Furnace temperature is measured by following methods namely (i) Electrical pyrometers (2) Thermo electric pyrometers (3) Radiation pyrometers (4) Optical pyrometers and (5) Photoelectric cells.
An important radiation pyrometer is discussed as under.
Some years ago Siemens devised a pyrometer by which the temperature was inferred from the increased resistance of a platinum wire coiled on a cylinder of fire-clay and exposed to the heat it was desired to measure. Any increase in the temperature of the wire causes a corresponding increase in resistance, the law of which is known. The resistance at 0°C being ascertained, it can also be calculated for a higher temperature, and as the resistance can readily be measured the temperature is easily calculated. This is the principle upon which the electrical pyrometer of Siemens was based, but the instrument has not stood the test of time. The resistance offered by the platinum to the passage of the current was not found to remain constant after several successive heatings, unless the coil spindle was made of porcelain and the encasing tube of platinum, the former of which is of too fragile and the later too costly for industrial use.
Thermo electric pyrometers
Pyrometer is an instrument used to measure very high temperatures such as the temperature of a furnace. It worked on the principle of thermo electric effect. A thermocouple o Pt and an alloy of Pt-iridium is usd. Two long wires of these two materials are threaded through fine porcelain tubes and their ends fused together at the bottom. This arrangement is placed in an outer porcelain tube having two terminals at the top. A moving coil galvanometer can be connected to these two terminals. The thermoelectric current produced can be detected by the galvanometer and the galvanometer scale is calibrated. Pyrometers are common used to check any variation in the temperature of finances in Thermo-electric Pyrometer metallurgy.
Radiation pyrometer is a temperature device by which the temperature of hot body is measured by measuring the intensity of total heat and light radiation emitted by a hot body. A radiation pyrometer works on the principle of focusing the energy from all wave lengths of radiation upon a sensitive element thus causing a force to be developed in the element which may be a resistance pyrometer or a thermo couple. All the radiations are concentrated by concave mirror upon the hot junction of a thermo couple. Mirror placed in inclined position is used to indicate whether correct focus can be obtained.
The optical pyrometer is a non-contact type temperature measuring device. It works on the principle of matching the brightness of an object to the brightness of the filament which is placed inside the pyrometer. The optical pyrometer is used for measuring the temperature of the furnaces, molten metals, and other overheated material or liquids. It is not possible to measures the temperature of the highly heated body with the help of the contact type instrument. Hence the non-contact pyrometer is used for measuring their temperature.
The optical pyrometer is shown in the figure below. It consists the lens which focuses the radiated energy from the heated object and targets it on the electric filament lamp. The intensity of the filament depends on the current passes through it. Hence the adjustable current is passed through the lamp.
Photoelectric cell or photocell, device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material. A battery or other voltage source connected to the electrodes sets up a current even in the absence of light; when light strikes the semiconductor section of the photocell, the current in the circuit increases by an amount proportional to the intensity of the light. In the phototube, an older type of photocell, two electrodes are enclosed in a glass tube—an anode and a light-sensitive cathode, i.e., a metal that emits electrons in accordance with the photoelectric effect. Although the phototube itself is now obsolete, the principle survives in the photomultiplier tube, which can be used to detect and amplify faint amounts of light. In this tube, electrons ejected from a photosensitive cathode by light are attracted toward and strike a positive electrode, liberating showers of secondary electrons; these are drawn to a more positive electrode, producing yet more secondary electrons—and so on, through several stages, until a large pulse of current is produced. Besides its use in measuring light intensity, a photomultiplier can be built into a television camera tube, making it sensitive enough to pick up the visual image of a star too faint to be seen by the human eye. The photovoltaic type of photoelectric cell, when exposed to light, can generate and support an electric current without being attached to any external voltage source. Such a cell usually consists of a semiconductor with two zones composed of dissimilar materials. When light shines on the semiconductor, a voltage is set up across the junction between the two zones. A phototransistor, which is a type of photovoltaic cell, can generate a small current that acts like the input current in a conventional transistor and controls a larger current in the output circuit. Photovoltaic cells are also used to make solar batteries (see solar cell). Since the current from a photocell can easily be used to operate switches or relays, it is often used in light-actuated counters, automatic door openers, and intrusion alarms. Photocells in such devices are popularly known as electric eyes.