Is there a bias function for shifting the digital temperature value by +1°C?

Yes. The bias function is disabled upon shipment from the factory, but you can enable it. After enabling the function, set the bias on the relevant channels to +1°C. You can also set a negative number.

1. To enable the function, see section 7.15 in the user's manual.
2. To set the bias, see section 6.12 in the same manual.

µR10000 Recorder Operation Guide
µR20000 Recorder Operation Guide