Inverted (upside-down) Bucket, Curd Incubator

In this Instructable an inverted (upside-down) bucket is used to build a temperature controlled curd (yogurt) incubation chamber.

A standard low-voltage DC supply is used to power a Nichrome wire heater element raising the temperature of the chamber. An old CPU fan is used to circulate the air within the chamber and a thermostat maintains the temperature around 35 deg C .

Measurements on the unit showed a current of 1.6 A with the 12 V supply and a heating power of 19.2 Watts. The maximum temperature rise was found to be 25 deg.C above ambient. With the thermostat added in series and suitably adjusted the temperature trip points were 33 deg.C (lower) and 40 deg C (upper). The average temperature would meet the ideal, 35-45 deg C temperature range for setting curd.

In Bangalore where I live the minimum temperature ranges between 16-20 deg C over the year and this chamber should be able to comfortably maintain a suitable temperature for setting curd.

The highlights of this scheme in addition to its simplicity, low-cost and absence of any electronic circuitry is the inherent safety, as no high voltages are used and the 12 V DC is isolated from the mains.

1. 5 Liter thick walled plastic bucket
2. 110/220 V Ac to 12 V DC 2 A power supply ( a typical modem supply would be suitable)
3. 1.5 meter length heater wire straightened from a 220 V 1000 W heater coil (approximately 8 Ohms )
4. CPU fan 3 inch , 12 V DC
5. 4 inch water-heater thermostat
6. Bubble wrap plastic sheet
7. Cardboard chart paper

The coiled heater wire is straightened out by drawing it over a round metallic object and a length of 2 meters cut away.

A set of holes is drilled for threading the heater wire in an crisscross fashion.

The CPU fan is fixed on the inner side of the bucket using one inch spacers and suitable screws. The fan is oriented to blow downwards for circulating the hot air within the bucket.

The heater wire is laced in a crisscross fashion with heat-shrink sleeve added at the portions around the holes. This avoids direct heating of the wire and the bucket plastic.

The thermostat is fixed on top of the inverted bucket with the stem entering into the chamber.

The heater wire and CPU fan are connected in parallel and then connected in series with the thermostat. The connecting wires are brought out to the top.

Bubble wrap plastic sheet is wrapped around the outer surface of the bucket. This provides a thermal jacket preventing undue heat transfer to the outside.

Craft cardboard paper is wrapped around the entire thermal jacket and provides a neat aesthetic look to the thermal chamber.

The lead wires are soldered to a standard female jack which is used for connection to the 12 V DC power supply.

A vessel containing warm milk mixed with the curd starter bacteria is placed within the chamber.

The chamber is placed on a non heat conducting surface (plastic foam sheet).

The vessel can be of one liter capacity with a maximum height of 100 mm ( 4 inches).

The chamber power is switched ON and left for the curd to set overnight .

The curd once set is refrigerated.

*pencil only for illustrative purposes