David wrote:
We have a mains powered CO tester in the same room as our log burner.

However I am wondering if there is a sensitive portable CO tester which is
good.

https://www.amazon.co.uk/Handheld-Mo...ector-0%EF%BD%
9E1000ppm/dp/B08CSLLXVC/
is an example after a quick search.

The main question is if these detectors are more accurate/sensitive than
the ones designed for permanent installation.

There are 'personal gas detectors', eg:
https://uk.rs-online.com/web/c/?limi...noxide&sra=oss

I think I would trust those that have calibration against some standard over
random-brand Chinese ones from Amazon.

(for a similar reason, I don't buy unbranded smoke detectors from ebay)

A randomly selected one:
https://docs.rs-online.com/6faf/0900766b803e8cee.pdf
is specced at:

Type of sensor:electrochemical, specifically for CO.
Calibration of the sensor:calibrated by the manufacturer with asource
at 205 ppm.
Measuring range :Nominal: from 0 to 1000 ppm (2000 ppm for amaximum
period of 5 minutes).
Accuracy:+- 5% of the reading +- 5 ppm (from 0 to 1000ppm).
Response time: 70 seconds to 90% of the
measurement
Long-term drift:5% per year, depending on the frequency of use

Theo