Chapter 7-PRINCIPLES OF MEASUREMENT 



Aboard ship, it is necessary to measure the 

 viscosity of fuel oil and of lubricating oil. The 

 viscosity of an oil is usually expressed as the 

 number of seconds required for a given amount 

 of oil to flow through an orifice of a specified 

 size when the oil is at a specified temperature. 

 Devices used to measure the rate of flow (and 

 hence the viscosity) are called viscosimeters 

 or viscometers . 



The viscosimeter furnished for shipboard 

 use is a Saybolt viscosimeter with two orifices. 

 The larger orifice is called the Saybolt Furol 

 orifice; the smaller one is called the Saybolt 

 Universal orifice. The Furol orifice is used for 

 measuring the viscosity of relatively heavy 

 oils; the Universal orifice is used for measuring 

 the viscosity of relatively light oils. 



A Saybolt viscosimeter consists of an oil 

 tube, a constant-temperature oil bath which 

 maintains the correct temperature of the sample 

 in the tube, a 60-cc (cubic centimeter) graduated 

 receiving flask, thermometers for measuring 

 the temperature of the oil sample and of the 

 oil bath, and a timing device. A Saybolt vis- 

 cosimeter is shown in figure 7-38; figure 7-39 

 shows details of the viscosimeter oil tube. 



The oil to be tested is strained and poured 

 into the oil tube. The tube is surrounded by the 

 constant-temperature oil bath. When the oil 

 sample is at the correct temperature, the cork 

 is pulled from the lower end of the tube and the 

 sample flows through the orifice and into the 

 graduated receiving flask. The time (in seconds) 

 required for the oil to fill the receiving flask 

 to the 60-cc mark is noted. 



The viscosity of the oil is expressed by 

 indicating three things: first, the number of 

 seconds required for .60 cubic centimeters of 

 oil to flow into the receiving flask; second, the 

 type of orifice used; and third, the temperature 

 of the oil sample at the time the viscosity 

 determination is made. For example, suppose 

 that a sample of Navy Special fuel oil is heated 

 to 122° F and that 132 seconds are required for 

 60 cc of the sample to flow through a Saybolt 

 Universal orifice and into the receiving flask. 

 The viscosity of this oil is said to be 132 sec- 

 onds Saybolt Universal at 122° F. This is usually 

 expressed in shorter form as 132 SSU at 122° F. 



Saybolt Furol viscosities are obtained at 

 122° F. The same temperature (122° F) is used 

 for obtaining Saybolt Universal viscosities of 

 fuel oil, but various other temperatures are 

 used for obtaining Saybolt Universal viscosities 

 of oils other than fuel oil. Thus it is important 



that the temperature be included in the state- 

 ment of viscosity. 



OTHER TYPES OF MEASUREMENT 



Thus far in this chapter, we have been 

 largely concerned with basic principles of 

 measurement and with widely used kinds of 

 measuring devices. We have taken up many of 

 the devices used to measure the fundamental 

 variables of temperature, pressure, fluid flow, 

 liquid level, and rotational speed, and we have 

 considered the measurement of the properties 

 of specific gravity and viscosity. For the most 

 part, we have dealt with measuring devices that 

 might be considered as basically mechanical in 

 nature. 



Before concluding this chapter, it might be 

 well to point out that many other kinds of meas- 

 urement are required in the shipboard engineer- 

 ing plant. While it is true that many of the prin- 

 ciples of measurement discussed in this chapter 

 apply to measuring devices other than those 

 described here, it is also true that a specific 

 application may require a measuring device that 

 is not precisely the same as any device we have 

 considered. Where appropriate, other types of 

 measuring devices are discussed in other chap- 

 ters of this text, as they relate to some particular 

 kind of machinery or equipment. In some in- 

 stances, the student may find it helpful to come 

 back to the present chapter to renew his under- 

 standing of the basic principles of measurement 

 we have considered here. 



NAVY CALIBRATION PROGRAM 



The calibration of all measuring devices 

 begins with and is dependent upon the basic 

 international and national standards of measure- 

 ment just discussed. Obviously, however, we 

 can't rush off to the National Bureau of Standards 

 every time we need to measure a length, a mass, 

 a weight, or an interval of time. Therefore, the 

 National Bureau of Standards prepares and cali- 

 brates a great many practical standards that 

 can be used by government and industry. Gov- 

 ernment and industry, in turn, prepare and 

 calibrate their own practical standards. Thus 

 there is a continuous linkage of measurement 

 standards that begins with the international 

 standards, comes down through the national 

 standards, and works all the way on down to 

 the rulers, weights, clocks, gages, and other 

 devices that we use for everyday measurement. 



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