. Electronic apparatus for biological research . t t (c) (d) Figure Approach characteristics of controlled bodies heated from a lower temperature, (a) Asymptotic approach of a body to the maximum temperature produced by a fixed wattage heater; (b) overshoot caused when a booster heater of large thermal capacity is not switched off till control tempera- ture is reached; (c) ''damped oscillation^ produced by a thermostatically con- trolled single heater of large thermal capacity; {d) approach of a small thermal capacity heater, controlled by mark-to-space—heating is at maximum rate until c
. Electronic apparatus for biological research . t t (c) (d) Figure Approach characteristics of controlled bodies heated from a lower temperature, (a) Asymptotic approach of a body to the maximum temperature produced by a fixed wattage heater; (b) overshoot caused when a booster heater of large thermal capacity is not switched off till control tempera- ture is reached; (c) ''damped oscillation^ produced by a thermostatically con- trolled single heater of large thermal capacity; {d) approach of a small thermal capacity heater, controlled by mark-to-space—heating is at maximum rate until control temperature is reached, followed immediately by optimal control cycle, we must also consider approach to a lower temperature from a higher one at the switch-over point. This is, in general, regulated by the insulation of the body from the ambient, and as it is not impossible to construct an environmental chamber which will lose only a few degrees per hour with its heater switched off, it follows that for such equipment one should have only the minimum of insulation compatible with the necessary cooling rate, bearing in mind that the heater is going to have to be appropriately bigger, and that the machine will be less economic to run. Provided the equipment is always required to run above ambient temperature it is not advisable to introduce active cooling for this (see below). Continuously variable heater system In all the foregoing the effect of the control apparatus has been to distri- bute the action of a fixed-power heater in order to obtain the required mean power necessary for a temperature lower than that produced by continuous 403
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