Now-a-days, electronic measuring instruments are required for all the applications like industrial, medical, biomedical, Aerospace, etc. They are based upon basic principles of measuring resistance, temperature, pH, viscosity, pressure, humidity, etc. They are based on transducers, which convert physical quantity into electrical quantity, which are later on measured and calibrated.
Optical pyrometers are used to measure :
High temperatures
The name gives the answer. Pyro is Greek for fire, and a pyrometer is by definition an instrument for measuring high temperatures — option 3.
The principle it works on. Every hot body radiates, and the spectral radiance at wavelength λ is given by Planck's law:
\(L_\lambda=\dfrac{C_1}{\lambda^{5}\left(e^{C_2/\lambda T}-1\right)}\)
An optical pyrometer measures that radiance and infers T from it. It is therefore a non-contact instrument — nothing is inserted into the hot zone at all.
How the classic disappearing-filament instrument works. The observer views the hot object through a telescope, and superimposed on the image is the filament of a calibrated tungsten lamp. The lamp current is adjusted until the filament disappears against the background — at that moment the two brightnesses are equal, and the lamp current, already calibrated against a black body, reads the temperature directly. A red filter is used so that the comparison is made at a single wavelength, typically 0.65 μm, which makes the match sharp and the calculation valid.
Its range, and why it starts high. Practical instruments cover roughly 700 °C to 3000 °C and beyond. The lower limit exists because below about 700 °C a body does not glow visibly — there is nothing for the eye to match. That is exactly why option 2, low temperatures, is wrong: a thermocouple, RTD or thermistor is used there instead.
Why options 1 and 4 fail. Radiant intensity is what the instrument senses, not what it measures. Brightness is merely the intermediate variable; the calibrated output is temperature. A device for measuring light intensity as an end in itself is a photometer or radiometer.
Where non-contact measurement is indispensable. Molten steel and glass, furnace interiors, rocket exhausts and rotating or moving objects — anywhere a probe would melt, contaminate the process or be impossible to attach.
The main source of error. Planck's law describes a black body. A real surface has emissivity ε < 1, so it radiates less and the instrument under-reads. Emissivity correction is the chief practical difficulty in pyrometry, and it is why two-colour (ratio) pyrometers exist — taking the ratio of radiance at two wavelengths largely cancels the unknown emissivity.
Hence, optical pyrometers are used to measure high temperatures.
Similar Questions
Read the following statements regarding errors in measurement :
(a) Gross errors are mostly due to lack of knowledge and care on the part of the experimenter.
(b) Systematic errors are mostly caused by inherent short-comings of the instruments and components used in measurement.
(c) Random errors are due to sources and causes that cannot be specified for their effects with.
(d) Gross and systematic errors are non-correctable.
Which of the above are correct ?
Blood pressure is measured by
Assertion (A) : In a well-type manometer, ratio of cross-sectional areas of well and the tube is not the factor that determines the scale of the manometer.
Reason (R) : Accuracy of a manometer is not affected by the shape or size of the tubes.
Electrocardiogram is a graphical representation of electrical activity of heart muscle myocardium, in the form of
For the quantitative measurement of acidity or alkalinity, the instrument used is
The amplitude of an electrocardiograph signal is usually of the order of
In the bio-electric phenomenon, ERG is used to detect
An ammeter of 1 mA possesses a resistance of 100 ohms. This ammeter is to be converted into 1A ammeter. The value of shunt resistance required will be :
Followings is the sequence of blocks in spectrum Analyser :
A. Detector & video Amplifier
B. Fixed Frequency Active filter
C. Attenuator (Adjustable)
D. Mixer & Tunable Detector
Choose the correct answer from the options given below :
Read the following statements regarding biomedical instrumentations :
(a) Sphygmos is used for blood pressure measurement.
(b) EMG represents a record of the electrical activity of the brain.
(c) ERG represents the record of potential from the retina.
(d) ECG represents the record of electrical activity of brain.
Which of the above statements are correct ?
Important Questions from Miscellaneous
The ration of the number of girls and boys in a school is 8 : 7. If the percentage increase in the number of girls and boys is 10% and 20% respectively, what will be the new ratio?
The cheapest means of transport is:
The property of Catenation is most readily predominant in:
Which newspaper edited by Bal Gangadhar Tilak, was one of the strongest critics of the British rule?
Which of the following is NOT a cause of food and water contamination?
