Liquid Culture Systems for in vitro Plant Propagation

Bioreactor design for propagation 45
2.2 Temperature measurement and control
The temperature can be regulated between 15-35 �C and measured in the
inner vessel by a commercial 100 ohm platinum temperature sensor
(Termoelektro). The sensor is inserted into a stainless steel tubular pocket
dipping into the medium. Platinum elements are considered to be the most
stable commercial sensors available (Heyerdahl et al. 1995). The steel tube
creates a sterile barrier that allows for removal of the sensor when the inner
vessel is taken away to the laminar flow hood. The controlling computer
continuously measures this temperature and compares it with the set point.
Any deviation causes the controller to either increase or lower the
temperature of the circulating water in the jacket to compensate. By this
arrangement it is the medium temperature that is controlled.
Some bioreactors control only the water jacket temperature. This is less
than ideal since significant temperature differences may occur between the
jacket and the medium, mainly depending on how much heat is gained or
lost through the lid or gained through stirring. Heat transfer through the lid is
mainly dependent on the temperature difference between the medium and
ambient air.
The water in the jacket is circulated by a 12 V garden fountain pump
(Eheim, Type 1046, Serial nr. 90081/92011, 5 W, 5 l min -1 ). The water
passes through a small self-made coaxial tube heat exchanger and which also
contains a 100 W electric heating element (Watlow Cartridge 8 x 16 mm,
1/2"NPT). The heating element is switched on and off in a 1 sec continuous
cycle. By altering the on/off time-ratio, the controller can adjust the mean
applied power from 0 to 100 W. Flow-controlled cold tap water flows
through the heat exchanger primary side and thus cools the jacket water. The
tap water valve is of a self-made pinch type and consists of a 12 V
reversible-direction motor with an eccentric disk that opens or closes the
tube in a continuous way. This motor is on/off controlled and runs for a
predetermined time when actuated.
Thus the temperature controller can act on two actuators in parallel:
heating power and cooling water flow. Our control strategy is to use the
heating element mainly; the water flow valve is used only to maintain the
applied heat power within a range of about 20-80%. The controller mostly
operates the valve only after a temperature set point change, until new heat
equilibrium is obtained.
When the inner vessel is taken away for work in the laminar flow hood,
the temperature sensor is withdrawn from the pocket and placed into the
water in the outer vessel. In this way the circulating water temperature is
maintained at about the right temperature and ensures the quickest return to
set temperature when the vessel is assembled again.