V O L U M E 2 3 , NO. 4, A P R I L 1 9 5 1 Table XI.
643
Time to Maximum Modulus
Time t o Max. Mod. a t B O o J-., Lot
Set.
1,I :O
Jlin. 60
I / R 50
L R BO
R
60 40
‘I’ahle X11.
Sanlple
Coniparison of Standard Stress-Strain a r ~ d Loop Tests
Standard T e s t , Tline t o Max Mod. at 600% L I 50 a t 40 at 260° F. 280’ r.,Min.
-
L I BO
LR 50
I / R 30
bet co
The loop test has heen adopted by this laboratory as an integral part of a revised evaluation system for compounds. I n the operation of the revised system, the loop test is made and cures for the remaining tests are determined from the results. CONCLUSIONS
.A hysteresis loop test made at low stress over a series of cures provides an extremely accurate measure of state of cure. The test is accurate and reproducible when made on test piece5 ivhirh have I)eeri conditioned and are tested under controlled con-
ditions of temperature and relative humidity. Furthermore, the type of test machine used and the autographic recording of the data practically eliminate the human element in testing. The accurate molding of the test pieces eliminates possibilitiee of error in gaging and compensating found frequently in the standard stress-strain test. The initial load data a t 50, 100, and 150% elongation on the shortest cure in the series provide a scorch test which is accurate enough for practical factory control. The ratio of initial modulus at 50% elongation to modulus on recovery a t 50% elongafion provides a measure of hysteresis loss or heat build-up. lleasurements made in the range of 50% elongation appear to be a more accurate measure of the properties of tire stocks, bemuse the test range more closely approache? the conditions under which tire stocks operate in service. Finally, the use of this test provides a considerable saving i n testing time. LITERATURE CITED
H., I n d i a Rubber W o r l d , 115, 61 (October 1946). (2) Fletcher. W. P., Rubber Chem. & Technol., 23, 107 (1950). (3) Holt et al., I n d i a Rubber World, 118, 513 (July 1948). ( 4 ) Mernler, “Science of Rubber,” p. 564, New York, Reinhold Publishing Corp., 1934.
(1) Dillon, 3 .
RECEIVED October 5 , 1950. Presented before t h e 57th International MeetC H E M I C ASocrL Ing of the Division of Rubber Chemistry of t h e AVERICAK F T T , October 11 t o 13, 19.50, Cleveland, Ohio.
Automatic Sample Changer for Windowless Gas Flow Counters W I L L I . 0 1 N. NYE
AND
.J. D. TERESI, Stanford University, Stanford, Calif.
A machine was desired which could obtain statistically significant data more readily from experiments involving the use of soft &emitting radioisotopes. This paper describes the design, construction, and operation of an automatic sample changer for use with gas flow-type windowless counter tubes. The instrument records the time in seconds and automatically changes each sample after a predeter-
W
I T H the ever-increasing use of i~idioactivt.cwhxi as a tool in research, the need has arisen for ail automatic. sample changer to be used with a windowless gas flow rouriter chamber. Although automatic sample changers are a t present commercially available, their design limits their use to the windowtype counter tubes. This paper describes the development of a simple automatic. sample changer which can be used with windo~dessgas flow counter tubes for the measurement of a-particles or soft p-particles.
mined number of counts have been made. A feature is included which preflushes two samples in 10 seconds while a previous sample is in the counting position. The geometry of each sample well was perfectly reproducible and the measurements of the same samples at various intervals of time were also reproducible. This machine has proved a great timesaver without the sacrifice of accuracy.
At any rate this would not be objectionable under normal operations of a Geiger-Muller counter since the measurement of 2 samples in 15 seconds with a standard deviation of 37, would require a radioactivity of about 80 counts per second for each sample. For a standard deviation of 1% the activity of each sample would have to be about 660 counts per second. These activities are much greater than those normally measured in research. Counting Chamber. The variables involved in counting chamber characteristics are the dimensions of the electrodes, the gas mixture, and the type of materials used in construction.
DESIGN CONSIDER4TIONS
Preflushing Feature. =\ny counter tube in which the ionizi6g gas mixture is introduced as a steady stream presents the problem of admixture of air with the counting gas during the interchange of samples. This has been obviated by designing the apparatus so that as one sample is in the counting position the next two are being preflushed with the counting gas mixture as shown i n Figure 1. The preflushing time was 10 seconds. This time interval is less than the time required bv the mechanical movements of the apparatus for proper function (about 15 secondq).
