Mariion Bufalini and Joseph E. Todd University of Cincinnati Cincinnati, Ohio 45221
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An Automatic Toepler Pump
M a n u a l operation of a Toepler pump is extremely tedious and time consuming. Thus, it is very desirable to operate such pumps automatically. Automatic commercial Toepler pumps that are available usually have three tungsten wires sealed into the pump: one into the exhaust arm, one into the air/ vacuum arm of the mercury reservoir, and one sealed near the top of the mercury reservoir but which extends t o the bottom of the mercury reservoir. The mercury makes electrical contact between the tungsten wires which are connected to a solenoid relay control circuit. Although such pumps eliminate the problems associated with manual operation, they have two disadvantages.
When the mercury is allowed to rise into t,he piston chamber, a spark will result when the mercury makes electrical contact between the tungsten wire sealed into the exhaust arm and the tungsten wire a t the bottom of the mercury reservoir. In many applications this spark may affect the gases being collected or pumped through the exhaust arm. Also, the relay control unit is fairly expensive. Prices for commercial units range from $53485. The higher price includes the solenoid valves. The above disadvantages were circumveutcd by designing a transistorized relay control circuit with a photocell sensor to operate a Toepler pump automatically. The cost was only about half that of the commercial relay control units. Figure 1 shows the circuit for the relay control unit. Figure 2 shows the holder for the photocell and light
q-2Tz \
Pt Figure 1.
Circuit diagram.
Figure 2.
Photocell-light holder.
bulb. Figure 3 is a Toepler pump which shows the placement of the photocell-lamp holders. The relay contacts in the circuit diagram are drawn for the situation which exists when the mercury is in the reservoir. At this point the mercury has cut off the light beam from Lt on P2,and the light beam from LI is focused on P I . Relay R2 is deactuated and relay R1 is actuated, and as a result, relay LR is latched in the position shown in the diagram. This opens Sz which is the air leak solenoid valve. As the mercury rises into the piston chamber, the light beam from Lz will again focus on P2, and R2 is actuated. Even
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though this breaks the circuit, the air leak valve, SZ, will remain open since LR is latched in its position. The mercury will continue t o rise in the pistou chamber until the light beam from Ll which is focused on PI, is cut off by the mercury. This deactuat,es R1 which in turn act,uates LR. This causes LR to unlatch its
Figure 3.
Toepler pump.
positiorr aud relatch in the other position. When this occurs S,,which is a solenoid valve connected to a mechanical pump, is opened; and the mercury mill begin to lower iuto the reservoir. When the light beam from L, is again able to strike PI, relay R1 will be actuated. Although this breaks the circuit, S1 will remain open since I,R will be latched in this position until the light beam from Lz is prevented by the mercury from striking P f . When this occurs, the cycle is repeated. The air leak and pumping rates are controlled with stopcocks. Since the size of the glass tubing at the PI and PI
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photocell cutoffs was different, it was necessary to be able to adjust the intensity of the light beams. The inside of each holder was painted flat black and then a coat of aluminum paint was applied to the side of the holder containing the light hub. The inteusity of the light beams could be decreased iudependeutly with the variable resistors, VR, and VR2. In order to collimate the light beam, the light bulbs were painted with aluminum paint except for a circle of approximately the same diameter as the glass tubing iu the cent'er of the bulb. The holders were made from ceramic tubes which are normally used to line tube furnaces. Rubber stoppers were used to hold the screw light socliet,~and as covers at the other ends. The photocells were put tangent to the glass tubes as shown in Figure 2. The holders were split in half for ease in placement and were wrapped with masking tape after they were in position. By addiug a second transistor and a pot,entiometer to each half of the circuit, a variable sensitivit,y photoelectric relay system can be constructed. This system offers the advantage of response to extremely small light levels in addition to the capability of being adjusted to operate at selected light levels. The Toepler pump can then be operated automatically by usirtg only 3-w light bulbs, which virtually eliminates heating of the photocell holders by radiation. The authors will gladly supply a circuit diagram for this more sensitive photoelectric relay system upon request.
Journal of Chemical Education
Parh List
St, S2
Hoke, 2 way, normally closed S90A120 solenoid
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Sigma llF-1000G-Sil SPDT relays Potter and Brnmfield KBliDG 4PDT latch in^- relay . Motorola 2N176 PNP transistors TI,T9 International Rectifier Coro. B21I ohotovoltaic cells P,. PGeneral Electric S6 125 v, 6 w pilo; lamp Type 6-13 L;; L; candelabra screw light socket A 24-v hattery B 9-v battery V R X VRI , 1000-ohm variable resistors SPST t,oggle switch, standardvariety C,, C2 Rotary 6P switch, standard variety (3 poles we used D as offpositions)
R,, R, LR
