Instrumentation
May
1949
A reminder that t h e inherent errors of instruments should be kept constantly in mind by all users by Ralph Mi. Munch
M
plant tells of a young technical man who was assigned to test steam pressures at various points in a waste-heat steam generating system to determine the cause of certain operating difficulties. He care fully checked a suitable Bourdon tube tyP e pressure gage with a dead weight gage and found it accurate over its entire range, but when he connected the gage to the system to take measurements he noticed that the pressure read ing rose sharply and then gradually came to an equilibrium value. He knew that it was improbable that the pressure was changing in this manner so asked his superior for an explanation. After a few questions, his superior explained the effect of temperature on the calibration of a Bourdon type pressure gage and instructed the young man to use a pigtail between the steam line and the gage and to fill the trap with water before attaching the gage. Used in this manner, the gage remained at approximately room tem perature and gave equilibrium readings quickly with no tendency to drift. Moreover, the values read were not subject to the systematic error caused by calibrating the gage at one tempera ture and using it at another. Everyone who has seen gages in stalled on steam lines has noticed the pigtails used between gage and line, but few have thought of the reason for using them. Next to measurement of pressure {Continued on -page 92 A)
OST instrument users recognize that all instruments are subject to errors of one kind or another; all too few of them consider the implications of this fact as completely as they should. Since no measurement is absolutely accurate, the limits within which a measured quantity is known should be stated. Likewise, when instruments are to be installed to measure process variables, a reasonable estimate of the accuracy with which the variables must be known should be made. Errors of measurement are of two types, systematic and ran dom. Where the measured value must represent the best obtain able value for the variable being measured—for instance, where it is 1Λ be published, where it is to be used as the basis for de termining the amount or quality of material bought or sold, or where theoretical calculations are to be made—both types of error must be kept within acceptable limits. The systematic error in a quantity can be discovered by using several funda mentally different methods of measurement. When two or more such methods agree within the limits of accuracy of the individual methods, it is improbable that there is significant sys tematic error. Statistical methods can be used to evaluate random errors where sufficient data are available. Common sense also is a great aid in discovering errors. If measured quantities are not consistent with all other data, there must be an error. Even the simplest instruments are subject to error. A friend of mine who is in charge of the instrument department of a large
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Instrumentation with a simple Bourdon type gage, temperature is probably the most common plant measurement. Thermocouple-type temperature instruments are among the more common types of temperature measuring devices. Resistance thermometers are used less often. It is interesting to compare the errors of these two types of instruments. Equipment for measuring temperature with a thermocouple consists of a thermocouple, extension leads, and an instrument to measure the e.m.f. generated by the couple. Common thermocouples are iron-constantan, Chromel-Alumel and platinum-platinum rhodium. One or both of the wires of each of these types of thermocouple is an alloy. The e.m.f. temperature characteristics of the couples depend on the composition of the alloys and on the purity of the metals used to make the couples. Standard tables of e.m.f. against temperature are available. The characteristics of thermocouple wire and thermocouples available from suppliers match these very closely. However, for the most accurate work, it is desirable to calibrate each couple. Thermocouples must be protected against corrosion and against action of atmospheres which alter their electromotive force-temperature characteristics. Serious temperature measurement errors often result from improper placement or immersion of thermocouples. The thermocouple must be connected to a cold junction kept at a known fixed temperature or to the terminals of an instrument containing a cold junction compensator, by extension lead wire of matching electromotive force-temperature characteristics. Neglect of this requirement may lead to large errors. For most accurate wTork the thermocouple e.m.f. is measured by means of a potentiometer type instrument—for plant work, usually a recorder. Use of potentiometer-type instruments makes possible measurement of the e.m.f. of thermocouples differing greatly in resistance without error. A potentiometer is essentially a device for measuring an unknown e.m.f. by determining what fraction of the e.m.f. of a standard cell it is equal to. It consists of a calibrated voltage divider and its working cell, along with a device such as a galvanometer or thermionic amplifier for determining what portion of the potential drop along the voltage divider is equal to the unknown e.m.f. A standard cell is provided for calibration purposes and a cold junction compensator to avoid the necessity for maintaining the cold junction at a fixed temperature. Many recording potentiometers standardize automatically at frequent intervals so that they are not likely to be out of standardization except before the first automatic standardization, after the instrument is turned on or when the working eel} is worn out. A manually standardized potentiometer, on the other hand, may be far from standardized if no one thinks to perform this simple operation for some time. The accuracy of either type is only as good as the standard cell.
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Resistance thermometers avoid many of the sources of error inherent in thermocouple temperature measurement. The most common types of resistance thermometers are the copper, nickel, and platinum resistance Thermohms. These do not need special extension lead wire; ordinary copper wire is satisfactory. The Whcatstone bridge used to measure their resistance requires no standard cell. Alternating current from the power line can conveniently replace the working cell. And last but not least, no cold junction compensator is required. These factors make resistance thermometers especially valuable for jobs where somewhat greater accuracy is required than can be obtained using thermocouples. The cost of instruments increases rapidly as the accuracy required increases. Therefore, it is wise to purchase instruments which comfortably meet but do not greatly exceed the required accuracy. To maintain accuracy requires constant cheeking by a capable instrument department provided with the facilities for calibrating and repairing the types of instruments under its jurisdiction
The JohnVan Range Co. PIONEER F A B R I C A T O R S O F STAINLESS
METALS
DIVISION OF THE EDWARDS MANUFACTURING CO. Branches in Principal
701-731 CULVERT STREET
Cities
CINCINNATI 2. OHIO 92 A
