Automated temperature control of a multiple number of sample tubes, or test tubes, is required in many applications such as, for example, biological or biochemical material stability studies, enzyme reactions, enzyme kenetics, DNA/RNA denaturation, freeze-thawing of biochemicals and biologicals, and bacterial transformations, etc.
Typically, the temperature is controlled by immersion in liquid baths or by holding tubes in dry blocks that have heating and cooling elements for regulating and controlling temperature.
The dry block designs generally employ flat metal blocks that are heated and cooled, for example, by a Peltier Element or by pumping heating or cooling fluids through bore holes in the metal blocks, or heated by electrical heating elements.
The typical flat block design utilizes a planar arrangement of wells for sample tubes which are held in a vertical position as illustrated in FIG. 7. This arrangement will lead to an ununiform heating of the sample tubes. Because sample tubes B and C (FIG. 7) are near heaters 1, 2 and 3, they will receive twice the heat load of sample tubes A and D. That differential heat load is maximized when the block material is a poor conductor of heat. Using highly conductive material, the differential heat load can be minimized during certain conditions, for example, steady state and low influx of heat. When the system requires a high rate of change in temperature, a minimum of 250 watts (depending on the weight of the block) is required to induce the desired rate of temperature change. Under these conditions, edge effects appear in the distribution of heat. Certain laboratories have measured temperature variations as much as 5.degree. C. in sample wells or bores (particularly comparing the edge wells to the center wells) during a heating or cooling cycle. See, Resendez-Perez, D. and Barrera-Saldana, H.A., "Thermocycler Temperature Variation Invalidates PCR Results", Biotechniques, 9, No. 3, p. 286-292 (September 1990).
Any flat block design that uses more than one heating element can experience further uneven distribution of heating/cooling when using a single control device. This result is due to the different tolerances in the heating or cooling ability (watts/square inch) of the individual heating elements. Using a single control device with the block illustrated in FIG. 7, sample tube B can receive an additional 20 watts of heating power more than sample tube C because the tolerance between different heaters can be high as 4%.
It is desirable to control the temperature of all the sample tubes within 0.5 degrees Celsius or less for static conditions and for dynamic conditions where temperature changes at the rate of up to about 1.degree. C. per second. Improvements in dry block designs to achieve this goad are still being sought.