Construction of a Low-cost Mobile Incubator for Field and Laboratory Use

Incubators are essential for many microbial methods, particularly for culture-based analysis of drinking water. Here we address a need for low cost incubators for use in locations with limited infrastructure. We will detail the construction of an adaptable low cost transportable incubator using commonly available materials.

This incubator operates under a range of environmental conditions, and performs similarly to laboratory-based models. This method can be used for any activity requiring an incubator that can maintain a constant temperature in locations with or without a reliable access to grid-based electricity. It is possible to use different components than those described here as long as they fulfill the electrical requirements, although different components may influence how the incubator performs.

The main inventors of this version of the incubator are Jurg Sigrist and Christian Ebi, technicians from our laboratories. Before assembling the heating unit, gather one 280 by 250 millimeter support plate, two 60 by 60 by 25 millimeter axial fans, four 20 millimeter long spacers with an internal diameter of 4.25 millimeters, one luster terminal with three pins, four M4 and one M3 screw nut, eight M4 and one M3 washer, and four M4 and one M3 screw. Next, drill the appropriate anchorage holes into the support plate, for securing the axial fans and the luster terminal.

When all of the holes have been drilled, use two M4 screws, two screw nuts and four washers per fan to anchor the axial fans in the center of the support plate, using the spacers to maintain space between the fans and the support plate. Use M3 screws, screw nuts and washer to anchor the luster terminal to the support plate, and secure the fan cables. Then connect the fan cables with the luster terminal.

Connect the positive and negative cables of each fan together. To assemble the control unit, gather the universal enclosure, on/off switch, DC/DC converter with an input voltage range of nine to 36 volts, and an output voltage of 12 volts, a proportional integral derivative temperature controller with a 12 to 35 volts direct current operating voltage, a 12 to 15 millimeter cable gland with a crimping range of two to 7.5 millimeters, a temperature sensor platinum 100, and an AC power supply. Use a drill and a jigsaw to mill the openings for the PID temperature controller, on/off switch, and cable glands into the enclosure, and place the on/off switch and cable glands.

Connect the positive cable of the AC power adaptor to the on/off switch, and the negative cable of the AC power adaptor to the negative voltage input of the DC/DC convertor. Use a cable to connect the on/off switch to the positive voltage input of the DC/DC convertor. Connect terminal one of the PID temperature controller to the DC negative wire from the heating unit connection, and to the negative voltage out terminal of the DC/DC convertor.

Connect the DC positive wire connected to the heating unit to the terminal four of the PID temperature controller, and to terminal two of the PID temperature controller. Connect terminal two of the PID temperature controller to the positive voltage out terminal of the DC/DC convertor. And connect terminal five of the PID temperature controller to the command wire connected to the heating unit.

Connect the temperature sensor to terminals 10, 11, and 12. Then use hook and loop tape to anchor the DC/DC convertor to the bottom of the enclosure, and close the universal enclosure. To set up the incubator electrical core, gather two 100 by 200 millimeter 12 volt 20 watt self-adhesive heating foils, and connect the DC negative wire from the control unit to the luster terminal, and with one conductor of each of the heating foils, and the negative wire of each fan.

Then connect the positive wire coming from the control unit with the positive cable of each fan. And connect the command wire from the control unit to the remaining two conductors of the heating foils. To assemble the incubator, gather an incubator shell, and support rack.

Place the incubator shell on its side with the incubator door on one side of the shell, and place the support plate with the heating unit at the bottom of the incubator shell. Place the support rack on top of the heating unit, leaving a minimum of 10 centimeters between the heating unit and the support rack. And place the temperature probe on the support rack, and secure it.

Drill holes into the door of the incubator to allow insertion of the cables. Close the incubator tightly and connect the incubator to the power source. Then, power on the incubator, and adjust the settings of the PID temperature controller as experimentally appropriate.

For the representative incubator setups, the time to reach the set temperature in the incubators was influenced by the ambient temperature and the material of the incubator shell. At an ambient temperature of about 27 degrees Celsius, the three incubator setups reached the set temperatures in a similar amount of time with a performance of a standard incubator. In cold environments, the incubators with thicker shells reached the target set temperatures more slowly than under a normal ambient temperature, but in a similar amount of time to each other, while the incubator with the thinner insulation never fully reached the set temperatures.

In a warm environment, the three incubator setups reached the target temperature in under 10 minutes. However, when the set temperature of 37 degrees Celsius was lower than the ambient temperature of 39 degrees Celsius, none of the incubators could lower the temperature, resulting in overheating for all three incubator setups. In similar environments, the three incubator setups consumed 0.22 to 0.52 kilowatt hours per 24 hours less energy than the standard incubators tested, and under all setups and conditions, the growth of E.coli and total coliform was successful, and comparable to the growth observed in standard incubators.

The choice of the shell is critical. An incubator with a more insulating shell will perform better in terms of time to reach the set temperature, and in power consumption. It is advised that the construction and the wiring of the electrical components be performed by a person skilled in the electrical field.