S. 2. LEWIN, New York University, Washington Square, New York 3, N. Y.
T h i s series of articles presents a survey of the basic principles, characteristics, and limitations of those instruments which find important applicatias in chemical work. The emphasis ti on commercially available equipment, and approximate prices are quoted to s h the order oJmqnitude of cost of the various types of design and castruction.
6. kecording Devices It is the object of chemical measuring instrumentation to sense a property of a system, such as temperature, pressure, hydrogen-ion activity, eto., and convert it into a form that can be recorded as useful data by the chemist. The output of the instrument, which is termed the "readout," may take a great variety of forms, including mechanical, optical, and eleotrical effects. One of the most characteristic features of modern instrumentation is the widespread use of specialized devices that make the recording of read-aut signals automatic, continuous, and rapid. These devices are oalled recorders, and they have played s. large role in the revolution in Iaboratory instrumentation that hits occurred in the past two decades.
is then utilized to move a writing meehanism. This mav consist in the movement ~f n pen holder, 1l.r drflwtim < , f a mirror from which .I light i m m is rent ~ t c domo r photosm~ir~vv aurfner, c,r f h r rotntiou of :! motor that drives a writing instrument. The writing instrument may also take a variety of forms, including a capillaryfeed or ball-point pen on conventional paper, s stylus on heat-, cbemiesl-, or messure-sensitive oaoer. a, lieht beam on on a chart, and for convenient interpretation of the data, the chart is u~ually driven a t a constant rate, so that the data is spread out on a Linear time axis.
Basic Construction The fundamental components of a recorder are represented schematically in Figure 1. The signal from the instru-
SIGNAL
"
CHART RECORD
The simplest and cheapest recorders are those in which a writing mechanism is directly attached to a standard D'Arsonval meter movement, so that the meter needle deflection is used to create the written record. A diagrammatic sketch of this type of recorder is shown in Figure 2. These reoorders are relatively inexpensive ($200 to $400), and can be used for recording data in any rtpplication where a milliammeter or voltmeter would be employed. Since the meter coil moves the writing pen, the minimum current that can be reliably recorded is limited by the inertia. and friction due to this mass. For practicalpurpases, these recorders are generally not used for greater sensitivies than 1 milliampere full-scale deflection, although direct-writing instruments are listed by manufacturers with sensitivities as great as 50 microamperes full-scale. With any given mewr nlavm.t:nt it is, of rourse, pass i l k to r w o d 13r~r.r vurnntr t h , ~ n1h.n of rhr. I,x?i< full-+..~Ie n r.aitivi1y bv attnrhiug an appropriate shunt (i.e., pardel resise anee) across the instrument. Voltage ranges higher than the minimum voltage charaoteristii of the basic movement me obtained by the use of multipliers (i.e., series resistance). Deflection Systems
Figure 2. Schematic diogrom of o D'Arronvoi meter movement modified for pen-ond-ink recording.
DEFLECTION SYSTEM
DirecbWriting Recorders
There is currently available a very great variety of recorders that can be used for laboratory purposes. These cover a wide span of capabilities and prices, ranging
A cross-metional view of the writing mechanism of a typical directcurrent milliitmmeter recorder is shown in Figure 3. This is the graphic recorder of the Esterline-Angus Co., Indianapolis 6, Indiana, and is available in 0-1 ma and ( t 5 ma basic movements. The pen feeds ink onto the chart paper by capilla~yrise in the fine tubing of the pen element. Drain tubes below the inkwell carry away spilled ink and keep the meter coil and magnet air-gap free of extraneous material. In this device, the pen point lies on the radius of a cirole, the center of which is the verti-
TIME
Figure 1. The fundamental component ports of a recording device.
ment, whioh is the input to the recorder, may require amplihstion before it can actuate the deflection system. This amplifier may not only be neoessary to raise the amplitude or power of the input signal, but may also eonveit that signalinto a more useful form; e.g., a dc input may be "chopped" into ac, an so signal may be rectified or demodulated, the wave form and frequency of an ac signal may be transformed, etc. The output of the amplifier
Figure 3. Cross-sectional view of m e writing melanism of the Esterline. Angus Co. graphic recorder.
