Chemical Instrumentation S. Z. LEWIN, N e w York University, Washington Square, N e w York 3, N. Y.
T h i s series of articles presents a survey ofthe basic principles, charackristics, and limitations of those inslruments which find important applications i n chemical w o ~ k . The emphasis is on commercially at~ailablee q u i p ment, and approzimate prices a l e quoted to show the order of magnitude of cost of the various types of design and construction
6. Recordinq Devices
effect of the current, as well as the Peltier effect. tend to introduce errors in temerat the measurement when annreeiable
Since recording devices are instruments designed to produce deflections which are proportional to the input signal, the characteristics of the signal source are of primary importance in determining the choice and use of a recorder. The following are the types of signals most commonly encountered in laboratory work. 1 . Large Signal, Low Swrce Impedance. A low source impedance in this connection means t h a t appreciable currents (of the order of milliamperes) can he drawn from the source without significantly affecting the source voltage. For example, if i t is desired to monitor the voltage applied to a lamp in a photometer, s, heater in s. bath, the electrodes in an electrodeposition cell, the coil of an electromagnet, etc., the additional current that might he drawn by any recorder is generally negligible compared to the ourrent already flowing. In this case, a direct-writing recorder can be used, and since these devices are essentially moving-coil (D'Arsonwl) meters with a w i t i n g attachment, they must he employed in accordance with the principles that govern the use of all ammeters and voltmeters. That is, an ammeter is connected in ~ e r i e swith the load, as shown in Figure 14, and its effective resistance (parallel resistance of coil and shunt, if any) must he small compared r i t h that of the Load. A voltmeter is connected in parallel with the load, and its effective resistance (coil plus multiplier, if m y ) must he large relative to the load. I n many cases, small signals from sources of either high or low impedance are fed into amplifiers whieh are designed to create an outnut that is suitable far application to dkechwriting D'Arsonval rerorders. 8. Small Szgnal, Low Source Impedance. An example of this type of source is the thermocouple. A copper-constantan oouple generates only 41.5 X 10-6 volts per degree Centigrade difference in temperature between the hot and cold junctions, and the total resistance of the couple is of the order of ohm8 (this depends on the diameter of the wires). If the thermocouple is in good thermal contact with its surroundings, and the surroundings have a large heat capacity, the current drawn from the thermocouple will have only a small effect on its voltage. [The heating
only small torques in electromagnetic meter movements; hence, a. very sensitive movement must he used if n direet-defleetion measurement is to he made. Hence, an oseillograph (galvsnometer recorder) is an appropriate instrument for this application. Alternatively, the signal may be balanced against s. fitandard voltage in a potentiometer circuit, so that no current a t all is drawn from the signal source a t balance. I n this case, a servopotentiometer recorder with a high-gain feedback-stabilized amplifier for balance detertion, would be desired.
SHUNT
POWER SUPPLY
I
Figure 14. In applying D'Arwwol meter diredwriting recorders. ommeterr must have small effective resistances relative to lood; voltmeters must hove large effective rerirtonces.
