K-TC and K-AN8: grounding the module before calibration

Calibration laboratories calibrate K-TC and K-AN8 modules on the bench, away from the logger and the CAN bus that normally give the module its ground. Without a ground reference the readings drift during the calibration and the certificate comes out wrong. The K-TC32 and the K-AN8 have a ground screw on the front panel for this. Connect it before you start.

Why the module must be grounded

In a vehicle or on a test rig the module shares a ground with the logger and the supply through the CAN connector, so every input is measured against a stable reference. On a calibration bench the module is often powered from a bench supply and connected to nothing else. The inputs then float relative to the calibrator, and the amplifiers see a shifting common-mode voltage. On a K-TC the cold junction reference moves with it. The symptoms are readings that drift between channels and between runs, offsets that change when you touch the module or the cables, and a calibration that does not repeat.

Which modules have the ground screw

On the K-TC32 the screw sits below the last row of thermocouple sockets, under TC28, and carries a coloured ring. The K-AN8 has the same screw in the same place on its own, different enclosure.

Before you calibrate a K-TC32 or K-AN8

1. Power the module from its normal supply on the CAN connector: supply on pin 9, power ground on pin 5. See K-TC pinouts or K-AN8 pinouts.
2. Connect the ground. Fit a ring terminal under the front panel ground screw and run it to the calibrator's reference ground. Tighten the screw so the terminal cannot move.
3. Let the module settle. Leave it powered at the bench temperature before the first reference point, so the module and, on a K-TC, the cold junction are stable.
4. Calibrate in K-CAL. Follow K-TC: calibrating channels and updating firmware or K-BOX: calibrating inputs and updating firmware, which the K-AN8 follows in K-CAL, for the reference points and the Calibration tab.
5. Check the result. Read one known value on every channel after the calibration. Channels that still differ by more than the module's accuracy point to a missing or loose ground, not to the calibration.

K-TC8 and K-TC16

The K-TC8 and K-TC16 are the older design and have no ground screw. Some of them calibrate without any trouble, because the bench setup happens to give them a good enough ground; others drift. There are too many variables in a bench setup to say which in advance, so treat every K-TC8 and K-TC16 the same way: give it a ground reference through the power ground on pin 5 of the CAN connector, tied to the calibrator's reference ground, and check the result as above.

Related guides

• K-TC: calibrating channels and updating firmware
• K-BOX: calibrating inputs and updating firmware
• K-TC pinouts
• K-AN8 pinouts
• K-Series: connecting and powering modules