Need help reading EC meter!!!!

I bought a digital waterproof Ec/ppm tester from martini model ec-60. I dont understand how to read it. I am sure I am just retarded but I though it would just say EC then a number by it. On this model there are two diff. calibrations you can go with. I am trying to tes my tap water to see how it is. here is what is displayed on it. First calibration says ATC 0.23 PPT 17.6 C what does ATC mean and what is 0.23 PPT? I know the 17.6 c is the temp. On the second caliration it says ATC 0.47 MS 17.6 C Can somebody help me I am trying to understand what this is so i can adjust my hydro res. Thanks any help would be appreciated is there a formula I have to do to get EC from those numbers i listed?

I'd assume it means microsiemens. If it will measure tds, go with that; ppm seems easier to me.

It will measure tds but how do I convert that to EC or PPM?

Those aresome coplicated readings.My ppm meter has lights on it that light up the wand to the ec or ppm or cf measurements showing me my ppm of like 500-1600 ppm. The tds is your ppm same thing so if the tds is 500 your ppm is 500.Ill have to look to find the US conversion of ppm to EC or tds to ec but if you know the ppm or tds then you dont have to have the ec