Other examples of small signals from low impedance sources are the outputs of strain gauges, resistance thermometers, thermal conductivity detectors, and barrier layer photocells. 3. Large or Small Signal, High Souree Impedance. I n many important experimental situations the internal impedance of the source is so great that the recorder must make its measurement b y drawing only extremely minute currents. If the signal voltage is E,, the source impedance is R,, and the input current to the recorder is I, the effective input voltage to the recorder is: E, - IR,. It is evident that the relative error in the record produced
by the recorder increases as E, becomes smaller and as IR, becomes greater. Examples of signals from high impedance sources commonly encountered in chemical work are the outputs of pHsensing electrodes, Geiger-Mueller counters and ionization chambers, phototubes and photomultipliers, and high resistivity conductivity cells. In such oases it is generally necessary to employ an electrometer amplifier, or equivalent circuitry, between the t r a n s dueer and the input to the recorder. Often, this preamplifier is made a part of the recorder. Whwe it is a separate unit, its output can he fed into any suitable recording instrument, either direcewriting or nervo-potentiometer. I n addition to the magnitude of the signal, and the internal impedance of the source generating it, the rate of variation of thesignal with timeexerts a determining influence on the type of recorder that can he employed. If very rapid variations are to be faithfully recorded, an oscilloeraoh is the inetrument of choice. Such
shock-tube experiments, some mass spectrometers, and electrode mechanism studies. With slowing varying signals, such as the output,^ of eolorimeters, polsrographs, furnaces, titrimeters, gas chromatographs, etc., direckwriting or servo-mechanism recorders are appropriate. However, i t should be recognized that there is an inherent time constant characteristic of any writing mechanism, and the input signal is damped or d i s t,orted by any recorder to an extent determined by the ratio of the frequency of the ~ i g n a lvariation to the time constant of the recorder. Thns, in recording spectrophotometers, the ahsorption hands of sharp peaks tend to he distorted much more than broad peaks, because the effective frequency of the input signal due to a sharp peak is large, whereas that of a broad peak is small. There are often other factors than those treated above that play an important role in determining the choice of a recording device. Among these are the initial cost of the equipment, the cost of the recording paper, the type of environment in which the inst,rument will he used, and the t,echnical skill of the instrument users. There is now commercially available such a vsrietv of recording devices that the laborittory scientist should have little difficulty in finding instruments suited to his specifio needs. The following paragraphs describe the commercial recorders that have found the most widespread use in laboratory work. Several other manufacturers than those cited offer recording instruments, but they are designed primarily for process control or military applications.
Volume 37, Number
I , January 1960
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Chemical instrumentation Sargent
E. H. Sargent and Co., Chicago 30, Illinois, manufneture a. very versatile and convenient servo-potentiometer penwriting recorder (Model MR, $1725). The servo-merhanism is of conventional design, hut it is provided with a variety of controls to permit its utilization for measuring currents and voltages over a wide range of values, to adjust or anppress instrument zero, and to vary the chart s p e d and direction. The potentiometer slidewire is a threeturn helical winding, connected to a hatte:y mpply consisting of eonventionnl carbon-zinc dry cells (such as are used in flashlights and portable radios) in parallel with mercury cells. Thc latter are a
relatively new development, consisting of an amalg:mated minc anode and s mercuric oxide cathode, imbedded in a pota~sium hydloride-containing immo-
HOURS
Figure 15. cell.
Discharge curve of a typical mercury
hilized electrolyte, This typo of primary cell has the prnperty of maintaining a quite constant cell potential until i t is
almost completely discharged, and is now widely used to supply s. reference voltage while delivering moderate currents (up to 25 ma per ema of hattery volume). The familiar Weston cadmium reference cell has a more reproducible and temperature-insensitive voltage than the mercury cell, hut cannot be employed to deliver current if its voltage is to he constant, The discharge curve of a t,vpiral mercury cell is shown in Figure 15. By using carbon dry cells in parallel with the mercury cells, it is possible to draw considerable currents while maintaining a relatively constant voltsge. That is, the emban cells supply most of the current, while the mercury cells work only as much as is necessary to maintain a constant output voltage from the psr;~llrl combination. The slidewire is in serira with a standa.rdisat,ion adjusting resistor and voltagedropping resistor*, as illustrated in Figure 16. The role of these resistors ma>- hc. explained most simply in terms of a numerical example. Suppose the total working battery voltage is 3.000 volts, and the slidewire resistance is 11100 ohms ( = 1K). If i t is desired to have t,hr. apan of input voltages that can he lralanred on the slidewire cqual to 100 millivolts, then enough additional resistma* must he added to the slidewire circuit to arrount, for 2.900 volts. Hence, if a t,otsl reristance of 2Q3000ohms ( = 2 Q K ) is plared in series with the slidewire, the current flowing through each resistor will he 0.1 milliampere, and the IR-drop ( = E) :WTOPS the slidewire will he 0.100 volt.
