Remote Processing Units (RPUs) for tank control are dedicated, self-contained computer controllers, with modular expansion capability.

Each RPU has 16 temperature inputs. Each RPU has 8 digital outputs on board for driving valves, and may be expanded to drive additional valves.

An RPU can support up to eight control tanks Mounted on each tank is a VSU (Valve Switch Unit), which switches valve power (24 or 120 VAC) and serves as a collecting point for the sensor cables.

RPUs record data at ten minute and hourly intervals. The hourly data is kept for 72 hours on-board, in case the PC may be down. Eight channels of digital 'status' history is kept for each of the eight control channels. This data is also recorded at ten minute and hourly intervals.

Tanks may be controlled using a PID (Proportional with Integral & Derivative) algorithm, which can eliminate the temperature cycling normally seen with 'on-off' controls. During cold stabilization, a special de-ice mode is available for faster, more accurate stabilizations.

Temperature sensors are silicon ICs, with numerical calibrations accurate to 0.25 degrees C. Replacement sensors are calibrated by keying the numbers in at the PC.

The LCD display cycles through the Tank numbers and Temperatures.

The RPU's programs are written in Forth and assembly code.