How Does Varian CONTROL The Temperature?

This VT regulation is a "black box" to many users - in this article we want to try clarifying a few issues. The VT parameter interface is simple:

Using the "temp" utility hides most of these parameters from the user. Note that even though our probes are equipped with thermocouples that are accurate to 0.1 degrees or better, we can't really tell the EXACT temperature WITHIN the sample, because of

In critical cases it may be worth calibrating the temperature under the given conditions (sample height, solvent, salt concentration, gas flow, pulse sequence / decoupling).

For more please see Varian NMR News 1998-10-02.

How Does VT REGULATION Work?

VT control involves much more than simply turning on the heater if the temperature is too low, and switching it off when the temperature is too high: such a simple algorithm can lead to excessive "overshooting" early in the regulation process and excessive rates of change in temperature (see also the article below), and it could also lead to oscillating temperatures. Our VT controllers (both the Oxford VTC and the newer Highland controller) use a much more sophisticated, so-called "PID" algorithm to dynamically regulate the heater current.

"PID" stands for "Proportional / Integrative / Differential" regulation - an algorithm that can be adjusted to the characteristics of the regulation circuitry (which is all the hardware involved - heater characteristics, heat capacity of the VT gas, gas flow, heat capacity and geometry of the relevant probe components), such that the regulation is efficient / fast (within minutes), does not produce excessive overshoot (not more than a few degrees, depending on the amount of temperature change) and suppresses oscillations. The characteristics of such a regulator can be adjusted by setting the P, I, and D components. In practice, these values are pre-set in the software and are transmitted to the VT controller once prior to every experiment requiring VT control. For experiments using the Oxford or Highland VT controller (GEMINI, MERCURY and UNITY families of spectrometers), the acquisition software uses a PID value of "440" (P=4, I=4, D=0), while the solids VT controller for Varian MAS probes uses a value of "171". In the standard software, this is NOT under user control, and with few exceptions there is NO NEED to alter these values, see the following article.

"Stepped" VT Regulation:

When starting VT control, older spectrometers and all HAL-based systems (VXR-S, UNITY, UNITYplus) directly send the desired setpoint to the VT controller, irrespective of the current temperature. This method has several disadvantages because different probe hardware (characteristics of the ("regulation circuit") and different temperature intervals would in principle require adjusting the PID values to the current situation, in order to obtain optimum results. One can observe these problems through:

This means that

Note that "stepped" VT changes are NOT implemented on HAL-based systems, i.e., on VXR-S, UNITY and UNITYplus systems you should avoid performing huge jumps in temperature. You could insert one or several dummy (1-scan) experiments between experiments at much different temperatures, or (in manual operation) change the temperature step-wise with a series of

commands. There is a user library contribution "psg/vtfuncs" that permits user entry of the PID values via a global parameter "pid" - but this should be used with care, and no assistance can be given for these adjustments, beyond the information given above (change only one digit at a time, and only by one unit).

For an article on temperature control and understanding temperature regulation see Varian NMR News 2001-09-15.
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