A Practical Inexpensive Hydrostatic Switch Cutoffs in the water supply of chemistry laboratories can cause serious problems, particularly when reactions are left on reflux for extended periods without attendance. Our laboratories, located in buildings still under construction, frequently have problems of this sort and the chemist finds himself unable to leave the I& while running a. reaction. Having this problem in mind, we developed a simple safety device that responds to changes in weter pressure, interrupting current to heating haths and other equipment whenever the water pressure drops below a -& preset minimum. The heart of the system is 8. 5-cc hypodermic syringe cut near the needle end and suspended in the g1a.s apparatus, shown in the figure, hy s. rubber ring sa that it clean the glass walls by about 1mm. A hole of about 5 mm in diameter is cut in the cylinder wall opposite to the left elbow to facilitate water flow. The syringe piston, cut to about 2.2 cm,serves as the variable pressure controller, depending upon the amount of mercury it contains, (about 18 g will control the normal water flaw for a, condenser) by making and breaking contact with the Nichmme electrodes (minimum diameter 2 mm). The figure shows the device in close-switch position and also illustrates the low pressure position of the glass tube stop which prevents the piston from binding in the open-switch position.
404 / Journol of Chemical Educaiion
--,
--
