temperature value in its memory and displays it. The red led (over) lights up and the amber led goes off. The hooter is actuated for 3 seconds. This indicates successful completion of measurement. The measuring temperature is latched and display on the instrument.
If no plateau is detected then the measurement goes through the complete cycle of M.I.P time set and after the pre determined time average of all the peaks which are within 5 degree Celsius are taken and by digital filtering technique temperature measured at the end of the set time duration is stored in the memory and displayed LED, lamp, hooter signals.
Model : MELTEMP-MK-1
Output : 4 to 20 mA
Input : 230V, 5A single phase AC
Resolution : 1 degree Celsius
Accuracy : 0.1% of full scale reading, +/- 1 count
Ambient : o to 55 degree Celsius
Supply : 230V AC, 50HZ, single phase
Type of TC : S/R/B or K type
Measuring Range : For S/R type TC 1200-1750 degree Celsius
For K type TC 650-1350 degree Celsius
For B type TC 1200-1820 degree Celsius
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat into electric power. They are inexpensive and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius (C) can be difficult to achieve.
Any junction of dissimilar metals will produce an electric potential related to temperature. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals.
Cold Junction Compensation:
Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of the junctions—normally the cold junction—is maintained at a known reference temperature, and the other junction is at the temperature to be sensed.
Having a junction of known temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitive device such as a thermistor or diode to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.
The Type E thermocouple is suitable for use at temperatures up to 900°C (1650°F) in a vacuum, mildly oxidizing or reducing atmosphere. This thermocouple has the highest EMF output per degree of all the commonly used thermocouples.
The Type J may be used, exposed or unexposed, where there is a deficiency of free oxygen. For cleanliness and longer life, a protecting tube is recommended. Maximum recommended operating temperature is 760°C (1400°F).
Due to its reliability and accuracy, Type K is used extensively at temperatures up to 1260°C (2300°F). It's good practice to protect this type of thermocouple with a suitable metal or ceramic protecting tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric furnaces, tube protection is not always necessary when other conditions are suitable; however, it is recommended for cleanliness and general mechanical protection.
This nickel-based thermocouple alloy is used primarily at high temperatures up to 1260°C (2300°F). While not a direct replacement for Type K, Type N provides better resistance to oxidation at high temperatures and longer life in applications where sulphur is present.
This thermocouple can be used in either oxidizing or reducing atmospheres, though for longer life a protecting tube is recommended. Because of its stability at lower temperatures, this is a superior thermocouple for a wide variety of applications in low and cryogenic temperatures. It's recommended operating range is— -200° to 350°C (-330° to 660°F).
Types S, R and B:
Maximum recommended operating temperature for Type S or R is 1450°C (2640°F); Type B is recommended for use at as high as 1700°C (3100°F). These thermocouples are easily contaminated. Reducing atmospheres are particularly damaging to the calibration. Noble metal thermocouples should always be protected with a gas-tight ceramic tube, a secondary tube of alumina and a silicon carbide or metal outer tube as conditions require.
REMOTE LARGE DISPLAY UNIT
REMOTE STATUS INDICATOR WITH LAMPS The lamps indicate "READY", "MEASUREMENT IN PROGESS" AND "MEASUREMENT OVER" , using the relay outputs of the Model AUP-500. These indications are of great help to the operator, as they indicate whether the thermocouple is properly inserted into the lance before taking a reading and also whether the reading was completed without thermocouple burn out; in case the thermocouple becomes open before the sampling process is complete "Measurement Over" will not be indicated