from devices that require large input signals to others that draw less than a micromicrowatt from the input signal; from recorders costing less than $100 to some priced a t several thousands of dollam. Rates of chart travel from inches per day to hundreds of feet per minute are commercially available; every type of chart ruling, from linear to hyperbolic functions, can also be obtained from commercial sources.
cal axis of the meter coil. Hence, as the meter deflects, the pen draws the arc of a oirole on the chart paper. A typical chart record is shown in Figure 4. To permit visual interpretation of these records, the chart paper is ruled so that one of the coordinates is curved with the radius of curvature of the deflecting system. The utilization and evaluation of charts of the type shown in Figure 4 requires considerable mental gear-shifting on the
Volume 36, Number 12, December 1959
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Chemical Instrumentation part of persons accustomed to the usc of rectilinear graphs. Consequently, consider;rble effort has been expended in de-
mechanical linkage employed by Texas Instruments, 1 , Houston, Texa~, to convert curvilinear motion into a rectilinear displacement. The pen arm is rep?aented by the line MT, with t,he writing point a t T. The axis of the galvanom~ter is at G, and as the galvanometer roil deflects, the point P travels along the arr of a circle described about G as the eent,er. As P deflects, one end of the pen arm, M, is constrained to move in a track toward or away from G. Under this condition, the pen point, T , can only move up and down along s, line perpendicular to the traok of M. Thus, the curvilinear motion of I' is transformed into a rectilinear motion of T.
In this arrangement, the deflect,ion of the gdvanometer coil is coupled through an auxiliary rotating wheel to the pen point,, to give t,he desired trigo~~ometrio ronvrrpion.
Figure 6. Principle of curvilinear-to-redilineor linkage w e d in Curtisr-Wright recorders.
Figure 4. Emmpler of the chart paper used with ~ ~ w i l i n e recording ar de~icer.
vising systems for producing a rectilinear tracing with this intrinsically curvilinear deflection mechanism. Figure 5 shows the principle of the
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Journcll of Chemicol Education
Figure 5. Linkage 9yrtem "red by Texor Instruments to convert curvilinear motion into rectilinear deflection. G is center of galvanometer coil; PG ir swinging golvonometer arm; MT is pen writing arm; T is pen point. PM = PT = PG. Alternatively, some effect ir achieved if PGIPM =
PM/PT.
The Curtiss-Wright Corp., Carlstadt,,
New Jersey, uses a different form of rectilinear linkage, as illustrated in Figure 6.
A unique approach to this problem has heen adopted by .Mass* Lahoratorirs. Inc., Hinghsm, Masmchusetts, who have altered the design of the galvanometer rail movement itself, as illnrt,rzted in Figure 7. Current flowing throngh th? coil causes it, to be pulled into the magnet air-gap, producing a to-and-fro motion instead of the angular rotation typical of the U'Arsonvnl (Continued on page A733)
Chemical instrumentation
ducing the chart record. The gdvanometerdeflection, heingrotational,wouldtend
is the fact that the writing mechanism is of very low inertia; it is possible to record
meter. The linkage transmits the motion of the coil to the marking pen, producing linear displacements on the chart, as shown.
Figwe 7. Design of galvanometer and pen linkage used by Morw Laboratorier to give rectilinear recording.
Another novel approach to the construcbion of a rectilinear galvanometer recorder has recently been announced by the Swedish firm Elems-Sehbnander (U.S. Agent: Siemens New York, Inc., 350 Fifth Ave., New York 1, N. Y.). The schematic diagram is shown in Figure 8. A very small weighing less than a milligram, is attaohed to the galvanometer coil. As the coil deflects, in response t o the How of ourrent through it, the nozzle is turned. A fine jet of specially filtered ink is forced through the noazle a t high pressure, and impinges on the moving chart paper, pro-
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Journal o f Chemicol Education
Figure 8. Ink-jet recorder ryrtem of Elemo-Schsnmder, jet; 4, amplifier; 5 writing liquid; 6, pump.
to produce a displacement on the paper that differs from linearity in proportion t o tan a, where ar is the angular deflection. However, this galvanometer consists of 8. single loop of wire, and the angular deflection is not linear with respect to the input current, but deviates in proportion to cos a. Hence, the relative error in the linearity is proportional to sin a/n, which is practically constant up to about 25 degrees. The principal advantage of this recorder
1, galvmometer;
2, nozzle; 3
current or voltage variations occurring as rapidly as 1000 cycles per second (i.e., speed of response to changes is 1 millisecond). The recorder is called the Oscillomink; the two-channel model is priced at $1812.
