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The System

The schematic above shows the basic microcalorimetric system. It consists of a bridge, R1, R2, R3, R4 supplied from a 15 volt rail, the current through the bridge being controlled by transistor Q1. R3, & R4 are reference resistors, R1 is the heater resistor and R2 is a standard resistor. R4 is a coil of wire of a metal or alloy with a reasonably high temperature coefficient of resistance. Some suitable wires are listed in the table below.
Metal resistivity coefficient of resistance max temperature 1/K° °C
Nickel 6.8 0.006 ~1000
Monel 400 48.2 0.0011 400
Stainless Steel 316 49.6 0.001 900
The resistance of the heater coil, R1, at its operating temperature should be 5 -10 times greater than the standard resistance R2. This is so that the major heat dissipation occurs in R1 rather than in the standard R2. R2 should be a sufficiently high wattage resistor with a low temperature coefficient. R3, & R4 should be in the same ratio as R1:R2 but can be much larger ( say 10 -100kohms). They too should be low temperature coefficient components but this is less critical than the requirement for R2. R3 or R4 can be made variable to enable adjustment of the temperature of the filament R1.
The out-of-balance voltage of the bridge is buffered by the unity gain amplifiers U3 & U4 and applied differentially to the inputs of U1. The gain of U1 is controlled by R8 & R9 and is adjusted to give precise operation without oscillation or hunting. The output of U1 is fed to Q1 which controls the bridge current. Inspection of the circuit will show how it operates: If the current through the bridge is too great, R1 will reach a temperature higher than desired. This in turn will depress the voltage at the junction between R1 & R2, which in turn will reduce the bridge current via Q1. Thus the circuit will self-adjust until the bridge is balanced and R1 = (R3/R4)*R2.
If the voltages across R1 and across R2 are monitored the total power dissipated from R1 can be obtained as P = VR1*VR2/R2. R7 is a low value resistor inserted to prevent the emitter voltage standing at ground potential and R5 allows a small current to flow through the bridge even when Q1 is off.


Correctly configured the microcalorimeter system can function as a programmable thermostat while simultaneously measuring power dissipation. It can thus be used for differential thermal analysis, for measuring specific heats, latent heats of fusion and evaporation, as well as heat losses though insulation materials. The thermal transfer through silvered and non-silvered vacuum jackets has been determined using the system. (details here. The author has also used the system for measuring the thermochemistry of the hydrogenolysis of aliphatic aldehydes on a Nickel catalyst. See Journal of The Chemical Society, Faraday Transactions 1, 1978, Vol. 74. (Download pdf here)

The main application dealt with here, however is the use of the microcalorimetric system as a precision temperature controller. Apart from the obvious feature that the power supplied to the heater is directly monitorable, the use of the heater filament itself as the temperature sensor simplifies the heater construction greatly and is a considerable advantage where wiring has to be kept to a minimum, for example in vacuum systems with limited numbers of lead-throughs. With the addition of computer control, a wealth of data can be derived from this heating system and used to provide sophisticated heating and cooling programmes. If the operational amplifiers are fast, the circuit is capable of providing very rapid heat pulses with no risk of burning out the filament.

A Microcalorimetric Temperature Controller

The schematic for a microcalorimetric temperature controller is shown below: If it is difficult to read, a pdf can be downloaded here

The circuit is a more elaborate version of the one at the top of this page. R18 is a high wattage precision 2 ohm standard resistor from Rhopoint and is fitted with a heatsink. The heater itself is not shown but is connected between PL4 and the 15V 5amp supply. The remaining bridge resistors comprise R66 and digital potentiometer U29 and resistors selected from the arrays attached to CONN2 and CONN6. This latter arrangement allows selection to be made for coarse temperature range and for allowance for variation of cold heater resistance. The bridge outputs are buffered by operational amplifiers U1a and U1b and applied to the differential amplifier U18a which drives the bridge current controller Q11. Amplifiers U18b, U18c, and U18d gather heater voltage and current data and feed these to the 12 bit A/D converter U17. This can be addressed by the microcontroller via the latch U16, which also controls the digital potentiometer U29. The circuit is capable of maintaining the heater at a particular temperature until addressed by the micro-controller with fresh instructions.