I bought a cheap TDS meter. I live and work in indonesia where it's not recommended to drink from the tap and most of the country depends on these water cooler jugs referred locally simply as a "gallon" (it actually has about 5 gallons) which is typically some sort of RO mixture and there's filling stations everywhere.

I measured the tap water and it read 26ppm then I did the "gallon" and it read 61ppm. Out of interest I bought some really low TDS bottled water advertised as 9ppm and measured it for accuracy and it picked it up exactly at 9ppm. So the unit seems to work fine. (I also did my fish tank to see if it would show a higher reading and it did at 443ppm yikes)

I was surprised to see the tap water was so low knowing it's not recommended to drink and the "gallon" water so high. Is there a way to find out the amount of bicarb/magnesium sulfate is in a water sample? I could use the low TDS water and reminerlize it but I was just curious what the water I have access to has in it and how consistent it is over time.

TDS tells you nothing about what is actually in the water or if it is safe to drink. Its only useful if you can make assumptions about where the TDS is coming from.

Even though your tap water is lower TDS, if its truly not safe to drink it could be due to bacteria, heavy metals, etc. I recommend not taking chances with it.

Is it possible to get a report on what's in your refilled "gallons" from the filling station? I would think they would be required to provide that if asked. The fact that it is higher tds than your tap water is not necessarily a concern, its fairly common to add minerals to purified water for flavor and consistency.

As to determining what's in your water, you can buy color change titration kits to determine hardness and alkalinity. Even going this route isn't going to truly tell you specifically what is in your water. To get that information you would need to get it from the filling station or send a sample to a lab.

BTW, my suggestion is that, if you can't get a report on what's in the "gallon" water, you either:

Buy the 9ppm water if its economical, consider that close enough to zero, and remineralize from there
or
Use a zero water or RO filter to filter the "gallon" water to as low TDS as reasonable, and again remineralize from there.

Also wouldn't hurt to just try the "gallon" water, and if it tastes good, call it good. If its not broke, no need to fix it.

I'm guessing that your concern is the possible scaling of the boiler. If so, assuming they are both truly potable, either your tap water or your 'gallons' would be fine. With TDS that low in both cases, it does not matter much what's in the water. If anything, many would argue that the water probably does not have enough minerals for optimum coffee flavour. That said, unless one has a discerning palate, either of your water sources probably would be good enough for most people.

I see you're also concerned about whether your tap water is potable. I would certainly not recommend drinking it even in coffee unless your water authority says it's OK after boiling. If so, you could just boil it and cool before putting it in your machine.

As Nate says, if it ain't broke... I agree with that. Many of us are really into coffee and our concerns for optimum water may not match yours if you are just wanting to produce some reasonably good coffee.

The filling station doesn't have mineral content posted they more have information that it doesn't have any of the bad stuff. I'm having inconsistencies when brewing and so my goal is more about taste and consistency than it is about scaling/rusting equipment. I have no plans on using the tap water (or the fish tank water) but I am going to continue to monitor it as well as the gallon to see how it changes but I'll probably end up using the low TDS water and reminerlize it myself or look at getting a RO system and reminerlize that.

If you talking about manual brew and good water for coffee you can buy Amidis in Indonesia gallon (19L) and mix with Thirdwave Water (3.8L/sachet) it's the easy way to get good taste and consistentcy, your cheap tds meter it's very useful for lowering the full strength TWW around 150-160ppm to lower 100-125ppm some Coffee Beans from Indonesian roastery better in that dilluttion.

The 9ppm I guess it's Cleo it wasn't bad for coffee just mix with other brand like Nestle several coffee roaster in Indonesia tell me either mixing Cleo 40%: Nestle 60% or the other way use your taste as guidelines.

You can also use the water recipe from mixing Baking Soda and Epsom Salt from BH with distilled water like Amidis.