I have 2 Maxi dosers with both in fairly heavy use as AWC and kalkwasser addition. I would like to have plenty spare head tubing on hand to replace tubing at about 50% of predicted life. What are the dimensions of the head tubing, specifically wall thickness? I don't see anywhere to order replacement tubing for the Maxi. Also, I don't see any obvious way to open the pump head to replace the tubing. There seems to be no instructions inn the manual.

Thanks in Advance, Bob