15.20 X lo-? cm? sec-1 for benzene and methanol, respectively
expected rates calculated from the equation ( 4 )
(5).
s
=
~
DMP A RT ( i ) l n h
where S is the diffusion rate grams sec-’, D is the diffusion coefficient cm2 sec-I, M is the molecular weight of the vapor, P is the total pressure dynes m r 2 , R is the gas constant, erg K-’ molecl, T is the operating temperature K, A is the cross-sectional area of the capillary tube cm2, L is the length (cm) of the diffusion path, X
=
~
where P and p , the
p-P vapor pressure of the diffusion component at temperature T, can be expressed in any convenient units. The diffusion coefficients used were 9.32 X lo-* and
With the fixed reservoir-type cell used, the gravimetric results showed a steady decline in the diffusion rate with time; these results with benzene were confirmed by the flame ionization detector. It is not possible to maintain steady conditions due to the concentration gradient in the space above the liquid level changing as the level falls. With the constant level diffusion cell, the parameter
(3
re-
mains fixed at its chosen initial value and concentration gradient effects are eliminated. The observed diffusion rates are found to be better than 95 of the theoretical values for continuous operating periods which are limited only by the capacity of the reservoir.
RECEIVED for review June 2, 1969. Accepted July 3, 1969. ( 5 ) G. A. Lugg, ANAL.CHEM.,40, 1072 (1968).
(4) J. Stefan, W e n Ber., 63,63 (2) (1871).
A Rapid-Response Variable-Temperature Thermostat Bath Employing Analog-Digital Control Circuitry Theodore E. Weichselbaum and Raymond E. Adams Sherwood Medical Industries, Inc., Subsidiary of Brunswick Corp., St. Louis, Mo. 63121 Harry B. Mark, Jr. Department of Chemistry, The Unicersity of Michigan, Ann Arbor, Mich. 48104 WITH the advent of highly accurate chemical instrumentation based o n both analog and/or digital logic circuitry such as those recently described for kinetic (1, 2) and electrochemical (3) measurement, experiments have shown the temperature control of the systems often become the limiting factor in the accuracy of the data (2, 4 ) . Also, it was found that for experiments, such as kinetic based analyses and differential thermal activation of enzyme reactions ( 4 ) , it was often necessary to change reaction temperatures (2, 4 ) . Thus, a rapidresponse variable-temperature thermostat bath was found t o be necessary and convenient. Previous designs of utility constant temperature fluid baths used for the thermostating of chemical reaction vessels, rate studies, electrolysis cells, etc., generally have employed a temperature-controlled switch, such as a mercury or bimetallic switch, t o the application of a heater (or cooling element) o n (or off) when the temperature of the fluid in the bath or cell varied from the preset value desired (5). Although the temperature-sensitive switch can be made to be very sensitive to small temperature[changes, it acts only as a n on/off switch for applying full voltage o r current t o the temperature element. Thus, “overshoot” in reaching temperature equilibrium cannot be avoided. This problem can be partially circumvented by employing a thermistor bridge temperature sensor and a (1) T. E. Weichselbaum, W. H. Plumpe, Jr., and H. B. Mark, Jr.’ ANAL.CHEM.,41,(3) 103A (1969). (2) T. E. Weichselbaum, W. H. Plumpe, Jr., R. E. Adams, J. C. Hagerty, and H. B. Mark, Jr., ibid.,p 725. (3) G. Lauer and R. A. Osteryoung, ibid.,40 (lo), 30A (1968). (4) T. E. Weichselbaum and J. C. Hagerty, Sherwood Medical Industries, Inc., unpublished results, 1968. ( 5 ) C. N. Reilley and H. T. Sawyer, “Experiments For Instrumental Analysis,” McGraw-Hill, N. Y . , 1961, Chapter 13.