Writing Mechanisms Several kinds of writing mechanisms are encountered in commercial direct-writing (Continued on page 754)
Chemical lnstrumentlrtion recordem. The most common is the penand-ink device, utilizing either a low-viseosity ink with capillary-action feed, or a high-viscosity ink wit,h a hall-point pen. The latter requiros a relatively high pressure contact of the pen against the psper, and is applicrthle only if large deflection forces arc available from the pen-drive mochsnism. The capillary-feed pen mechanism is shown in outline in Figure 3. I n some eases, the ink cartridge is a hermetrieally-sealed unit, so that evaporation and spillage am avoided. Cartridges arc disposable, and are avsihble in various colored inks. A source of supply of instmment pens for many makes of recorders is the American Recording Chart Cu., Los Angeles 22, California.
-
CONTACT FR'cT'ON
k
Figure 9. lnklerr writing system employed by Curtis-Wright. Electrolyrir current pagring through rtylvs at point of contact on paper oxidizer away the zinc coating at that point.
In soma reconlers, tho chart record is produced by electrochemical reaction. For example, the inklem writing system of
A734 / Journal o f Chemical Education
Curtiss-Wright r~carders utilizes sinrcmted paper and a metsl stylus, as illustrated in Figure 9. Current flows from the power supply through the metsl contact nhere it touches the paper, oxdising t,he metallic zinc eoatine rtt that mint.
the t,l~ieknessof the trace can he cont ~ d l e dby rrgulnting the magnitude of the cunmt. A line of graphic recorders based on dectrosensit,ivie 1,aper is produced by Alden Electronic and Impulse Recording Equipment Co., \T'esthoro, Mnssachusotts. In these, a fine stninlcss ~ t e c wire l serves as the stylus. Where it touches the psper, tho power supply callsea an dectrolysis euvront to pass, and frrrie ions are generated. These ions react with a dye nt the surface of the paper, producing a sepia-colored t,rare. Electric-writing styli tend to bc lightpr than pen-nnrl-ink mechanisms, and hrnec can respond to mare rapidly varying input signals. I'lugging up of the pen rapillary is a frequent sourec of trouble, as is the evapornt,ion and splashing of ink. These problems are xvokled hy the deatrir techniques; however, the latter rrquirr special p:tprrs that are 4-10 times more expensive than ordinsr.~ c h a t paprrs. Heat-sensitive papers are also ownsionally nsed in recorders. A stylus is employed with an electrieally-heatcd tip; x-here this tin touchcx the chart. chemicals
mechanism requires s greater pressure uf the st.ylus against the chart than does the electric t,ype. The paper used with hested styli is more expensive than that, used for ink-writing pens. Photographic papers have long been used with galvanometer recorders in which n light beam is the deflected agency. They hnve the obvious advantage of high specd and complcte freedom from inertial nnd frictional intrrart,ion with the deReetian mechanism. However, they require that. the r c r o r d ~ rbe loaded in a dnrkroom, and the paper must eventoally h e developed and fixed with the standard photographic chemical solutions. Ilecently, a nnmber of recorders have become available in which the inconveniences and delays involved in the wet processing of photographic papers hnve been eliminated. For example, the Honeyxvrll Visicorders, made by Heiland Division of MinnrapolisHoneyw4, Ilenver, Colorado, use IZodak Linagraph Direct Print paper. This paper (boff-rolored) is insensitive to visilde light, but is turned blue on exposure to ultraviolet light. Thus, s high intensity, mercury-vapor U.V. lamp serves as the light source within the recorder, and the rprord prints out directly into a. normallylighted room. Rinee these itre photngraphic recorders, they can follow high speed signals, up t,o 5000 cycles per second; and the chart fipecd can he ss fast as 14 feet per second. I'rcssure-sensitive papers are used with a "rhopper-bar" type of writing meehanifim t o produce permanent records. An cxsmple of this kind of recorder is t,hr
Chemical Instrumentation Itustrak of Rust Industrial Co., Manchcstcr, New Hampshire. This class of recorders consists ol a convnnt,ional D'Arsonvnl meter movement, with s, flat pointer that deflects across the chart paper, its broad side being parallel to the paper.
