The Peltier Plate
Our TA ARES rheometer is equipped with a Peltier plate as the lower geometry. It cools or heats the copper plate to the commanded temperature with a thermoelectric cooler (TEC). A TEC work of the Peltier effect and is often named a Peltier plate. Recently the instrument lost control on the temperature, so I dissembled the lower geometry to check the problem.
Phenomena and considerations
Phenomena: The temperature would not change toward the set value. The voltage supply of the thermoelectric cooler is stable at 15.4 V DC. This was suggestive of the failure of the thermoelectric cooler.
Consideration: the supports of TA Instruments charge a lot of money for replacing the whole lower geometry including the thermoelectric coolers, whereas a thermoelectric cooler alone is very cheap. For example:
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Therefore the most cost effective solution should be replacing the TEC with a new one.
How does a TEC work?
Thermoelectric Cooler Diagram -- from Wikipedia.org
Practical TECs are often made of an array of p- and n-type semi-conducting blocks, which exhibit unbalanced flow of charge carriers and hence a temperature difference at the two sides. If the hot side is attached to a heat sink, e.g. water circulation at room temperature, the temperature of the cold side can be as low as the power of the TEC can reach. If the temperature of the hot side is maintained, for example, at 25 °C, then the temperature difference of the two side a TEC1-12705 can reach follows the relationship on the figure below (from the datasheet provided by He Bei (HB) Electronic Components):
TEC1-12705 data
Alternatively, inverting the sign of voltage supply exchanges the hot/cold side, turning the TEC into a heating device.
TEC products are released with a range of powers. Their voltage supplies are mostly 12.0 or 15.4 V DC, so they differ in maximum current. Choosing a TEC with the right maximum current value is crucially important. If the current is too high, other part of the instrument’s electric circuit would be damaged. It would be best if we were told the power or maximum current of the original TEC equipped with the ARES rheometer, otherwise risk has to be taken by choosing a TEC with as small power as commercially available, that is TEC1-12705, with maximum current of 5 A and, under 15.4 V DC, power of 77.0 watt.
Another less important consideration is the size.
More details about the wisdom of picking a suitable TEC are available on the Internet (here, here and here for example). TECs are mostly used by computer DIYers when they overclocked their CPU and most introductions on the Internet are for such purpose. But the theory is the same in other applications.
Controlling temperature with TEC
The instrument firmware compares the set-point temperature and the current temperature read by the thermal sensor, and determines whether and how fast to cool or heat the lower plate. To achieve a “smooth landing” and steady-state control after reaching the set-point temperature, a PID (proportional-integral-derivative) controller is used. A PID controller is tunable to be either fast stabilizing, with larger overshoot, or small overshooting, with longer stabilization, by adjusting the proportional parameter (see below).
Image from Wikipedia.org
Originally the PID set to accommodate the property of the equipped TEC (e.g. resistance). If the TEC is replaced with a different one, the PID setting should be changed accordingly. The Orchestrator software provides the PID setting option (refer to the user manual).
My experience
Dissembled lower geometry
I am pleased to share my experience here. Sources told me that the original TEC of ARES has a power of 135 watt (equivalent with a TEC1-12709) . For the sake of safety I still chose a TEC1-12705 for replacement, which is only 77 in watt. I took the following steps to dissemble the lower fixture:
After dissembling the lower fixture, the TEC is ready to be replaced. Be sure you choose the right alternative following the considerations above. To identify the cold and hot side, simply apply an instant voltage between the two connection of the TEC and feel it. The TEC should be placed on the lower fixture of the rheometer with the right side faced up, and connected to the rheometer correctly. Otherwise the rheometer will cool the sample when it should be heated, vise versa. Moreover, enough amount of thermal grease should be evenly applied on both sides of the TEC. Assemble all things following the inversion of the steps shown above.
Dynamic temperature ramp result
Because the replaced TEC has a lower power than the original one, the PID setting, temperature range and ramp rate will not be the same as before. So a series of experiments should be conducted to find the best setting of PID controller, and identified new limits of the temperature control for future reference. For example, test the upper limit of ramp rate with a series of dynamic temperature ramp tests. On the right shows on example of 2.0 °C/min ramp from 10.0 to 40.0 °C after replacement of TEC. The result shows that the requirement is met at such a low ramping rate.
