Devices for heating a fluid chamber, in which a temperature sensitive or temperature-initiated chemical reaction takes place, typically comprise a heater and a temperature sensor. The heater is controlled thermostatically to maintain a desired temperature so that it can be sensed by the sensor. In some circumstances, a reaction requires the reactant to be exposed to two or more different temperatures sequentially. Predicting the precise amount of heat to be applied, the instant energy is to supplied and the rate of supply of energy (heat) is no always straightforward because a time constant for the response of a system depends on many factors, including the geometry and physical structure of the fluid chamber, the heater and the sensor.
In particular, in polymerase chain reaction (PCR) amplification of DNA, the reaction is cycled between two or more, (typically three), different temperatures and rapid transition between temperatures as well as accurate maintenance of a temperature after a transition, are needed for optimal results. Frequently, as in PCR, a fluid chamber is provided as part of a fluid container removable from a device (termed a thermocycler in the context of PCR) which comprises the heater and temperature sensor.
In such cases the actual temperature in the fluid chamber and the time constant for heating depend on: the nature of the fluid container; and the heat transfer coefficient between the heater and the fluid container. Generally, the actual temperature within the fluid chamber differs from the temperature being sensed by a sensor on the heater device. Additionally it is advantageous to cycle the fluid chamber rapidly, through the required temperatures in order to complete the reaction quickly. The aforementioned factors therefore can lead to the temperature in the fluid chamber failing to track accurately the temperature applied by a heater device.
Various heater devices have been proposed which typically aim to increase the heat transfer coefficient (by reducing the thermal contact resistance) between the heater and the fluid container. One technique uses heater blocks that enclose the fluid container, thus permitting heating and cooling of the container by air flow over the container. Another technique involves placing the container in contact with a first block at a first constant temperature and then placing the container in contact with a second at a second temperature.