Very low temperature (-25C) on ds18b20 precision

[yamadanao]

I'm busy building a multi-probe temperature sensor and log system using a couple of ds18b20 probes on a Raspberry Pi.

They will be monitoring freezer temperatures that should be between -23°C and -18°C.

How to test ds18b20?

I'm a little concerned about the precision of the probes though as the datasheets only give the precision between the -10°C to +85°C Range (±0.5°C).

There's an error curve on page 18 here, and another almost identical one on page 20 here. Both of them however start a 0°C and only go up.

As I'll be operating outside of this range, how/where can I find the expected accuracy?

The freezers that the probes will be monitoring are used for storing human tissue, and I'll need this before I can put forward a tender/proposal. Any Ideas?