Purpose
The aim of the experiment is to establish the calibration curve of the pyrometer.
Introduction
Optical polymers are some of the oldest and the simplest non-contact technique of estimating the temperature of a radiating object. It employs the use of color observation to estimate the temperature of a radiating object. Optical pyrometers are used as radiation thermometers to determine the temperatures of radiation wavelengths of both visible and infrared radiation bands (Chapra and Raymond 56). Optical pyrometers are classified according to their operating techniques and wavelengths. Some of the major classifications of optical pyrometer include; tow color pyrometers, disappearing filament pyrometers, total radiation pyrometers, multi-wavelength pyrometers, photoelectric pyrometers and two-wavelength pyrometers.
The optical pyrometers are further primarily classified into two main categories; automatic optical pyrometers and manually operated optical pyrometers. Out of all the existing types of optical pyrometers, only disappearing filament pyrometers and two-color pyrometers are manually operated. The other types of optical pyrometers are automatically operated. This experiment uses the manually operated disappearing filament optical pyrometer. The manually operated optical pyrometers are uses the operator’s eye as the comparator. It is made up the target, optical system, operator’s eye, measuring instrument and the reference unit.
Theory
The fundamental idea used in this experiment revolves around the principle of operation of the optical pyrometer. The optical pyrometer determines the temperature of a radiating object (glowing or hot body) by matching the brightness of the object to a calibrated light bulb. This technique employs the use of the human eye as the comparator to perform brightness matching. The brightness of the lamp is altered by adjusting the current to the lamp until the filament vanishes against the background of the target (Chapra and Raymond 56).
The light intensity generated by the reference bulb (bulb 1) is regulated by varying the voltage V1. The voltage of the reference bulb is varied using the VARIAC from 20 volts to complete brightness (110 volts). The second bulb is positioned between the reference bulb and the observer. Its temperature (T2) is established by observing the brightness of the bulb. The DC generator is used to alter the brightness of the second bulb which in turn alters the current through it. Varying the dc generator changes V2 which in turn affects I2. In case the temperature if the filament of the second bulb (T2) is greater than that of the first bulb (T1) then the filament of the second bulb will appear bright on a darker background. On the other hand, in case the brightness of the second bulb (T2) is equivalent to the brightness of the first bulb, then the filament of the second bulb will appear to be almost invisible and unidentifiable from the background (Lyzenga and Thomas 1422). The experiment seeks to establish the condition under which T1 and T2 are equivalent. It employs the use of 20 different values of bulb 1’s brightness to attain its objective. The temperature of the first bulb is computed for different sets of V1, and I1. The following equation is used to estimate and establish the value of the first bulb’s brightness, T1.
T1={I12 Rx/ (σ£A)}^1/4
Where σ= 5.67×10^ (-8) W/m2 K-4 is the Boltzmann constant; A is the light emitting surface area of the bulb 1’s filament (A= 8x10^-5m2); £= 1.00, and Rx is the ration of voltage to the current of the first bulb (Rx= V1/I1)
Apparatus
The experiment utilized the following apparatus and instrument to establish the calibration curve of the pyrometer; VARIAC, rail, three lenses, stands, four multimeters, two light sources (incandescent bulbs), DC voltage sources and filters.
Procedure
Before the experiment was started, it was imperative to get familiar with the equipment. The lab apparatus provided were checked so as to locate different components of the experiment. The DC and the VARIAC voltage source connections, operation and settings were understood before the experiment was arranged. Also, all the connections of the multi-meter were checked so as to identify the reading obtained from each of them.
The provided apparatus and equipment were arranged according to the diagram of the experiment layout provided in the lab manual. The first bulb was linked to the VARIAC that produces a maximum of 110 V (Foley 832). The slit was positioned between the first bulb and the lens. The second bulb was placed after the lens and was connected to a DC generator which produces a maximum of 10 V. The filter was positioned immediately after the second bulb. An ocular lens was located in a straight line right after the filter. It is of the essence to note that the first bulb, the slit, the lens, the second bulb, the filter and the ocular lens were all positioned in one straight line (Lyzenga and Thomas 1422).
The voltage of the first bulb (V1) was set on the VARIAC by measuring the voltage with the accompanying multi-meter. The reading of the current through the first bulb (I1) was taken and tabulated appropriately. The voltage across the second bulb (V2) was adjusted until the filament of the second bulb vanished in the background generated by the first bulb’s light. The reading of the voltage across the second bulb (V2) and the current through the second bulb (I2) was taken and recorded. These steps were repeated for V1 form 20 Volts to 110V in an interval of 5V. The equipment was turned off after all the readings were taken and recorded.
Results
The values of Rx
Presentation
Curve of I2 Vs. T1
Discussion
For values of V1 (20) and I1 (20.6), T1 was determined using the formula T1={I12 Rx/ (σ£A)}^1/4. The value of T1 was determined in parts. First, Rx was determined and recorded. The value of Rx was determined by Rx= V1/I1= 0.970874. Next, I12 was determined and recorded = 424.36. It was multiplied by Rx to obtain I2Rx=412. The value was divided by σ£A to obtain I2rx/σ£A= 9.08289E+13. This value was raised to power ¼ to determine the value of T1 as 3087.138. The step was repeated for different value of V1 and I1 and data recorded as indicated in the tables.
There is a limitation in the range of temperatures that can be determined using this method since the temperature depends on the brightness of the bulb. The brightness of the bulb is limited by some factors such as the current and voltage through it as well as the voltage source. I2 and T1 exhibit a linear relationship. There is a direct correlation between the two factors. Rx is linear since it is the relationship between V1 and I1. An increase in the value of V1 results to a corresponding increase in the value of I1.
Works Cited
Chapra, Steven C., and Raymond P. Canale. Numerical methods for engineers. Vol. 2. McGraw-Hill, 1998.
Foley, G. M. "High-speed optical pyrometer." Review of Scientific Instruments 41.6 (1970): 827-834.
Lyzenga, G. A., and Thomas J. Ahrens. "Multiwavelength optical pyrometer for shock compression experiments." Review of Scientific Instruments 50.11 (1979): 1421-1424.
