J. H. Larkins, J. D. Perrings, G. R. Shepherd, and 8. J. Noland
An Automatic Recording Multigradient
University o f California Los Alomos Scientific Laboratory Los Alamos, New Mexico
Capillary Viscometer
D a t a obtained from measurements of solution viscosities may be used for a variety of purposes. Products of industrial processes are often monitored and graded on the basis of their viscosities. Valuable dat,a concerning the size and shape of molecules may be obtained from measurements of the viscosities of their solutions. The capillary viscometer has been widely used owing to its sensitivity and accuracy, simplicity, and sound theoretical basis, and an automated version of the Ostwald viscometer has been described.' The Ubbeholde capillary viscometer and its modifications may be used for measurement of a series of dilute solutions a t each of several gradated rates of shear. Such measurements are especially valuable when applied to non-Kewtonian fluids. Measurement of flow times with a multigradient capillary viscometer is a tedious process requiring strict attention on the part of the operator for extended periods of time. Operator fatigue, particularly during the latter of a series of measurements, leads to errors in an inherently precise technique. Accordingly, an automatic photoelectric system was designed and constructed to perform t,he critical operator reading functions. Apparatus
The viscon~eterdescribed by Eigner2 consists of a reservoir, a series of four measuring bulbs which flow into a coil of 1-mm capillary tubing, a side tube for venting the lower end of the capillary to atmospheric pressure, and the reservoir. Measurements may be made by timing the downward passage of the meniscus as it passes lines scribed on the small sections of tubing above, between, and below the measuring bulbs. The timing operation is usually carried out by the operator who actuates a series of stopwatches, each of which corresponds to an interval between scribe marks. It has been observed that there is considerable difference between the transverse light transmission characteristics of an empty glass tube and of the same tube filled with a liquid. The focusing effect of the empty tube upon a light beam produces two bright lines separated by a dark region. Liquid in the same tube causes the central region to brighten. A meniscus
This work was performed under the auspices of the U.S. Atomic Energy Commission. GOLDFINGER, G., AND GREATBATCH, W., Imtrummts and Autornatia, 30,88(1957). 1 EIRNER,J., Doctoral Dissertation, Hrtrvard University,
passing through the beam will accentuate this difference. Pilot experiments were conducted in which a miniature incandescent lamp and a photodiode were placed on opposite sides of a glass tube. Fluctuations in light intensity during the passage of a meniscus were recorded on a strip recorder. Figure 1 illustrates the effect of linear meniscus motion upon the intensity of light reaching t,he photodiode.
Figure 1.
POSITION OF MENiSCUS Response of photodiode to linear movement of menirsur
The nature of the response was found to depend mainly upon the relative positions of the light source and detector, the diameter of the glass tube, and the intensity of the light source. Once these parameters were fixed, the response was found to be quite reproducible for each source-detector pair, both in air and when submerged in distilled water. The final instrument design is illustrated in Figure 2.
Figure 2. Diogmm of multigradient capillary v i ~ o m e t e r and housing. I i ) Photodiode.; (21 miniature incandescent lamps; 13) Lueite housing; 14) viscorneter bulb; (5) capillary tubing; (61 leads to incandescent lampsi 171vocuum releare vent; 181 Lvcite plater; (PI Lucite yokes; ond (101central vixometer barrel.
Volume 42, Number 10, October 1965
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555
Light sources and photodiodes are installed in Lucite housings, each pair corresponding to a viscometer timing locus. Housings are held in alignment by Lucite yokes which clamp around the central tube of the viscometer. Each photodiode is connected to a separate transistorized trigger circuit, as illustrated in Figure 3b. Passage of a meniscus causes the photodiode response to pass through trigger points C and D (Fig. I), energizing the relay coil. Relay 1 activates the clutch mechanism of timer clock 1. Relay 2 deactivates the first clock and activates the second. Relays 3 and 4 function in the same manner. Relay 5 deactivates the fourth clock and activates an end-of-run alarm. Drawing liquid back into the tubing causes these relay coils to be deeuergized in preparation for the next operation. Individual variations in the characteristics and positioning of each of the source-detector pairs may be overcome by providing controls for adjusting the intensity of each light source. Trigger points may then be set for the optimum value of each circuit. Once set, this adjustment appears to be stable over a period of several months.
operation. The use of high-sensitivity relays in conjunction with low-voltage transistors would eliminate the need for the 40-volt power supply.
Figure 310). Dl, Ds L R, R* RaRi T
Power supply for lomps.
- Zener Diodes, 7 5 v 1 0 won - Welch Allyn No. 2 Otoscope Lamp
- Resistor, 8 0 0 ohm 1 0 wan Resi.tor, 0.47 ohm 1 - Rheostat, 2 ohm 2 w o n - Fiioment transformer, Pri. 1 1 5 won
v, Sec. 6.3 v, 1.2 omp CT
(haif of recondory used)
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Figure 3(b).
Discussion
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Tngger ond relay circuits I 5 required).
Tronrlrbrs
The table presents the data obtained from a series of replicate determinations performed with distilled water and with a 20% aqueous glycerol solution. Greater precision is obtained in bulbs 1 and 2 than in bulbs 3 and 4. The source-detector pairs may be relied upon to respond repeatedly in the same manner to light intensity and, therefore, to linear meniscus position. The lesser precision in bulbs 3 and 4 may be ascribed to variations ih flow rate while the shortened liquid column, under a decreased pressure head and traveling a t a lower velocity, is passing through the lower portions of the viscometer. These results, however, compare favorably with similar data obtained by visual timing. The automatic viscometer eliminates the human facton of anticipation and fatigue-induced loss of reflex time. This device might, therefore, be recommended for use inlaboratories performing multiple routine viscosity determinations. Future versions of this instrument might well incorporate several suggested modifications. Mounting each source-detector pair in an individual yoke would eliminate several alignment problems encountered in the single-mount unit. Scribe marks between bulbs might be eliminated in a unit designed for automatic
Resirton (% wan, 5 % carbon, all valuer in ohms) 20K RI Rlr Ra 1K R3 - 3.3 K R4 - 1.8K Rs 5.6 K Rs 6.8 K R7 - 2.2 K Rs - 0.62 K
-
Other Com.onenb Ci Cz DI. D2 PD RI
Figure 31c). CrCI RlrRir A PL
-
Capacitor, 0.05 MFD 1 0 0 v Electrolytic Capacitor, 2 5 MFD 5 0 v Silicone Diode. 5 0 0 M A 1 0 0 v Photo-diode, ~ e rInst. . lN2175 Reioy. Potter and Brvmfleld MH3362.
Clock circuits.
- iimer Clocks, Standard Electric, 1 0 0 0 semnd - Relay Contact., SPDT, Poner and Brvmfleld M H 3 3 6 2 - Alarm Light and Bell - Pilot Light, 1 15 v AC
Viscorneter Performances Using Distilled Water and a 20% Glycerol Solution as Reference Liquids
Bulb No. 1 2
556
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Average flow time (set)
324.33 367 95
&O Mean deviation 0.060
n inn
Journol of Chemical Educai~on
Standard deviation of the mean 0.0876 022.5~
Average flow time (sed 594.61 674.93
20% Glycerol Mean deviation 0.105 0.075
Stmdard deviation of the mean 0.128
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