TO AMPLIFIER
.-STANDARD
ADJUST.
I -kid-; . CARaON-ZN
CELLS
;
L..--.-A
MERCURY
CELLS
Figure 16. Potentrometer didewire wim range selection by voltage-dropping retlrtorr.
If, now, i t is desired to increase the sensitivity of the potentiometer, and make its span correspond to, say, 10 millivolt~a, it is necessary to add an additional voltage-dropping resistor in series with the slidewire. To reduce the slidewire voltage to 0.010 volt,, the total series current must be reduced to 0.01 milliampere; hence, the added resistance must he 270K To decrease the sensitivity, and ineretse the span, say to 1000 millivolts, enough resistance must he removed from thc circuit to increase the slidewire current t,o 1.0 (Continued on page A101
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Journal of Chemical Education
Chemical instrumentation milliampere. Therefore, a series of precision resistors mounted on a elector switch can serve to define a variety of operating spans of a given potentiometer slidewire. The principle of adjusting or displacing the recorder sero may be understood by reference to the diagram in Figure l i . The zero-adjusting battery, E,, is connected in series with a variable resistor, RI, and a fixed resistor, R2. The portion of this battery's voltage that exists across Rs as an IR-drop depends on the ratio of RI to Ra, and therefore is continuously variable by adjusting R,. Resistor R2 is connected in series with the input voltage, El,, so that the net voltage applied to the potentiometer slidewire is (E,, EaJ or ( E t . - E*), depending upon the relative polarities of these two voltages. The fiense of the connection of Rx to the input can be reversed by the double-poledouble-throw reversing switch. If En, is addcd to El, (i.e., same polarity), this moves the roeorder zero upscale, which is to tho right in most recorders. If En? opposes Ei,, the recorder zero is moved down-scale, or to the left in most recordem. If the sero is moved beyond the mechanic4 stop of the recorder pen, the pen cannot follow to the electrical zero position, and hence this is the condition of a. swppressed zero. A suppressed zero is very useful when i t is important to follow small changes in a large current or voltage. By suppressing the zero, high sensitivity of the recorder can be employed, and the pen can be kept from going off scale bo t h right ~
+
DISPLACEMENT
INPUT
U N I T S SWlTCH
Figure 17. Simplified circuit diogrom rhowing principles of zero displacement m d mils selection in Sorgent Model MR recorder.
Another feature of this recorder is the provision of a "units" switch that permits the operator to choose a t will the proper circuit condit,ionfor measuring high or low ranges of ourrents or of voltages. The principle of this type of circuitry is illustrated in the diagram of Figure 17, upper right. The schematic shows a ganged switch, a.hieh is mechanically construci,ed so that both cantactors are moved simultaneously by the rotation of the central shaft. When the contactom are in the
A 1 0 / Journol o f Chemical Edvmfion
,,I" positions the input signal flows through a precision 1000-ohm resistor. The IRdrap, or voltage, generated across this resistor is proportional to this input current, and is measured potentiometrically by the servo-mechanism. This is the "mierortmperes" position of the function snitch. I n the "2," or "milliamperes" position, the input current flows through a 1-ohm resistor, producing an IIGdrop far milliampere currents that is the same magnitude as that produced a t position "1" for microampere currents. I n position "3," the input signal is applied directly to the slidewire; this is the setting appropriate for the low voltage ranges (1.25 millivolts to 2.500 volts). For measuring larger voltages, the "units" switch is rotated to the "4" position, in which a. 1-megohm ( = 1000K) resi~toris placed in parallel with the input, and anethousandth of this resistance ( = 1K) is connected to the slidewire. Thus, these rrsistors me employed as a. voltage divider, so t,hat '/,w of the input voltage is the effective input to the recorder.
?"
ADJUST
Figure 18. Principle of the manual $tandordirotion circuit.
