simonp wrote:I did a bit of an experiment last night, I tried just running through a pre-heat to see what happened with the outer cover on but not that back panel. I ran it till about 120C on the on-board temp sensor. This reached 150C during the cooling cycle. I haven't taken off the shield yet, but peering in the back there don;t appear to be any cracks in the JB Weld but it has (as before) changed colour after the heat from a a dark grey to a light-ish one.
I might try a full run with no beans and see what happens. I am wondering if the beans are putting a side load on the thermocouples which is in turn levering the spacers causing the problem? Maybe a slightly flexible end on the thermocouple gives an easier time?
The manufacturers data states a tensile strength of 3,960 psi and a bond strength of 1,800 psi. This bond strength is more than adequate to cope with the forces on the thermocouple and the significantly larger forces due to differential thermal expansion. The bond strength, however, obviously depends on good surface preparation, without this the bond strength can drop close to zero. I believe this is your problem. Others have not had the problems you are experiencing despite the fact their thermocouples experience the same forces.
simonp wrote:.... It is a shame the european and USA websites have different advice with regards to cleaning preparation
That is surprising, Simon. From a quick google it looks to me as though the JB Weld website mentioning IPA that you link to is a reseller and not affiliated with the JB Weld manufacturer. If you follow the"About us" link and the Maximex-Shop link you will see they sell many products not linked to JB Weld such as incontinence pants. They also do not seem to provide any links to the JB Weld manufacturer, the technical data or materials safety data sheets, or stockists for JB Weld. In contrast the UK site jb-weld.co.uk states that it is an official distributor and includes links to this information.
Wikipedia has a detailed article on JB Weld which appears to be written by the manufacturer as it provides lots of detailed information. This states, in three places, "do not use alcohol". As stated in previous posts the JB Weld manufacturers website states "DO NOT use alcohol".
Based on this quick google I would not trust the information or advice from the reseller.
After the problems you have had it may be best to take no chances and ensure the surfaces are absolutely clean by first removing any trace of JB Weld then cleaning a few times with very hot espresso machine cleaner to remove any remaining coffee oils. Then roughen the surface again and clean several times with acetone or lacquer thinners. Anything less than this may leave you (and me) wondering if the bond is going to fail on your next roast.