It’s essential to program the PID to maintain tube temperature to the desired set-point while avoiding undershoot/overshoot. Here’s an example of an undershoot/overshoot oscillation. Let’s say you set the temperature at 1700 degrees C, but the PID is not optimized. When the temperature drops to 1699 degrees C, the PID increases hydrogen flow. The temperature climbs to 1710 degrees. The PID reacts by reducing hydrogen. Now the temperature plummets to 1690 degrees. The differential gap has widened, with severe temperature swings. A quality PID that is properly programmed will tightly control the deposition temperature along the entire preform length. To achieve consistency and reproducibility along the preform length, I recommend a tight control of +/-1 to 2 degrees C.

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