I didn't explain well - the pen has modes - in tds mode it measures around 27 in ec mode it measure around 8. It comes with 2.7 ec solution which i use to calibrate the ec.It doesn't make sense to me. Since the pens are measuring EC (and not TDS directly) you should be able to derive the ppm TDS from the EC, full stop. 27/500 - 0.054. Is it close to 0.07? Yeah, but since it's a calculation and not a measurement, I'd want to know why the two values don't jibe.
You say the pen doesn't state the unit. OK, is there any online resource, or can you ask the company that made it?
If you care about precision and knowing whether the TDS is 25, 50, or 70, you should sweat the details, IMO. I'm all for inexpensive pens as long as you know what the output means. i.e., if it's inaccurate, do you know how, or by how much? Here's what I'd do personally (obviously up to you):
If you want to use the cheap pens, I recommend doing #1.
- Purchase a standard solution. 1413 uS/cm is the most common one. I usually verify against that and a lower one (23 uS/cm). Is that second measurement overkill? Maybe, but since I need to measure EC in the single digits, I want to be confident that it can measure precisely in that regime. And/or:
- Spend slightly more money and get a more precise meter. You can get an HM or Hanna meter for about $60.
In either case, you don't need to rely on the pen's TDS measurement if you know what the EC units are.
You don't have to do any of this, but if not, it's hard to say whether the TDS is "over the limit."
Possibly, or something else in tank. It's also likely a noisy number.