In order to provide means of adjusting for variations in the parameters of the potentiometer circuit due to aging of the batteries, temperature variations, etc., a standardization eirouit of the type shown in Figure 18 is provided. When the standardization switch is depressed, a cadmium amalgam standard cell is connected as tho input signal, and connection is made to a fixed point on the slidewire which would correspond exnctly to the voltage of the standard oell (1.0186 volts) if the slidewire current (and, hence, IRdrop) were correct. If there is m y unbalance signal, the amplifier activates the sorvn-motor and the pen moves across the chart. The standardization adj.dju& ing resistor is then varied until the unbalance signal is zero, as evidenced by the cessation of motion of the motor-driven pen. There is inherent in evey meohanical system a certain amount of inertia of the moving parts, and this creates, in the case of recorders, the problem of overshoot and oscillation ("hunting"). When the potentiometer is outof-balance, an output (Continued on page A19) Volume
37, Number I, Jonuory 1960
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Chemical instrumentation signal e-iisk which eawes the servo-motor to turn. AR the position of balance is appronehed, the signal to the motor becomes weaker. However, if the balance point is being approached rapidly, the motor may not have time to come to rest a t t h r point where the potentiometer unbalance is zcro, but may continue to turn for a bit, driving the pen and slidewire contactor past the bnlsnee point. This, in turn, creates an unbalance signal of opposite phase, causing the motor to turn in the opposite direction to correct this overshoot. Jf tho magnitude of the amplifier output is large, this condition will be pronounced, and large oseillationfi will be produced, as shown in Figure 19A. I f , however, the amplifier output signal is smsll, inertia and friction in the mechanical system may prevent the motor from driving the slidewire contactor to the true balance point, as illustrated in Figure 19C. In this case, there is adead band about the balance point in which deviation8 from balance produce signals that are too emall to result in motion of the pen. It should he reeonnized a t this point that the problem of amplifier and servo-motor design in this type of recorder is a ticklish one. It is important that the recorder have a wpid response to varying signals, in order that significant information not be lost in the process of roeordinq. This requires a motor producing large torquea, driving the pen rapidly across the chart, and an amplifier giving a large output to
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Journal o f Chemical Education
activate the motor. However, these conditions are just those which tend to produce overshoot and hunting.
C Figure 19. Effect of damping on recorder response to on abrupt change in signal. A. Underdamped. B. Critically damped. C. Overdamped.
B y appropriate design of the amplifier circuit, i t is possible to achieve tho eondition of critical damping, Figure 19R, in which the system responds to signals with satisfactory speed but with minimum oscillation and minimum dead band.
In the Sargent reeorder, the damping is controlled by manual adjustment of a. variable resiator that oontrols the power output of the amplifier. The higher the voltage span of the slidewire, the greater is the magnitude of the unbalance signal for a given deviation from balance, and the s m d e r must the amplification factor be t o give critical damping. In the recorder under discuseion, the pen speed ia 1 spcond for full-scale travel.
Figure 20. Open front view of Sargent Model MR remrder, with chart writing plote in extended position, and loke-up roll removed.
