After about day of using the 20mm tube with the 4mm nichrome and its too powerful - the element is too far up the tube so most of the straw gets hot and has less cooling effect. Im not sure if the extra few mm diameter of the main tube is making much of a difference other than allowing the wider tm2 stem in.
I have been taking some readings of the inner tube temperatures and its a lot higher than the outer temperature with both the 4mm nichrome + 20mm tube as well as the 2mm nichrome and 18mm tube - both are at least 15C higher than the outer thermocouple and gets higher as the setpoint is raised like to about 20C difference.
I did think the inner part would be cooler but it makes sense for it to be hotter as the air is trapped inside the coil so will build up heat unlike the outer side where it can dissipate into the room.
However if i take readings from inside of the straw then temps are much lower and are only about 5C out but that depends on how long the straw has been sitting in the tube for / hot it was to start with.
So maybe the best thing to do is assume the straw is the same temp as the tube and then add an approximate tube temp value to the home screen - i will have to work out what the temp differences are depending on the setpoint as the higher the setpoint the greater the difference - this will be unique per tube coil setups i think and would need to be tuned like the pid - it should be possible to automate this data gathering.
However i do have another idea - as i got about 20 of the 20cm tubes and finally a dremel
plus some diamond tiny drill bits i may try and drill a 1mm hole into the tube hopefully just enough for the thermocouple to pop in.
Also when out and about doing a food shop i spotted some glass drinking straws basically like these:
Only a few $ so couldnt get them home fast enough. There are 4 x straight and 4 x bent long straws with OD 10mm and ID 6mm - they are 10" long so nearly double the current 12mm OD purple straws i have been using and a bit too long.
But what i actually got the pack for was the smoothie/bubble tea straws which are also a bit long at 9" but have OD 14mm and ID 10mm
So out of my plain straws i have 6mm, 8mm and 10mm inner diameters - all of them have roughly the same wall thickness.
I wired up the trusty 18mm tube with the 2mm nichrome as im quite comfortable with it as its been my daily driver for a while now so thought best to use to compare the stems with.
At first i gave the smoothie straw a go - it took a lot of weed and took me a while to work through it - i preferred it to the tm2 hook stem thing - was a very tight fit with the 18mm tube but worked nicely and was good with a fine grind.
Out of curiosity i tried the long 6mm ID one - i wasnt expecting much but was very surprised as it was much more flavoursome than the 10mm and very similar to the 8mm one i have become accustomed to. Also as the draw was so tight it lasted a good session length and a few minutes longer at a much higher temp to finish it off. I tried with some pretty loose large bits and it worked really well - 5 min session with very little bud used.
I have 8 of the 6mm ID ones and the dremel with some diamond cutting disks so hopefully can cut one to a more suitable length without breaking too many as I think they are too long and cooling the vapour too much so killing a bit of the terps but not sure on that yet - it is a bit too long for me and prefer the feel of the shorter purple ones.
Next thing to try is a range of similar ID tubes but with 1mm walls rather than 2mm and also would be good to try ID of 7mm and 9mm.
So completely the opposite to how i was expecting things to go - never been much of a micro doser but that may be changing - i was thinking a much fatter stem was going to be better but it really is more about the chamber length than its girth
, now to find the time to tinker. Thanks again for all your detailed posts.