GALVANOMETER MOVEMENT 2 CHART FEED Figure 10. Chopper-bar recorder design. The striker it caused periodically to clamp t h e meter needle against the prerrure-sensitive p a p e r , marking the deflection a t t h e moment of clomping.
Once each two seconds a. cam (or solenoid, or equivalent mechanism) presses a striker against the paper, clamping the needle 0nt.o the paper s t a paint corresponding to its deflection. This produces a small dot a t that point, and as the process continues, a line c o n ~ i ~ t i nofg many of these dots is drawn. The tmcc is due to the puncturing of a apecia1 coating on the paper surface by the pressuro of tho metal contact. This type of writing meohanism is convenient, hut it is inherently much slower than the ink and electric writing types. Servomechanism Recorders A direct-writing recorder utiliees the force developed in a meter coil by the electmmagnetic interaction betheen coil and magnet, or an equivalent component, to operate the deflection mechanism. Unless that meehnnism is based on the weightless light beam, the farce needed to produce the deflection is considerable, and small signals cannot be so recorded. Lightbeam galvanometers (generally called oscillogrsphs) can handle extremely small signals, but they arc both very expensive (ca. $1000-$3000) and delicate, and leave much to he desired for routine use under a variety of laborntory conditions. One of the most significant and fsrreaching factors in chemical instrumentation has been the devrlopment of the technique of automation by means of the s~n~omechanism principle. Application of this principle to t,he de~ignof recorden has led to the production of instruments that have had profound effectson chemical lxboratory work. Thero are now dozens of important instrument,s in which a servomechanism recorder is an integral part; (Continued on page A736)
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Chemical Instrumentation e.g., spectrophotometers, gas chromstograph, radiation scalers, temperature recorders, etc. The servomechanism principle can be
Figure 11. Simplified diogram of a relf-bolansing potentiometer.
illu~traterlhy memd of n rirnple raamplr; irr Figure 1 I is iilawn n schematic dia~rarn of x self-ldanrinu oowntiomet~r,!inwit. If the potentiome&;(upper left portion of circuit, with Ez the unknown voltage and Ea the slidewire current source) is unbalanced, them is an unbalance signal (i.e., a current) flowing between points A and B. The direction of this current depends upon the direction of the potentiometer unbalance, vie., if the slidewire contactor is above the balance point, the current will he in one direction, if below the balance paint, the current will be in the opposite direction. This unbalance current is converted to ac by the electromechanical chopper, and fed into the amplifier. The output of the amplifier is an ac voltage that is applied to one of the sets of coils (e.g., the stator) of & phase-sensitive, reversible motor. The other set of coils (e.g., the rotor) is connected to the line sc, and always has a. fluctuating magnetic field associated with it. Consequently, the rotor will turn only if both ststar and rotor are energized by alternating signals that are out of phase with each other. In the present instance, the rotor will turn only if there is an unbalance signal coming from the potentiometer. The direction in which the rotor turns depends upon the phase of the ac output of the amplifier; if this signal leads the line ac in phase, the rotor will turn in one direction, if it lags behind the line, the direction of rotation is reversed. The rotor shaft is mechanically linked to the slidewire contactor so that rotation of the ahaft drives t h i ~contactor along the slidewire. The mechanical linkage is so arranged that the direction of drive of the sliding contactor is always in the sense that will reduce the amplifieroutput signal. That is, if the contactor happens to be above the balance point, the potentiometer unbalance signal will be of such a phase with respect to the line ac as to
A736
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Journal
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Chemical Education
Chemical instrumentation turn the motor in the direction that will move the contaotor down. If the contactor is below the balance paint, the unbalance signal will be opposite in phase to the previous case, and the motor drives the contactor upward, again toward the balance point. Thus, i t will be noted that the deviation of the potentiometer from halance produces a signal that sets in motion a chain of events the consequence of which is to move the slidewire contactor in a direction that reduces that signal. The contactor therefore continually seeks the balance point, for that is the only position in which the motor is not energised. The basic principle of the servomechanism may be generalized in the schematic form shown in Figure 12. The signal to be measured is compared with a reference standard, and the differenceis presented as an error signal to an amplifier, the output of whioh activates a signal generator to produce a correction signal that is applied to the source in such phase as to cancel the error signal.