The gas mixture used in the apparatus described herein was obtained by bubbling helium through ice cold absolute ethyl alcohol. Using a tungsten wire (0.004-inch diameter) as the center electrode and a brass tube (3-inch length, 2.25-inch diameter) as the cathode, the operating voltage was 1600 volts and the plateau with a 2% slope was 170 volts. The “geometry” of the counter was 33%. The term geometry, introduced previously (I), is defined as the proportion, after correcting for absorption, of the total number of disintegrations per unit time that are actually counted. The counting chamber diameter was chosen to accommodate the mounting plates (1.75-inch diameter) used in this laboratory. If smaller mounting plates are used, the chamber size
ANALYTICAL CHEMISTRY
644
can be smaller. However, a decrease in the diameter of thcl cathode results in a decrease in the geometry or counting efficiency of the chamber. CONSTRUCTION
Table I. Determination of Reproducibility of Geometry of Sample Wells
Figures 1 and 2 show most of the construction details. I t is essential that a rigid platform be used as a base, in order that the fixed shaft, C, maintains its spatial relationship with regard to the sealing plates, T and U . For this platform a 30inch length of 12 X 3 inch channel iron, with the drive motor mounted on one end and the sample changer on the other, was used. The end on n-hich the changer is mounted should be milled flat to within *0.002 inch. The sample-well table, L , must slide through the sealing plates bvithout too much friction (minimum clearance about 0.002 inch) and yet fit sufficiently close to prevent admixture of air with the gas (maximum clearance about 0.008 inch). It is difficult to mill or turn large pieces of rolled sheet metal to this close a tolerance; thus, better results would be obtained if these three pieces were made from cast or annealed materials. By using dissimilar metals, the friction between surfaces is decreased. The supports, Q, and separators, R, may be supplemented with shims to improve the fit. The drive motor is mounted on the opposite end of the channel iron base. The drive unit was the antenna reel assembly used by armed forces aircraft. This has the proper gear reduction ratio to give 35 r.p.m. and has a magnetic clutch which prevents coasting, a necessary feature where accurate placement is required. This unit, equipped with an %point sprocket (0.375-inch pitch), was connected to the changer with 5 . 5 feet of rollerless 0.375-inch chain. The manner of recording time for a sample to reach predetermined number of counts is illustrated in Figure 2. The synchronous motor turns only while the scaling circuit is counting. and the number of turns is recorded by the revolution counter in opposition to it. Since each revolution takes 1 second, the _counter records t,he number ol' 3econds required to reach the p r e d e t e r m i n e d number of counts. This method of recording time requires about one tenth the cost of timing d e v i c e s which p r i n t t i m e started and time finished for a 16-sample unit. For a larger number of sample wells the comparison is not so favorable, inasmuch as one revolution counter is' required for each sample well. T h e s a m p l e changer, as shown here, uses aluminum disks 1.75 inches in diameter and 0.031 inch thick. For other sizes, the dimensions of the sample well table and c o u n t i n g c h a m b e r may be changed to suit requirements. The electrical circuit shown in Figure 3 was the most satisfactory of several tried with the T r a c e r l a b Autoscaler. Suggested changes for use with other instruments are made below. I n the Tracerlab instrument there is a relay which is pulled down a t the full count and released on the half count. The latter position activates relay 4, and this is held in position with a holding contact. When the instrument reaches f u l l c o u n t , c u r r e n t flows through contact D on mounting strip 27, to contact on relay 4 to activate relay 5. This shuts off the clock motor a n d s t a r t s t h e sample-
Figure 1.
A.
C. D. E.
F. G. H.
I. J.
!
c, b $1 10 11 12 13 14 1,?
88
91 91 92
16
Table 11.
Reproducibility of Assay Values of Various Samples
Average Time for 1024 Counts, Sec. Sairiple Trial 1 Trial 2 Trial 3 Trial 4 220 ( 1 7 ) 214 ( * 4 ) 218 1 213 ( * 4 ) 219 ( 1 7 ) 223 ( * 5 ) 220 2 217 ( * 4 ) 253 ( 1 7 ) 243 ( * a ) 250 3 238 ( * 5 ) 1353 (*39) 1369 ( ~ 2 7 ) 1358 ( - 2 7 ) 1361 241 ( * 5 ) 236 (=:I 236 236 (*?I) "9 ( - 4 ) 209 ( * A I 212 207 ( * 4 ) 205 ( * 6 ) 207 (*.I) 204 206 ( * 4 ) 243 (*7) 245 ( - 5 ) 250 241 ( * 5 ) 222 ( - 7 ) 231 ( * 5 ) 230 9 227 ( * 5 ) 210 ( *6) 205 ( * 4 ) 210 10 203 ( * 4 ) 208 ( *6) 20; ( * 4 ) 206 11 208 ( * 4 ) 225 ( *7) 231 ( * 5 ) 234 12 227 ( i 5 ) 1343 ( *40) 1353 ( * 2 7 ) 1320 13 1354 ( - 2 7 ) 219 ( - 7 ) 221 ( * 5 ) 229 14 227 ( * 5 ) 260 (*8) 249 13 245 I =t;) 242 ( * 5 j 210 ( - 6 ) 219 I - . ! ) 213 16 202 ( * 4 j
b
F. Groove
(1/16
X
3/16
Trial 5 216 ( 1 4 ) 218 ( *4) 246( 1( *257)) 1364 237 ( * 5 j 208 ( 1 4 ) 210 ( * 4 ) 247 ( * 5 j 235 ( * 5 ) 208 ( * 4 j 207 ( 4 4 ) 238 ( * 5 ) 1325 ( * 2 6 ) 229 ( * 5 j 243 ( ' 5 ) 208 ( * 4 )
inch) in upper surface of lower sealing plate
Groove ( l / i e X 8/16 inch) in lower surface of upper sealing plate Holes c u t i n sprocket t o reduce wei h t Placement switch (Microswitch. SAT) Placement switch supporting a r m Clock motor (60 r.p.m., 60 cycles, 110 volts) Veeder-Root square case revolution counter form P-8 (1 of 16 shown) Fixed s h a f t Clock motor support a r m Placement switch tripping pin (1 of 16 shown) Adjustable collar Upper threaded locking collar Seconds counter table Upper ball bearing ( S o r m a - H o f f m a n B 5 4 l ) Rotating shaft
2 to 5.
