Cheers Victor but I won’t be mixing them will I? It should be the same green Mazda coolant, just the small amount that I reuse may be less effective due to it’s age. If I buy another 5 litres to use just 1 litre, it will mean 4 litres sit’s around for 2 or more years and probably past it’s shelf life.
Think I might save some that drains out, flush engine and radiator though with water, get out as much as possible, then pour in the new 5 litres. I’ll then be able to judge properly how much water, or old coolant to put in, or whether to go and get another 5 litres and waste 4 of them.
Oh yes…sorry I read that as your old stuff was different to the new. But yeah, that’s what I would do too…or maybe just a bit of a top up of distilled water.It won’t be a whole litre, that’s for sure.
that’s interesting, never tested for anything else but antifreeze on any car i ever had.
always bought the same stuff over the years.
should i be pouring in something else? 
Just pointing out that the tester tells you nothing about the state of the corrosion inhibitors, just the antifreeze concentration (specific gravity).
If you keep just topping it up, you’ll have next to no corrosion protection at all.