Figure 12. Generalized representation of the principle of the rervomechonirm. A correction signal is generated ond fed bock t o the source in such phore or tosoncel the error signal.
This servomechanism principle is applied in recorders in several forms. I n potentiometric recorders, the deflection system is the servo-motor, and a pen is driven across s. chart of paper by the sitme drive shaft that causes the potentiometer slidewire contractor to seek the balance point. The common linkage between pen and slidewire contractor is shorn schematically in Figure 13. Since the servomotor can deliver large torques, the penwriting mechanism can be as large and heavy as is desired; ball-point pens can be used in place of the capillary-feed type. In bridge recorders, the error signal is the unbalance voltage of a Wheatstone bridge, and the servo system automatically (Continued on page A740) Volume 36, Number 12, December 1959
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A737
Chemical Instrumentation seeks to balance the bridge, simultaneously driving a pen in synchronism with the bridge balance adjust. Several other types of servomechanism recorders will he described later. By the use of negative feedback sta-
mental situation. Direcbwriting D'Arsonvd recorders are appropriate for eurrents of theorderof milliitmperes, or potentials of the order of volts, and for dc to low-frequency ile signal variations. Servomechanism recorders are suitable for much smaller signals. Light-beam galvanometer (oscillographic) recorders are appropriate for low-level, high-frequency applications.
The value of recorders was early recognized in industry, and the great develop ment and proliferation of this type of instrumentation has been a direct conse quence of the economic advantages it offers in controlling and reproducing commercial processes. At present, many tens of thousands of recorders are in daily use in hundreds of industries, and the production of recorders is itself .a multimillion dollar industry. The laboratory scientist has been the fortunate beneficiary of these economic developments.
Bibliography Hansam, A. J. and DAMBICH,R. A,: "Graphic Recorders: Oscillographs" in "Industrial Electronics Handbook," Cockrell, V. D., ed., McGraw-Hill Book Co., N. Y., 1958, pp. 6-28 to 6-38. HARRIGON,T. R., "Servo-Operated Graphic Recorders and Function Ploh ters. Strain Gages," ibid., pp. 6-38 to Figure 13. Generalized representation of a senromechonirm recorder, showing linkoge between slidewire contactor and writing pen.
bilizod amplifiers, and high speed servomechanisms, instrument manufacturers have produced recorders that respond to input signals with a time delay of less than a tenth of a second. and thitt eive stable, low-noise, linear deflections fo; inputs as small as miorovolts. As the preceding discussions how, there are recorders available for every experi-
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Journal of Chemical Education
Since nearly every property of interest to the scientist can he converted into s. current, voltage, or resistance variation, a recorder can be employed to produce a continuous. hieh-resolution, Dermanent record of these-properties. ' he time is close a t hand when every chemist will find 8. recorder as indispensable a part of his daily work as is a. chemical balance.
Ef "--".
4'
Ho~zsocu, "Instruments for Measurement and Control," Reinhold Puhlishing Corp., N. Y., 1955. "Recorder Manual," s. reprint of sixteen articles from Instruments and Automation, 1959, Instruments Publishing Co., 845 Ridge Ave., Pittsburgh 12, Pa.
instruments nirrently available