The foregoing discussion has covered some of the important features of the electrical circuitry of the Sxrgent Model MR recorder. This instrument has also been designed with certain mechmical features intended to enhance its v~rsatility and convenience far general laboratory usp. (Continwd on, page A I 4 )
Chemical Instrumentation The paper chart may he driven forward and hackward, and by means of interchangeable gears, chart speeds ranging to 12 inches per minute are from available. A neutral drive position is provided on the chart drive that unrolls or rcralls the chart a t 20 feet per minute. The chart unrolls on a writing platform that is tilted a t either of two angles, 15 or 40" from the vertical, for convenience in identifying or marking the traces its they are made. The nlechanical design of the recorder is s h o r n in Figure 20. Either hall-point or capillary-feed pens can h? used. An auxiliary outlet is provided far powering other equipment; the power to the chart drive and to this auxiliary outlet is controlled by the same s ~ ~ t e so h , that synchronous operation is possible. This company also produces a lowercast recorder, their Model SR, price $675. The servo-pot,cntiometer is hasieally similar to the Model MR, but, to keep the price low, the flexibility and versst,ility of the lattar have been dispenmd with. The slidewire is excited by a single mercury cell, and there is no provision for standardization by the operator. If the battery is replaced a t six-month intervals, the slidewire calihration will be suhstmtially canstmt. Only one voltage span is provided, although other spans can he obtained hy suhstitution of rsnge resiston available a t extra cost. A single chart speed of 1 inch per minute is standard; other speeds can be obtained by use of interchangeable motors, also avaihhle a t extra cost. A zero displacement control with an upscale r m g e of one chart width is provided. Pen speed is 1 second for full-scale travel; accuracy is +0.25%; and hall-point or eapillary-feed pens may be used.
Fisher Scientific Co. Another recorder designed for flexibility and vrraatility in general laboratory use is the Fisher Recordall, price $1650. This is basically a standard servo-mechanism potentiometer recorder, with s, variety of controls for auulication to a lame ranee
voltii) or currents (5.5 microamperes to 0.55 ampere) directly, in osscntially the manner described above in detail in eonneetion wit,h Figure 17. In addition, the fallowing specialized input circnita are available: a thermocouple input in which the voltage signal fram a chromel-rtlumel thermocouple is a u t o m a t i d l y corrected for the temperature of the cold junction, and the recorder deflections are directly readable in degrees Centigrade over the interval 0 to 1000°C; a resistance thermometer input, for use with the Fisher niekel-wire resistance thermometer, ealihrated to make recorder deflections direetreading in the interval -100 to 320°C; and a resistance input circuit, giving measurements directly in ohms over ranges from 11to 5500ohms full scale. The standard instrument has a pen (Continued on page A18)
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lournul o f Chemical Educotion
Chemical instrumentation speed of 3 seconds for full scale travel, an accuracy of &0.5%, and 11 chart speeds ranging from 1/16 to 1 inch per minute. Provision is made for manual
cost (Model 71 Calibrated Zero Shifter, $90). The Recordall comes in x 6-foot tall metal cabinet on casters. The recorder chart is a t desk level, with the controls mounted above the chart, as shown in Figure 21.
Photovolt Corporation
Figure 2 1 . Firher Recorddl, in 6-foot metal cobinet. Operating controls are above chan; s t o m g e rpoce in cabinet below.
checking of the slidewire ealihration by the op~rator. A zero displacement control is wailable as an accessory a t extra
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Journal of Chemicol Fdumfion
A recorder that was designed for use in densitometry, hut that appears to have general utility for many laboratory applications, is the Photovolt Corporation's Varicord, price $590. This is a. servopotentiometer with several unique features. A sehematio, simplified diagram of its general principles of design is given in Figure 22. The circuit contains three slideu~ires, the sliding contacts of which are ganged together. The input signal flows through one of these slidewires (A, in the figure) in series with a variable resistor. A portion of this signal is tappcd off the slidewire by the sliding contact, A, and it is this voltage that is the input to the selfbalancing potentiometer (slidewire B). As the potentiometer eontactor is moved by the servo-motor, seeking the balance point, the contactors on the other two slidewires move in synchronism with it. Suppose, for example, that the input signal is larger than the voltage XY on slidewire B. The potentiometer contact, B, will start to move upward in the diagram (i.e., co~mterclockwine),in order to increase the voltage being opposed to the input. In so doing, the contact A moves upward (Continued on page .480)
Chemical Instrumentation also, decreasing the magnitude of the signal being transmitted t o the potentiometer. Hence, in the balancing process, there is subtracted from the input signal an
Figure 22. Schematic diogrom of design principles of the Photovolt Corp. Varicord. R,. response relector; ,:R coarse sensitivity control; &, flne sensitivity control; R,, response compensator rlidewire; Rsr potentiometer rlidewire; Rs, damping cornpentotor didewire.
