Question about digital TDS meter calibration???

I found a Zero00 Water Z-Tester, electronic water quality tester. I found it at Walgreens for $14.99, It says it goes from 0-9990 but it's been factory calibrated to 342ppm. What does that mean, it will only go to 342ppm accurately? Also, does anyone know how to recalibrate the meter?

look up calibration solution NaCl

For anyone interested, the cheapie TDS meters sold at Target, Walgreens etc. often under the brand name Zero Water are actually made by HM Digital.

if you are using them to test your source water the stock calibration of 342 is probably fine.

If you are using them to measure nutrient strength before you water then you should calibrate the pen with regular 1382 solution available at your local hydro shop.

Here are the real instructions probably not included.
http://www.tdsmeter.com/calibration_maintenance.html