Instrumentation Frequent checks against adequate standards are the only guarantee that a n instrument is giving correct results
bw Ralph W.Munch
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is the process of comparing the magnitude of a variable of unknown magnitude with that of a suitable standard. Whenever we measure, we compare, more or less directly, an unknown with a standard. Measurements can be made in terms of arbitrary standards. Such standards sufice for the work of an individual or a small group working together very closely. Arbitrary standards are often set up by the first workers in new fields. However, as soon as it becomes necessary or desirable to enable others to make comparable measurements, the standards must be made available to them. This can perhaps be illustrated by considering the development of means for measuring temperature. The earliest instruments used to indicate temperature were thermoscopes developed by several workers abort 1600. These were made by blowing a bulb of several cubic centimeters capacity on one end of a glass tube a few millimeters in diameter and 40 cm. long and immersing the open end in a container of water. The device was completed by expelling enough of the air from the bulb so that the water stood about halfway up the stem and attaching a scale to the stem. The scales were arbitrary and the instruments were affected by changes in barometric pressure as well as by temperature. From these very unsatisfactory devices, the liquid-filled thermometer developed. Liquids such as water or wine were first used in open tubes, then in sealed tubes. These improvements eliminated the effect of atmospheric pressure on the readings, but water was unsatisfactory because of its nonuniform coefficient of expansion nnd high freezing point. Wine daas have a lower freezing point, but is ti substance of variable properties. Alcohol and oils were also tried. A11 these substances wet the glass. A mercury ' thermometer was described for the first time in 1724 by Gabriel Fahrenheit, Mercury is the most satisfactory thermometer filling for the temperature range from its freezing EASUREMENT
February 1952
point, -39.8" C., to its boiling point, 356.7" C., or even higher if the thermometer is filled with nitrogen under pressure. The concept of a thermometer scale based on fixed points was originated by Ole Romer about 1702. On his scale, the freezing point of water was 7l/2", the boiling point of water, 60". The zero corresponded to the temperature of an ice-salt freezing mixture. Fahrenheit used this scale at first, but modified it to avoid fractional values. His scale finally evolved into the one we use today with the ice point equal to 32" and the steam point equal to 212". Another early temperature scale devised about 1731 and still in use today to a limited extent is the R b u m u r scale which designates the ice point as zero and the steam point a8 80". The modern centigrade scale with the ice point equal to zero and the steam point equal to 100" C. was devised about 1745 by Carolus Linnaeus. These three scales based on reproducible fixed points with a specific type of thermometer as the interpolation instrument represented a tremendous advance in the art of making reproducible temperature measurements. As science progressed and international trade became more important, it became necessary to more closely define the temperature scale. Lord Kelvin in 1848 devised his temperature scale based on the fact that the efficiency of a reversible heat engine is independent of the physical properties of the working substance. This is a much more fundamental temperature scale than any based on the change of any physical property of a substance with temperature. The International Congress on Weights and Measures held a t Paris in 1887 adopted the constant volume hydrogen thermometer with the ice and steam points to define the international temperature scale. This was done because it gave the best approximation to Kelvin's thermodynamic scale which could be realized in practice. I n 1927 and again in 1933, International Conferences on Weights
and Measures adopted a new international temperature scale designed to more closely approximate the thermodynamic centigrade scale. They abandoned the hydrogen gas thermometer, since it was subject to errors because hydrogen does not obey the gas law perfectly enough and because it is cumbersome to use. The boiling point of oxygen, the ice point, tht! steam point, the boiling point of sulfur and the freezing points of silver and gold were chosen as the fixed points of the new scale. A carefully defined platinum resistance thermometer was chosen as the interpolation instrument between -190' and 660" C. From 660" to 1063 ", a standard platinum-platinum 10% rhodium thermocouple is used and above that range a radiation pyrometer, As anyone can see, our ability to define precisely and reproduce a temperature has improved greatly since the time when the Florentine thermoscopes were the best available temperature measuring devices. Modern industrial instruments are generally precise and rugged. Even so, we must never forget that every measurement we make is based on some standard more or less well defined. We must be prepared to check any instrument we use against eome suitable standard. The more accurate the measurements we wish to make, the more accurate the standards we must have available and the more often we must compare our measuring instruments with them. Mere possession of suitable standards is not enough, we must know how to use them too. For the most precise standardization of other temperature measuring devices in the temperature range from - 190"to 660° C., a platinum resistance thermometer and Mueller bridge are required. This equipment must be standardized a t the oxygen, ice, steam, and sulfur points. This can be done by the maker or by the (Continued on page 78 A 1