SAMPLE CELL
-+
TH2
Ts P ERROR AMPLIFIER AND BRIDGE COLD REF CIRCUIT
CONTROL AMPLIFIER
BRIDGE-PD POWER SUPPLY
PHASE DETECTOR AND PULSE TRANS,
Figure 1. Schematic diagram of the constant temperature controller circuit which controls the magnitude of the effective voltage applied t o the heater element so that the thermal control is proportional to the error signal sensed by the bridge (6). However, temperature oscillations (discussed below) are still present and affect measurement results ( 4 ) . The dual thermistor bridges employing the analog-digital heater control circuitry presented here circumvents this problem. Also, temperature sensitive switches are quite difficult and time consuming t o reset to a new temperature value. However, the circuit described can be programmed to a new cali(6) “Silicon Controlled R.ectifier Manual,” 4th Ed., General Electric, Syracuse, N. Y . , 1967, pp 150 and 274, Chapter 9.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
1913
4
1914
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
brated temperature setting in a few minutes by simply changing the resistance value in the thermistor bridges by means of a switch. Thus, n o trial and error adjustment is necessary. The thermostat unit described here can be used at four preset calibrated temperature values; 25, 30, 37, and 56 "C, &0.01 "C,and can also be employed in a continuously variable mode from 16 t o 60 "C. CIRCUIT DESIGN
X 0
4c
:
3
Z d N
h"
X
8-303
,. .. .. .. ZZGC G h h h 0 0 0 0
3 X
The basic design of the unit can be described by seven individual modules as shown in the block diagram of Figure 1. The operation and characteristics of each individual module are described below. The complete circuit diagram of the total unit is shown in Figure 2. Constant Cold Reference. In this unit three 4-V thermoelectric modules, T E of Figure 2, are connected in series which furnish a constant-level cold source to the fluid reservoir of the bath (total volume of the fluid bath is 1000 ml plus the volume of the jacket system). These thermoelectric modules can be operated at either a 4.5 o r 9.0 A constant-current level depending o n the value of the controlled temperature desired. The change in the applied current level is accomplished by means of a 110-220 V transformer, T1, of Figure 2, and by switching the primary leads from 110 V at 9 A (position 1 of S T )to 220 V at 4.5 A (position 2 ) . For temperature values less than 40 "C, the cool reference is operated a t the high current level (9.0 A). For higher temperatures, 40 "C and above, the cool reference is operated at 4.5 A. A simple diode bridge acts as a full wave rectifier to convert the ac of the secondary of T l , to the dc to operate TE. Note also that a means is provided for quick cooling of the bath reservoir when changing from high to low temperatures. The quick cool switch, Sa, operates a solenoid valve which allows tap water to flow through cooling coils in the bath reservoir. During the quick cool operation, the cool reference switch, s?,is turned on 9.0 A and when the desired value of the temperature is approached, Sa is opened and the system comes to regulation. Regulated Power Supply. A well regulated 17.5 V dc power supply is required for the sensor bridge amplifiers, and the phase detector circuit. The power supply circuit employed is conventional in nature (see Figure 2), and employs a high gain operation amplifier (Amplifier 1, Fairchild Model 709) to sense voltage changes and has a current limiting resistor (Rt2)for added protection. Heat Exchanger. The three thermoelectric modules of the cool reference are placed between two metal plates. The cold side is in contact with a top plate and the hot side in contact with a lower plate. Both plates have a maze channeled in them to allow liquid to be forced through them and the thermoelectric modules are positioned under the input portion of the top maze. A 200-W cartridge heater is inserted in the output channel of the top maze and a precision thermistor is placed close to, but slightly downstream from the heater as shown in Figure 3. Tap water runs through the lower plate at about 1500 to 2000 mljmin t o carry away heat from the thermoelectric modules. The control liquid, which is circulated to the thermostated sample cell, is pumped through the top plate at 1000 mllmin where it is cooled as it goes through the first part of the maze and then heated to the desired temperature as it passes the heater. Sample Cell. The complete temperature-controlled sample cell is shown in Figure 4. It consists of a brass water jacket, I, (also shown in Figure 5 ) , assembled with a glass or quartz cuvette (sample chamber), 2, in a n insulated housing. The cavity between the water jacket and the cuvette is filled with a fine copper powder, 3. This effects a very efficient heat exchange from the water jacket to the cuvette. The brass water jacket is shown in Figure 5 . The water enters the inlet tube, 4 , which passes through the top closure, 5 , and the baffle plate, 6. The water then enters the inlet chamber, 7, ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
1915
WATER TO SOLENOID QUICK COOL VALVE
