C4 Precision measurement

What it is

C4 is a precision DC measurement front end. It reads the sensors that process control runs on — thermocouples, RTDs, thermistors, bridges and load cells — plus four-wire resistance and DC volts, all at the same time, through one 24-bit converter.

It's the card in almost every build. The smoker, the reflow oven, the kiln and the fermentation chamber all measure through a C4, and a spare input usually pays for itself: a load cell on a free landing shows the smoker's pellets bridging in the hopper.

Capabilities

Design values. Measured values replace these after bring-up.

• Six single-ended inputs, all live at once, for RTDs (PT100, PT500 and PT1000 in two, three or four wires), NTC thermistors and voltages.
• Two differential inputs for thermocouples (types K, J, N, T, E, R, S and B, with cold-junction compensation on the card), bridges and load cells.
• A buffered pH probe input, powered by the card.
• Four-wire resistance to about 95 kΩ and DC volts from 0 to 2.5 V, through a 24-bit converter.
• Inputs that survive +15.5 V and −10.5 V.
• Not isolated.

Build with it

• Reflow oven. An E1 switches the heaters while C4 follows the board's thermocouples through the profile.
• Pellet smoker. C4 reads the pit and the meat, and a load cell watches the hopper.
• Fermentation chamber. A load cell follows a beer by the weight of the CO₂ it gives off.
• Yogurt incubator. Done at a pH, not on a clock.
• And the sous-vide bath, espresso retrofit, coffee roaster, mash tun, pottery kiln, heat-treat oven, greenhouse, aquarium and vacuum pump-down.

Calibration

C4 is calibrated against a 10 kΩ standard resistor and a 10 V reference. To recalibrate it yourself you need a certified four-terminal 10 kΩ standard, plus a 10 V reference if you use the absolute-voltage modes; the card's manual lists the rest. How calibration works