Laboratory Water Bath

My dry bath that I showed in the previous instructable felt a bit lonely so I made a wet water bath to give it company. I duplicated the previous dry bath construction except for a few changes: Replaced the aluminum block with a stainless steel pan that will hold water, used two of the ebay PTC heaters instead of one, did not use a thermal cutoff switch, and added a fuse.

Two PTC heaters rates at 100 W (110V) were attached with a thin layer of silicone GEII glue to the bottom of a stainless steel pan from the Daiso "dollar" store. The cured glue withstands a 200-300 C so should be ok for this application. The gaps between the heaters were filled with rubber strips from an inner tube tire and then covered with sheets of the same rubber. All the rubber was glued with the silicone glue. The rubber holds the heaters in place as well as insulate the bottom to some extent.

I used an exterior grade plywood which withstands water quite well. The two long sides were cut to measure from 1/2 inch plywood, and the front and back were cut out of 1/4th inch plywood. The front panel hd a cutout made for the temperature controller. The insides of the panels were painted with multiple coats of acrylic paint. The panels were edge glued and screwed. The stainless steel pan with the attached heaters was placed into the enclosure and the gaps at the top were filled with acrylic white filler. The white filler was also used to seal the open edges of the plywood at the top and to create a layer of insulation at the bottom of the pan. The filler was sealed with black acrylic paint. The exterior was then painted with a coat of black acrylic paint.

The wiring is relatively simple. The live wire from an AC cord goes to a fuse and then to a one pole switch. The live wire from the switch is connected to the AC power screw terminal of the temperature controller as well as to one of the relay screw terminals of the controller. The neutral wire from the power cord is directly connected to the AC power terminal of the temperature controller. Twin wires from the two heaters are also connected to the neutral terminal of the temperature controller. The remaining twin wires from the two heaters are connected to the second relay terminal.

Earth wire was left unconnected.

The temperature sensor was then wired to the requisite screw terminals.

The temperature sensor wires from the controller were soldered to an RCA jack on the back. And the temperature sensor was soldered to an RCA plug. A fuse was placed into the fuse holder and the system was plugged in, switched on with the front power switch and tested at different temperature settings after filling the pan with water.

After the tests were complete, a bottom panel of 1/4th inch of pre-painted plywood was added to the base. A couple of additional coats of paint were used to seal everything, labels were glued on and the whole thing was 'painted' with a couple of coats of polyurethane varnish.