Hi Matthew,
Ignore any info about the latest OAT technology antifreezes - that stuff is far too modern for us

The testers measure the density of your coolant - ethylene glycol and propylene glycol based antifreezes are as good as each other as an antifreeze but they have different densities and that's why they need different testers - and you need to know which one you have in the coolant.
Inhibitors are dissolved salts which keep the coolant slightly alkaline - we all know how acids eat metal so keeping the coolant slightly alkaline stops the coolant becoming acidic and eating the engine from the inside. From memory a fresh coolant mix should be around 8.5 pH and if it gets down to 7.5 pH it's time to replenish the inhibitors. They used to sell the inhibitor top-up separately but if you go into a shop and ask these days
they'll probably look at you as if
you're dumb
The alkaline inhibitors react with any acids that are formed in the coolant so they are 'used up' - the coolant pH can be tested with test papers...
http://cgi.ebay.co.uk/UNIVERSAL-TEST-PA ... 5ada76b553
and keeping the inhibitors sweet is the main reason for not using tapwater in the rad - de-ionised water is cheap enough anyway.
I did a bit of research into what comes in the bottle from the car shop - my conclusion was that the 'neat' Bluecol - not the 'ready-to-use' stuff contains ethylene glycol, the inhibitors, and the blue dye only - so at least 97% ethylene glycol - so the mix percentage you want for the protection it gives can be worked out refering to the tables at...
http://en.wikipedia.org/wiki/Ethylene_glycol
This gives the proportions by weight when the whole world and his dog measure liquids by volume - but there's not a great deal of difference so these tables are close enough to use as volume proportions.
There's stuff about propylene glycol at...
http://en.wikipedia.org/wiki/Propylene_glycol
and a general overview at...
http://en.wikipedia.org/wiki/Antifreeze
I have some of the chemicals that are usually used as inhibitors (I was pissed off because the inhibitor product for the Granny was no longer available and I was going to mix my own ! ) so if there's a real chemist on BXClub rather than an amateur like me they may be able to make a decent recommendation.