(All the photos available to this subject)

In my master thesis I built a thermostatic unit for EPR with biological samples.
The controlled temperature range was 15oC to 65oC in steps of 5/100 K.
The temperature stability was as good as ±1/100 K. The whole setup (apart from the EPR spectrometer) constisted of a regulation unit, the thermostatic fluid circulation and temperature measuring unit.

The thermostatic fluid circulation

The total volume was about 1000 ml. The circulating fluid was Silicon oil. The pump was an ordinary one for hydraulics. There was an ordinary car fuel filter after the pump to make sure no magnetical impurities would reach the cavity. A water cooling unit decreased the oil temperature in order to enable the temperature regulation by heating. Next was the pre-heater (photo), which should aproximate the final temperature. It consisted of a copper plate with the tubing for the oil attach to it and 5 W resistors distributed on it. The main stage of the heater was a 2 W resistor in an acrylic block together with the NTC (photo) of the regulator and the Pt1000 for the temperature measurement (photo), all distributed in an apropriate distance (important!) (photo). Finally, the silicon oil reached the sample holder, which was built with glas with an insert for a sample (photo).

The regulation unit

(front view, rear view and top view)

The sensor (NTC) was placed in a Wheatstone-Bridge with metal resistors and separated stabilized power supply. The "bridge-difference" is used also to manipulate the PID. Namely, the capacitor is shorten for bigger temperature changes, which limits over-/undershooting. A PID regulator does the main work. It controls the main heater stage with a power between 5 and 80 W. A second PI regulator drives the pre-heater stage. It doesn't look at the Wheatstone-Bridge but on the output of the main heater stage and trys to keep it in the middle of the range of the main heater. The power stage of the PI regulator drives two heaters. The one is a two state heater of 165 W, which is switched one when the other one, a continous heater, hits his maximum power of 10 to 240 W. When the 165 W are switch on, this continous heater is driven down to about 75 W in order to keep the total power of the pre-heater stage constant.
NTCs are placed close to the resistors of the heater to sense overheating and cutting the overall power. This is necessary since overheating would happen on leakage of the circuit.

The temperature measuring unit

The temperature measuring unit consisted of a Pt1000 (a "Pt100" with 1000 Ohms), a current source, a device to invert the current through the Pt1000 and DMV.
Inverting the current through the Pt1000 enables to cancel the error due to the thermoelectric effect.

Remote control of the regulator

The control unit had a analog interface for remote control, enabling setting the temperature with a computer.

Maretzek, António F.