c
TAP WATER /
OR
00
%
TOP
PLATE
BOTTOM PLATE COOLING PLATE
Figure 3. Heat exchanger sub-assembly
-
!I71 V! N
C
O
V
f
P
I N S U L i T I i Z ObTER H~ISIIIG :I61
-PC\h3EREG C3PPER
A
!31
(19) UPPER WATER JACKET
LIGHT APERTURE BLOCK (15)
kc
81 TEMQERA'URE
SENSING AND COYTROLL VG PROBE 53CKETS
SECT'CN A-A TEMPERSTLRE C 9 T R O L L E 3 SAMPLE i i L L
Figure 4. Temperature controlled sample cell
which is formed by the upper header, 19, and opens into the inlet heat transfer tubes, 8. These tubes are 0.015" wall thickness brass. The tubes enter the lower header, 9, which forms the transfer chamber, 10, with the bottom closure, 11. The water enters the transfer chamber and is directed upward through the outlet heat transfer tubes, 12, as shown by the arrows. The water then enters the outlet chamber, 13, and out the outlet tube, 14. This arrangement gives a minimum temperature gradient throughout the water jacket. The entire assembly is made of brass and is sweat-soldered together for a leak-proof maintenance-free assembly. The entire capacity of the water jacket is 17 ml. Figure 4 shows the light path which is formed by two brass light aperature blocks, 15, that are assembled with the insulated outer housing, 16. These blocks serve as a light "window" aperture, 17, and the round holes serve as sockets, 1916
,~
(9) LOWER
1
WATER JACKET HEADERl
~tl+~~t
1
TEWPER-'IRE .3UT91-LE3 SAYPLE i E L ~ h 7 E P .AWE-
Figure 5. Temperature controlled sample cell water jacket 18, to receive the control thermistor probe and a Rosemount 108MA platinum temperature sensing probe. Both probes are in cylindrical metallic housings that fit these holes closely. They are coupled thermally to the brass aperture block and the cuvette with a coating of Dow-Corning 340 silicone heat sink compound, The platinum sensor is coupled to a Rosemount 414L bridge. The bridge and sensor are calibrated to give 1 mV/"C into a 10K load to an absolute accuracy of 1 0 . 0 1 "C. Absolute temperature and temperature variations of 10.01 "C can easily be read with any readout instrument capable of resolving 10 pV with accuracy of 0.005%. Instruments of
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
this quality are available in the form of dc differential voltmeters (John Fluke 893A) and digital voltmeters (HewlettPackard 3460B). The control thermistor is an integral part of the Digecon temperature control unit and is calibrated for use with each individual unit. The sample cell itself is mounted in the spectrophotometer and is replaceable with sample cells of different reaction chamber capacity by removing the probes from their sockets. The platinum temperature sensor and the 414L bridge are sold as an accessory option installed in the spectrophotometer with the output available at the back panel connector. If a readout instrument specifically designed for this output is desired, a Digital Readout designed by Doric Scientific Corp. DS-100-K2-110 is available. This instrument reads out in degrees centigrade to 1 0 . 0 1 “C. Its accuracy is 0.01 % full scale +0.01% of the readings. Its resolution is 0.01 “C; therefore, a total absolute accuracy of zt0.03 “ C can be achieved by utilizing this readout in conjunction with the temperature sensing option installed in the Digecon spectrophotometer. The temperature sensing option can be used to monitor the calibrated temperatures of the unit, and also allows the operator to select easily other control temperature points with the variable control. Control Bridge Amplifier Circuit. The precision thermistor THl, which is physically within the heat exchanger, is electrically in one leg of the control bridge circuit. The resistors in the bridge, as well as the resistors in the amplifier circuit, are 1 resistors with a temperature coefficient of 50 ppm. The bridge is calibrated at a fixed point for each of the control temperature points--i.e., 25, 30, 37 and 56 “C. Therefore, linearity and close matching of resistors is not needed. A temperature coefficient of 50 ppm or better should be observed since a small change in resistance due to ambient temperature change will upset the calibration. When the temperature bath changes, the thermistor resistance changes and a voltage change appears across the bridge which is amplified by a high gain differential amplifier (Analog Devices Model 801). By changing resistance in the bridge arm opposite the thermistor by means of Sg to SS,the bridge is set for different temperature values. Thus, a wide range of controlled temperatures, both fixed and continuous, can be obtained. The bridge is actually calibrated for four fixed temperatures, 25, 30, 37, and 56 “ C (resistors R4 to R7). A variable range from 16 to 60 “ C is provided for using a variable 150 K , R13,potentiometer in the variable position, S4. Phase Detector and Pulse Amplifier. The phase detector circuit consists primarily of five “And” gates (a to e) and two transistors, Qz and Q4, and is used to sense the phase of the ac line voltage and to turn on the heater (by means of the solid state triac switch, Tr) at the proper time during each half cycle of the ac to give the power necessary to maintain the desired