a m a m t that is a funotion of the input signal, the amount increasing rapidly as the signal increases. This nlskes i t
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Journal of Chemical Education
possible to crcato a recorder output function that is related to the input function by any characteristic from linear to logarithmic. That is, if the "response selector" resistance is large compared to slidewire A, the variations in position of contact 4 will not significantly affect the signal to the potentiometer, and the recorder output will be linearly related to the input. If the response select,orresistance is small, large inputs will be attenuated markedly compared to small inputs, and the recorder will be close to logarithmic. The magnitude of the signal transmitted to the potentiometer slidewire is controlled by the cosrse and fine oontrols shown in the figure, which also serve as a zero displacement control. The third slidewire, C, fierves an a variable resistance, to modify the amplification factor of the amplifier, so that the degree of amplificstion is caused to decrease in proportion as the input signal increases, in such a. fashion as to maintain critical damping far all signals. Another unusual feature of this recorder is the method employed to "chop" the potentiometer unbalance signal into alternating current. A completely non-meehanical circuit is employed, based upon a photoconduotive CdS photocell. The principle of this chopper is illustrated in Figure 23. The photocell is illuminated hy li,oht from a neon lamp, which goes on and off 120 times each aecond because i t is powered by the 60-cycle line. Hence, the resistance of the photoeonduetor undergoes a. periodic decrease and increase, as it responds to the level of the light intensity. The current passing through
the circuit duo to the input signal therefore &a varies, and the transformer passes an a-o signal of line frequency, hut of amplitude proportional to the input signal magnitude. The recorder has fullscale spans continuously variable from 5 millivolts to 10 LINE A C 60 C P S
$
4
INPUT
Figure 23. convener.
Principle
d the
photocondustive
volts. Pen speed is 2 seconda for full scale travel. Reproducibility is ly0 of full scale. An adjustable a-c signal IS injected into the control winding of the servo-motor, creating a. "jitter" in the pen motion which decreases the effect of friction in the moving parts. Standard chart (Continued on page A H )
Chemical Instrumentation use of appropriate goarr. The strip chart is G inches wide and ieeds out on a horizontsl platform, as shown in Figure 24. The nen is of t h r citnillnr\~-f~ed tvne.
Figure 24.
0
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0
0
0
0
I I
0
I
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1
0
I
0
0
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Srnoll, voriable response recorder
IVaricordl of Photovolt Corp.
This company has recently announced an integrator accessory for th? Vwicrieord recorder (Integraph Model 49, price $420). One of the recorder slidewires is utilized to give an input voltage to the intcgrntor that is proportional to the recorder pen position a t any time. This voltage is applied to an oscillator circuit so as to rontrol the iroqurney of oscillation. This oscillstor signal is then el~ctronienlly compared nith a constant referenre irequenry, and t,hc difference, or bent ,freqaenc?,, is fed
A22 / Journol o f Chemicol Education
Figure 25. Vartord recording of a paper chromotogrom of rerum albumin scanned denritometricdly. lntegraph anochrnent produced the record at top of chart. Every tenth count appears or o pip above the line; other counts ore pips below the line. Peaks, areas, and percentoger ore, from leH to right: -pgiobulin, 31. 17.3y0; 0-globulin, 19. 10.6%; a-2-globulin, 16, 8.9%; a-l-globulin, 4, 2.3y0; albumin. 109, 60.9y0.
through appropriate circuitry to a pipping pen that makes a record of the pulses. A decade glow counting tube is incorporated into thc circuitry to give a. distinguishing pip for every ten regular pips. An ennmple oi a typical Varicord tracing, and the record simultaneoudy produced on t h ~
chart by the Integraph attachment, is reproduced in Figure 25. Nezlr Conclusion of the survey of corn~ncrciallyauailable laboralo~yrecorders. &larch and Aprik ( h b i m & r s and Pholomders.
