An Automatic Constant Flow Regulator for Low Gas Flows LESLIE SILVERMAN, Department of Industrial Hygiene, Harvard School of Public Health, Boston, Mass.
T
mm. on the inclined scale. With the desired flow found, contacts B and C are set to within 1 mm. of the mercury meniscus.
HE regulator described was designed to maintain a constant flow of 0.5 liter per minute through gas recorders for low concentrations of hydrogen sulfide and carbon bisulfide, but it can also be used for other gases and vapors. The
Motor Conversion A universal motor is easily converted into a reversible motor. An old Victrola motor was used here, but a laboratory slow-speed stirrer or mixer can be used. The brush leads are disconnected, the wires are brought outside the housing, and two leads are fastened to the brushes. A shunt-wound motor can be reversed in a like manner. This gives four connections, as shown in the upper left of Figure 1. When F 1is connected to A1 and Fz to AD, the motor will run in one direction, and &Then the field leads are interchanged-that is, F1 t o A*, and Fz to A1-it will run in the opposite direction. These leads are connected to the relays, as shown in Figure 1. The relays operate in pairs, essentially the same as double-pole relays. When connection AB is made, the left-hand pair closes and the others close when connection AC is made. The handle from a brass Hoke needle valve, similar to the control valve shown, was removed and a piece of 0.94-em. (0.375inch) brass tubing of 0.31-cm. (0.125-inch wall) was soldered to it. This valve was then coupled directly to the motor-worm wheel shaft by using a piece of vacuum hose and two light cotter pins. This gave a flexible coupling, so that the motor shaft and valve
regulator will automatically compensate for increasing or decreasing line resistance and will also take care of variations in voltage or water pressure, depending upon the suction source. (This apparatus was built in conjunction with the development of recorders by Philip Drinker for the American Viscose Corporation.) The apparatus is shown in Figure 1. It consists of en X-type inclined manometer connected across a capillary flowmeter and a by- ass valve operated by an electric relay. The manometer is maie of 7-mm. (4mm. inside diameter) glass tubing with the ends flared for 00 rubber stoppers. A piece of platinum wire is fused into the lower part of the manometer for a contact and the Nichrome wires shown form the other two contacts needed. The flowmeter is calibrated directly using a wet gas meter. Only readings on the rising leg need be taken. Using the ratio 1 to 8that is, the sine of the angle is l/g-it was found that a 1-mm. capillary, 95 mm. long, gave a rising leg deflection of about 15
FIGURE 1. DIAGRAM OF -4PPARATTS 292
ANALYTICAL EDITION
MAY 15, 1940
stem did not have to be aligned exactly. If the valve sticks, the cotter pins will shear off and prevent damage to the motor. The valve base was mounted rigidly, so that it would not turn with the stem. I n other designs, it should be possible to couple the motor directly to the rheostat and thus get current, rather than airflow regulation.
Operation I n operation the desired flow is set by the control valve and the by-pass valve is set approximately halfway open. The by-pass will then take care of either high or low deviations from the desired rate.
293
By using the variable resistance, the correct motor speed can readily be determined. Too high a motor speed will cause continued oscillation of the manometer; too low a speed will cause sluggish response. The relays used were obtained from an old piece of electrical equipment and operate directly from a 110-volt source. Hence, condensers were placed across the contacts to reduce arcing; 0.25-mfd., 110-volt, is adequate capacity. The relay switch should be opened when the apparatus is not in use. A small bottle can be placed in the line after t)he flowmeter to trap the mercury if any relay contacts fail to open.
Laboratory Electric Stirring Motor ( oiiverbe
A
E. B. HERSHBERG 3Ieniorial Laboratory, Harvard University, Cambridge. Mass.
W I I E variety of stirring motors is on the market, and
yet few possess the characteristics desired in the organic laboratory. Small electric motors develop but little power a t low speeds and direct drive is usually unsatisfact'ory for any but the lightest and most constant loads. If t'he character of the mix changes rapidly, or if the load is re- . leased suddenly, the motor may speed up to a point where breakage occws. For this reason laboratory stirring motors should incorporat,e some type of speed-reduction mechanism which allows increased torque a t low speed. The method of mechanical speed reduction is usually the determining fact'or in the cost, marketability, and life of a stirring motor. The simple belt and pulley drive device has been neglected in favor of more exclusive and complicated designs with possible patent features. The mediocre results experienced with a number of these over a period of several years have led t'o the development of the motor and pulley assembly with electrical speed control described below. This unit has been found to be very satisfactory for driving wire stirrers (1). The series-wound motor is particularly suited for this purpose because of its favorable speed-torque characteristics. Sparking a t the brushes has not been found to be a serious fire hazard and the ordinary organic l a b o r a t o r y p r e c a 11 t i o 11 i
suffice. Shaded-pole induction motors are admittedly safer, but their essentially constant speed characteristics make them useful only in connection with a step pulley where a
FIGURE1 . BODY A.
B.
Push fit bearing seat, 22-mm. (0.8661-inch, -0.0000-inah, 0.0008-inch) bore Set screw, brass, %/a inch, 24 threads, or pipe tap
+