temperature. The time duration of turn-on during each half cycle of the ac line voltage is proportional to the output voltage from the control bridge operational amplifier, and, therefore, is inversely proportional to temperature sensed by thermistor, TH1 (and THz, as explained later) which determines the input voltage to amplifier 2. As the temperature goes up, the resistance of THl goes down, and the output voltage of amplifier 2 goes down. A full wave rectified signal (from the bridge circuit of transformer T, of Figure 2) see point A of Figure 2, is applied to And gate a. This is an inverted negative logic And gate and, thus, produces a positive output pulse (at point B of Figure 2) whenever the full wave signal goes to zero as shown by wave form B of Figure 6, (the wave form as a function of time for all designated points in Figure 2 is shown in Figure 6). This positive pulse is fed to And gate b, which produces a zero output at point C of Figure 2 which is in turn fed to the input of And gate c. And gate c produces a positive pulse on capacitor, c13 (at point D of Figure 2). This positive
A
yo I I
td I TURN OFF TIME OF 02 Vtc* VOLTAGE AT WHICH 01 TURNS ON
Vcc’ PEAK VOLTAGE DEPENDING ON TYPE LffilC MODULE USED AN0 SUPPLY VOLTAGE t B PARALLEL COMBINATION OF THE RESISTANCE OF IO CR241 AND R 2 5 ASSUME BASE R E S I S ~ A N C EOF o4 OFF :m
e--&.* VCC
1 . LCI3
K
Q = KtC13
1
,
= PERIOD IN WHICH ENERGY I S BEING APPLIED TO HEATER
[&-kl
SECS
Figure 6. Time sequencing and wave form at designated points in the controller circuit pulse, transmitted though c13,appears on the base, point E of Figure 2, of Q4 and turns the transistor off. Thus, the collector of Q4 goes to zero causing And gate d to produce a positive voltage output (point F of Figure 2) which locks And gate b on zero volts (see wave form C of Figure 6) even though the positive pulse input to gate b fram gate a has returned to zero. The length of time that Q4 remains off is determined by the time constant of c13 and the total resistance of the transistor, Qz, and the 200 K potentiometer, RZ5, parallel combination. For each control temperature desired, there is a specific voltage on the base of QZwhich determines the base current. This base current controls the collector current which is a part of the discharge current of c13. With a constant voltage applied to its base, QZ will maintain a constant collector current. As the charge on C I Sleaks off, part through RzS and part through Qz, the collector voltage will decrease and the apparent impedance of QZ will decrease exponentially with the collector voltage. The rate at which the impedance decreases depends o n the magnitude of the base voltage, Although the impedance of Q2 is an exponential function, a constant equivalent impedance “Z,” made up of the above mentioned parallel combination, can be determined for each voltage applied to the Qz base. Transistor Q2 operates at, or very close around a point on its base voltage us. base current curve, for each temperature point. This equivalent impedance is approximately inversely proportional to the voltage on the base of Qz. This voltage is, as mentioned above, inversely proportional to the temperature environment of THl i THz. As c13 discharges through Qz (and R25), the voltage at point E drops and eventually reaches a value for which Q4 turns back on (see E of Figure 6), and results in the collector of Q4 (point F of Figure 2) to be positive. This makes the output of gate d (point G) return to zero. This produces a negative going pulse (at point H of Figure 2) through capacitor, c14, which swings the output of gate e momentarily positive (at point I of Figure 2) which turns on transistor Q3. The resulting pulse through Q3 is amplified by the UTC transformer, T3, (point K of Figure 2) and turns on the triac, Tr, applying power to the heater as
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
0
1917
~~
~
Table I. Determination of LDH Volume of reagents = 3.0 ml. Time held in reaction chamber = 30 seconds, including stirring time at 30.0 "C Monitored temperature readings in cuvette Reading (reagents at 26.5 "C) Time from complete Reading stirred in reaction addition of (reagents at 30 "C chamber for reagents, sec. 3 ~ 0 . 5"C) 30 seconds 34 29.84 28.8 36 29.92 29.32 38 29.98 29.51 40 29.98 29.83 42 30.00 29.94 44 30.01 29.98 46 30.00 29.99 48 30.01 30.00 60 30.00 30.00 120 30.01 30.01
