Flue pipe External temp v internal temp

So I have double walled insulated flue pipe inside going to the outside which then is Class A pipe...Disclaimer....I think I am recalling this all correctly from listening to my installer....the question is, I have an IR thermometer, is there a formula or a way to know using the IR what the temp is inside by shooting the outside? I can get an outside pipe temp of course but wanted to have an idea of what might be the temp inside.

I asked a similar question a while ago and got a few good insights

http://www.hearth.com/talk/threads/double-wall-pipe-flue-temp-vs-outside-surface-temp.100818/

Yup, good insight...kinda what I thought people would say...no way to really do it. I am sure I am fine, my first clean out came up with zero ugly creosote and that was From October to a week ago, just fine ash that was easily brushed out

is a probe easy to install?

Definitely get a flue probe....... they only require drilling a hole through the pipe and sliding the probe in. You will feel much better knowing what the actual flue gas temps are doing.

Two bits in my case . . . one smaller than the other . . . I forget the exact sizes.

A 5-minute job with the first four minutes being me worried about somehow messing things up and destroying an expensive piece of doubewall pipe and so I read and re-read the directions.

Don't forget to remove it before you run a brush down the pipe. Just saying' (oops).....