INDU'STRIAL A N D ENGINEERING C,HEMISTRY
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Instrumentation National Bureau of Standards. After the initial standardization, fairly frequent checks of the ice point suffice t o indicate that no serious changes in calibration have taken place. An occasional check of the steam point as well should be made when results must be as accurate as possible. Checks a t the oxygen and sulfur points are rarely needed, although it is desirable to make them since this provides good training in the accurate use of the equipment. When properly standardized and used, the platinum resistance thermometer reproduces the international temperature scale to within &0.02'. The instrument to be calibrated against the standard resistance thermometer should be compared with i t in a well-stirred liquid bath. For less accurate work, secondary standards such as mercury in glass thermometers or thermocouples calibrated against a resistance thermometer may be used. The standard platinum-platinum 10% rhodium thermocouple reproduces the international temperature scale within 0.2" from 660' to 1063' C. Properly calibrated, it can be used from 0' to 1500' C. with an accuracy of &0.5" to 110' C. and f 2 ' a t 1500'C. Thermocouples are usually calibrated by placing the couple to be calibrated in an electrically heated tube furnace and reading the electromotive force of the two couples simultaneously if two potentiometers are available, or alternately if only one is available. The two junctions should be brought into good thermal contact either by welding them together or by wrapping together with platinum foil. Where thermocouples are installed in plant equipment, it is often better to test them in place than to remove and check them in a calibrating laboratory. The reason for this is that the thermocouple wires become inhomogeneous in use. When the wires are inhomogeneous, the electromotive force developed by the couple depends on the temperature gradient along the wires. This would not be the same in a calibration furnace as in a plant installation. The preferred method for calibrating the working couple is to place the standard couple in the same well with it and read its electromotive force with a precision portable potentiometer. If there is not room for the standard in the same well as the working couple, it may be inserted into the furnace in a similar well placed beside the one containing the measuring couple. This method of comparison gives an over-all calibration of the sensitive element and the measuring instrument. When discrepancies are found, it is desirable to check the couple and the measuring instrument separately to localize the trouble so that suitable repairs or replacements can be made. Some types of instruments are more easily standardized than temperature measuring instruments, others less easily. Pressure measuring instruments can be (C07 inued on page 80 A ) 78 A
standardized fairly easily with sufficient accuracy for most work, since relatively inexpensive, easily manipulated standards are available. The mercury manometer can be used to measure pressures from a few inches of mercury up to 3 atmospheres or more. An accurate scale must be used, suitable corrections for the temperature of the mercury and the scale must be made, and the proper value of acceleration due to gravity must be used. For higher pressures, dead-weight type pressure gages serve as standards up to the highest pressures used commercially. Flow meters of small capacity can be standardized easily by weighing the material passing through them in a given time for liquids or by using a gasometer of suitable size to check gas flows. To perform an accurate check of a flow meter installation for very large flows becomes quite difficult or impossible for most users, If the flow meter is of the orifice type, one can check the differential pressure measuring device and measure the orifice and inspect i t to see that it has dot been improperly installed, but an over-all check is usually impossible. Devices for measuring chemical concentration such as pH meter5 and infrared gas analyzers must be standardized against samples prepared to have known concentrations of the material to be measured. Standard buffer mixtures are used as pH standards. Sometimes the instrument can be standardized against these directly. In other cases, samples must be withdrawn from the process and measured with another instrument which has been standardized against buffers. To standardize an infrared gas analyzer, gas mixtures having known concentrations of the component to be measured must be synthesized. This requires a good deal of equipment and very careful work. Users of instruments should remember that instruments must be checked a t intervals depending on the type of instrument and the accuracy desired. il check a t one point on the scale even if this is the only point where accuracy is desired is not enough. Instruments should be checked a t a t least two points, one on either side of the values to be measured. More points should be checked if the instrument is to be used over a wide range of values. Correspondence concerning this column will be forwarded promptly if addressed t o the author. % Editor, Industrial and Engineering Chemistry, 1156-16th St., X.W., Washington ti, D. C.
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