shown by wave form L of Figure 6. The triac automatically turns off when the voltage reaches zero at the end of each half cycle. Thus, the amount of power required to maintain the bath a t the desired temperature is regulated by the duration of turn-on of time of Q4 during each half cycle, the triac turn-on is governed by the time constant of the discharge of C13. This time constant is governed by the resistance of Q2 which is inversely proportional to the resistance of the thermistor, THl. The potentiometers, R I 1 and R z s , set the upper and lower limits of the time constant. Q 5is a n NPN and is used as a diode. A change in ambient temperature was found to have a n adverse affect o n various circuit components, so temperature compensating circuits or components were inserted where necessary. Rz, Q5,D12, 0 1 3 , Dl4, and D15 are such components. Error Amplifier and Bridge. The circuitry described thus, far would be sufficient provided the sample cell could be placed in the same position as THl a t the outflow from the heat exchanger. However, the thermostated cell is, in practice, a remote system and the fluid from the bath is circulated through it. Thus, if the ambient temperature in the vicinity of the sample cell is different from the thermostated output temperature, T,, of the heat exchanger o r the reactions in the sample cell are exothermic o r endothermic, the temperature of the sample cell, T,,, will be different than the desired value, T,. A temperature error, &AT, where AT = T , - T,,, would, therefore, occur in most cases. This temperature error can be eliminated, however, by adding the error amplifier and bridge circuit shown in Figures 1 and 2. A second thermistor, TH,, placed in the sample cell jacket is used as one arm of a bridge (similar to that employed with T H J which is connected to a differential amplifier circuit, amplifier 3. The output voltage of this high gain error amplifier (Analog Devices Model 801) is proportional to the magnitude of AT and is applied through R S 6to the summing point, SP (see Figures 1 and 2) of the control amplifier. Thus, the actual input to the control amplifier is the control bridge voltage plus an error signal voltage. The output voltage, which is now a sum of both signals, controls the time period of the heater (as described above). This time period will then be that necessary to correct T, to a new value T,' necessary to bring AT to zero. This corrected output temperature of the heat exchanger, T 8 ' ,will be equal to T , (the desired temperature setting) plus AT and the temperature of the sample cell, T,,, will be equal to T,.
1918
The control amplifier acts as a weighted summer with respect to the control thermistor bridge signal. The output signal of the error bridge thermistor amplifier damps the heater response to large error signals which prevents temperature oscillation in the bath fluid. This oscillation arises from the fact that rate of fluid circulation by the pump is finite, which introduces a delay in temperature response to the error signal at the sample cell. The control thermistor bridge signal has a gain of approximately 480 a t the output of the control amplifier. The error bridge signal gain of 215 is only multiplied by one, the control amplifier, thus a weighting factor of 2:l is obtained. This weighting factor allows thermistor, THl, to maintain primary control of the heater switching even when large temperature errors are present. Thus, extremely large power surges to the heater are prevented. DISCUSSION
This temperature control system has been tested extensively during kinetic studies (2, 7) and has shown that temperature control in the reaction mixtures in a 5-ml volume cell (2) was maintained at better than 10.01 "Cat all times. I t was found also that the temperature could be switched from 25 to 30 "Cin less than 5 minutes and from 25 to 56 "C in less than 20 minutes. On cooling, it was found that 19 minutes were required to switch from 56 to 37 "C using the quick cool (tap water temperature was 20 "C). About 30 to 40 minutes were required without the quick cool. Tests were also made to determine the time required to bring cold reagent sample solutions to the desired reaction temperature (as measured in the cell) in the stirred reaction chamber (2). It was found that an initial 23 "C temperature sample came to 30.00 "C in less than 2 minutes with continuous stirring in the reaction chambers, and an initial 19 "C sample took less than 3 minutes. I n common analytical practice, as for example in the determination of lactic dehydrogenase and blood urea nitrogen (2), we held these reagents to approximately 30 "C (10.5 "C)in a thermostatically controlled bath. Table I shows typical short-term temperature data measured in the cuvette. Also shown in Table I is a lactic dehydrogenase (LDH) determination where the reagents were at a room temperature of 26.5 "C. The mixed reagents and serum were continuously stirred for 30 seconds before entrance into the cuvette. The summing resistors and the components which determine the response of the system were chosen to give optimum performance with our sample cell. The cell should have no more than 18 to 24 inches of insulated hose connecting it to the heat exchanger, and a flow rate through it from 750 to 1000 ml per minute. For the lower flow rate, the error signal through R S 6has to be reduced. This is accomplished by potentiometers RS2,R53, R54, and Rss. The results of using this control with a different sample cell have not been determined because it would depend on the design of the cell. Flow rate, thermal transfer area, material used, thermal transfer to TH2,and insulation would be some of the factors involved.
RECEIVED for review April 10, 1969. Accepted August 12, 1969. (7) T. E. Weichselbaum, J. C. Hagerty, and H. B. Mark, Jr., ANAL. CHEM., 41, 848 (1969).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
