A Simple Water-Still ~ i a r m In one of our laboratories, the source of distilled water is a steam-heated still that delivers into a large carboy. This takes several hours t o fill, a fact that is easily forgotten. Overflow, with consequent flooding, is therefore quite likely. Obviously, some kind of alarm is desirable. A float system that operated amercuryswitch was first tried but was not suited t o the narrow neck of the carboy. T o avoid contamination, this neck is closed by a well-boiled-out rubber stopper that carries glsss tubes for connection to the still far draw-off of water by siphon action and for pressuizing to start this action. Two additional boles were bored in the stopper, to support two 4-mm a d . X 150-mm long glass tubes, each of which carries a 10-mm-square bright platinum foil electroiie. The electrodes, which are arranged facing one another and approximately 10 mm apart, come in contact with the water only when the level is some 15 min from overflow conditions. Miniature twin flex is used t o conned the electrode system t o the monitoring device, the circuit ofwbicb is shown in the left figure.
Mon lorlng device. B.9V oanery: 0,diode 1N4001: L, relay (see text) P. piazo pu sing buzzer; 0,. 4,transislor 2N4123; 03. Rans.star MPS-A 13. R,, resistor 1 MR: R2, v a m o e re* stor 10dl;Rg, resistor tsee text,: R,, variaole resistor 100dl.s,.SPST swwh (POWER,: s2.SPST pushbunon switch (TEST)
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To obtain a hieh .. inout imoedanee. transistor Q, is connected as an emitter follower. If the electrodes are clear of the water, '&.and henre tmniisto;()z, is rut off. Whm wnrer begins ro hridgetheelecrrodrs, its poor condueranre is sufficient to turn on QIand 0.. Relay L then pulls in and energizes piezoelectric hunzer H. This dwire emit3 nn intermittent highpitctlrd squeal that is fuu irritatmg tc, he igntrred. The rrlueal can he sn,pped only hy openmg rwrrrh SLor hy siphoninl: uff water so that the electrodes are clear of its surface. Pushbutton S1 allows the operation of the alarm to he tested, while control Ra adjusts the sensitivity. A 9-V supply is ample to sound the alarm when the water has risen approximately 1mm above the hottom edges of the electrodes. The value of Rg is chosen to suit the rating of L. For the 6V-330 Q relay actually used, Ra is 150 Q. The monitoring device is contained in a 4- X 3% X 2'18-in. aluminum minibox that is mounted adjacent t o the carboy. The even simpler alternative monitor circuit is shown in the right figure; i t has not been tested extensively, but appears t o work well. A single Darlington-type transistor is used. The eventual plan is to install an electromagnetic steam valve that will stop distillation automatically when the alarm sounds. ~
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John 1.Stock
u n i ~ e r sof i ~Connecticut Storrs. CT 06268
814
Journal
of Chemical Education
