Chapter 7-PRINCIPLES OF MEASUREMENT 



entire spectrum of radiant energy, while optical 

 pyrometers are sensitive to only one wavelength 

 or to a very narrow band of wavelengths. 



Figure 7-11 illustrates schematically the 

 general operating principle of a simple radia- 

 tion pyrometer. Radiant energy from the hot 

 object is concentrated on the detecting device 

 by means of a lens or, in some cases, a conical 

 mirror or a combination of mirror and lens. 

 The detecting device may be a thermocouple, a 

 thermopile (that is, a group of thermocouples 

 in series), a photocell, or some other element 

 in which some electrical quantity (emf, resist- 

 ance, etc.) varies as the temperature of the 

 hot object varies. The meter or indicated part 

 of the instrument may be a millivoltmeter or 

 some similar device. 



An optical pyrometer measures temperature 

 by comparing visible light emitted by the hot 

 object with light from a standard source. A 

 common type of optical pyrometer is shown in 

 figure 7-12. This instrument consists of an 

 eyepiece, a telescope which contains a filament 

 similar to the filament of an electric light bulb 

 and a potentiometer. 



The person operating the optical pyrometer 

 looks through the eyepiece and focuses the 

 telescope on the hot object, meanwhile also 

 observing the tin glowing filament across the 

 field of the telescope. While watching the hot 

 object and the filament, the operator adjusts 

 the filament current (and consequently the 

 brightness of the filament) by turning a knob on 

 the potentiometer until the filament seems to 

 disappear and to merge with the hot object. 

 When the filament current has been adjusted 

 so that the filament just matches the hot object 

 in brightness, the operator turns another knob 



slightly to balance the potentiometer. The poten- 

 tiometer measures filament current but the dial 

 is calibrated in degrees of temperature. As may 

 be noted from this description, this type of opti- 

 cal pyrometer requires a certain amount of skill 

 and judgment on the part of the operator. In 

 some other types of optical pyrometers, auto- 

 matic operation is achieved by use of photo- 

 electric cells arranged in a bridge network. 



MEASUREMENT OF PRESSURE 



Pressure, like temperature, is one of the 

 basic engineering variables and one that must 

 frequently be measured aboard ship. Before tak- 

 ing up the devices used to measure pressure, let 

 us consider certain definitions that are important 

 in any discussion of pressure measurement, 



PRESSURE DEFINITIONS 



Pressure is defined as force per unit area. 



The simplest pressure units are ones that 

 indicate how much force is applied to an area 

 of a certain size. These units include pounds 

 per square inch, pounds per square feet, ounces 

 per square inch, newtons per square millimeter, 

 and dynes per square centimeter, depending upon 

 the system being used. 



You will also find another kind of pressure 

 unit, and this type appears to involve length. 

 These units include inches of water, inches of 

 mercury (Hg), and inches of some other liquid 

 of known density. Actually, these units do not 

 involve length as a fundamental dimension. 

 Rather, length is taken as a measure of force 

 or weight. For example, a reading of 1 inch of 

 water (1 in. H2O) means that the exerted pres- 

 sure is able to support a column of water 1 inch 



DETECTING ELEMENT 

 (THERMOPILE, PHOTOCELL, ETC.) 



PEEPHOLE 



INDICATING INSTRUMENT 



(MILLIVOLTMETER, POTENTIOMETER, 



ETC.) 



147.56 



Figure 7-11.— Simple radiation pyrometer. 

 135 



