Laboratory Dry Bath

The laboratory dry bath went together surprisingly quickly. The bio-community lab that I am a member off were throwing away aluminum blocks to hold test tubes. I grabbed them before they ended up in trash and one of them had holes which were a perfect fit for 1.5 ml tubes. I had also bought a few PTC heaters (positive temperature coefficient) from a vendor on ebay for less than $2/heater (110V, 100W in 2 inch long packages). These heaters have a maximum temperature so you do not have to worry about leaving them unattended. So in theory if I stuck this heater on the bottom of the block I should be able to get a useful heated block "bath". I tested this and the heater took the block to 95 C after about 30 mins. This looked promising. I also had a temperature controller from ebay so connected this in and it worked at controlling the temperature of the block.

So essentially over one weekend I assembled this compact dry bath which would maintain test tubes at the right temperature.

The design was simple. The PTC heater glued to the base of the aluminum block with silicone glue. The heater and block covered with rubber from car tire inner tubes to provide some level of insulation. And the block encased in a wooden box with space for the controller. And all painted in the classic black ...

The PTC heater had a flatter aluminum side that I glued on to the bottom of the aluminum block with a thin layer of silicone glue and little sand bags as clamping weight. After the glue had cured, I added two layers of rubber strips (from an old band saw tire), which were the perfect width and thickness, around the heater. Again the heavy bags were used to keep the strips in place. Finally, I covered the bottom with two layers of rubber sheeting rescued from an old car tire inner tube.

The silicone glue is resistant to high temperatures and the rubber can handle 200 C without too much hassle. Tested the block by connecting the wires to 110V AC to see of the heater still works. Works.

I applied a layer of rubber around the sides of the heating block. Two layers may have been better ...

For the enclosure, I measured the height of the block with all the rubber on the bottom plus added the height of the temperature controller (about 1 inch) and cut a long piece out of scrap exterior grade 1/2 inch plywood. I then cut this piece so that it would cover the two sides. Then measured and cut two pieces for the remaining two sides. I cut out the holes on the back piece for the wire, switch and RCA jack. On the front piece I cut a rectangular gap for the controller. Glued and screwed the sides with the aluminum block in place.

A white roofing compound was added to the top of the enclosure to seal it from moisture.

The wiring is relatively simple. AC 110V wire from an old laptop supply was brought in through the hole in the back panel and wired to two pins on the power switch. Two additional wires (harvested from the heater) were wired to the central two pins of the power switch. The switch was then attached to its location.

I had a thermal switch that I decided to add just for the heck of it. This was added in series into the live AC wire after the power switch and the thermal controller power in terminal.

The heater was connected to the neutral wire on the controller power terminal. The neutral wire from the switch was also connected to the same terminal. The live wire from the thermal switch was connected to the second power terminal on the controller and a jumper wire was connected from this live terminal to the relay switch terminal. The other relay terminal was connected to the heater.

The temperature sensor wire was cut and soldered to an RCA terminal on the back panel. The sensor was soldered to an RCA plug.

The controller was then held in place with double sided tape and wooden wedges. A support block was added to keep the heater pressed into the aluminum block and the bottom was sealed with the bottom panel.

The box was given a coat of acrylic paint and labels were printed and attached. Will add a layer of varnish later on.

The system was tested at 37 C, 65 C and at 95 C for 3 hrs each with an external thermomemeter. The temperatures were held within 2 degrees as I had set a hysteresis of 2 C. Will be changing this to 1 C. At 95 C, the outside of the enclosure got to about 45-50 C so could have done with more insulation around the block.

... one more handsome addition to my lab.