B.
1 2 3
Standard Deviation 12.0 *2,1 12.1 *2,1 -2.1 -2.1 t2.0 *2.0 i2.0 iZ.1 *2.2 i2.0 -2.0 -2.1 *2, 1 *2.1
Plan Assembly Detail of Automatic Sample Changer
1 t o 3 a n d 4 to
7. 8. 9.
Time for 2048 Counts, Second89 92.5 90.6 92.5 89.4 90 88 89 88 89,; 93 89
' \Yell
K . Lower threaded locking collar L. Sample well table .]I. Sample well (2 of 16 shown) s, Sprocket separators ( 2 of 3 shown) 0. Sprocket (112-tooth J/a-inch pitch) P. Fixed s h a f t base ( f k e d shaft brazed t o base, then turned down in lathe) Q. Counting chamber support column ( 2 of 3 sliomn) R. Sealing plate separators (2 of 3 shown) S. Counting chamber T. Lower sealing plate c. Upper sealing plate 7. Gas inlet w.High voltage terminal s. Gas outlet Y . Lower ball bearing (Xorma-Hoffmann B b 4 l )
(Placement -witch and its supporting a r m not shown)
V O L U M E 23, N O . 4, A P R I L 1 9 5 1
645
I
Figure 2.
Elevation Section of Automatic Sample Changer Letters refer t o s a m e p a r t s as in Figure 1
24
28
Figure 3.
Electrical Circuit for Automatic Sample Changer
Sensitire d.c. relays ( S P S T normally closed 10,000 ohm.) 9. Relay (110 volts 60 cycles SPST normally open) ?. Relay (110 volts' d.c. D P S T , normally open) .i. Relay (110 volts: d.c.: D P D T ) ti. Sensitire d.c. relay ( S P D T , 3000 ohms) 7 . Relay, counter t y p e (110 volts, 60 cycle\) 8 . Selenium rectifier (36 volts 3 a m p . ) !). Transforiiier (110 volts t o 36 volts1 10, 11. 12. Toggle switches ( S P S T ) 13. Resistor ( 2 5 ohms, 10 v a t t s ) 1-1. Resistor (0.47 megohm, 0.5 watt) 15. Resistor (10 megohms, 0.5 w a t t ) 16, 17, 18. Electrolytic condenser (10 mfd., 250 rolt.;r 19. Electrolytic condenser (120 mfd.. 230 volts) 20, 21, 2 2 . Paper condenser (0.5 mid.) 23. S e o n bulb ( S e o n 16) 24. Table-rotating motor (24 volts d.c. 3000 r.p.m.) 25. Synchronous motor (60 r . p , m . , ' l l O h t s , GO cycles) 20. Mirrorwitch (SPDT) 28. Selenium rectifier (I50 Ilia.i 2 7 . .\loitnring strip (6 terminal) 2Y. P u s h b u t t o n swirch (normally era1method for the determination iylparathion in technical preparasolid and liquid. It is based on ,lorimetrio determination of the pSpecial problems caused by free y, dimethylparathion, and liquid
O F ANALYSIS
p-nitrophenyl thiophosphate, has years as an insecticide. A method is ( 1 )isvaluable for the determinar _ l_.l__l_ I,.-."."..I___ tion of small amounts of parathion in spray and dust residues, but i t cannot so well be applied to technical preparations and commercial formulations. Parathion is an ester of p-nitrophenol, and on saponification by boiling with 1 N alcoholic sodium hydroxide p-nitrophenol is easily split off. The p-nitrophenolate ion shows %very&rang absorption in the
violet regia.. .._"__ -_.__ ...-. This absorption is used for spectroscopic determination. As the malar extinction coefficient a t this wave length is about 20,000 om.-', which is about one half the value found for the magenta dye used by Averell and Norris (f), this method allows the detection of amounts of about 10 t o 20 micrograms. It can thus also be applied for the detection of traces in air and spray residues and may, by its simplicity, compete with the former method in a number of cases. I
INTERFERING SUBSTANCES
Technical parathion, if prepared according to the methods known in the literature, may contain as impurities p-nitrophenol, O-ethyl 0,O-bis(p-nitrophenyl)thiophosphate, p-nitrophenetal, and triethyl thiophosphate. p-Nitrophenol. Variable quantities of unreacted p-nitrophenol (free p-nitrophenol) may be present and have t o be determined separately because on saponification i t will be also found and attributed wrongly to parathion.
