Well, calibrate it and see what it is.
Then, find something in your house, a household liquid, like a bottle of vinegar, and see what the ppm is. Then, once a week, check the vinegar to see if it is about the same.
This is from a article I wrote for Stealth Hydro, that might help you:
pH Meter
Well, Roseman has taken a giant leap into the technical space age and I finally broke down and purchased a pH Meter. I visit and frequent a few hydroponics forum websites and I was being made fun of by a few members for still living in the dark ages and using the test strips. I thought I was in the space age because I grew using hydroponics instead of soil, but I guess you can always improve on anything. I've used the pH test strips for three years now, and they have always done a great job and are very economical too. In those three years, my goal in using the test strips was to stay within a safe pH range of 5.5 to 6.5 and target half way between the two. Honestly, anything in between is acceptable and works well for me. But I have to tell you, after receiving and using my new pH meter, well there is an acceptable pH within a safe range, and there is a precise accurate, more efficient pH.
Comparison Shopping
I'll have to admit that I started by doing some comparison shopping, and I found meters from $69 to $299. No where could I find one for less than $69, except the one offered at StealthHydro. I asked the owner at StealthHydro did he know anything abut the pH meters, and he said he only had experience with one of them, and that was the one they have on sale at StealthHydro for $59.95. I also learned that many meters are sold without the batteries and without the Calibration Solution, which is needed but sold separately. And after my 5% discount for mentioning "growtips" it was a very easy decision. And it did come with the batteries, a micro screw driver and the Calibration Solutions.
My Experiment I had two separate reservoir tanks bubbling with some very mature plants, and I was able to do some comparisons of different ph levels using my new pH meter and each tank. I ran one tank at a higher pH of close to 6.8, and one tank at a lower pH of around 6.0. I learned very fast that my plants drank exactly a gallon of water and nutrients, when I kept my pH exactly at 6.2 to 6.4. They ate and drank less when the pH was above 6.6 or lower than 5.8. I don't think I could have been as precise and accurate with my experiment just using test strips.
If you are serious about hydroponics growing, and if you seek the precision and accuracy that the test strips can not measure as precise, then I highly recommend you purchasing the pH meter.
Easy Calibration
I've been asked is there an easy way to calibrate it for precise accuracy. I decided to have me a shot of Scotch while I was mulling it over and then it came to me. Use a shot glass! I'm just kidding, I don't drink liquor, but I did use a shot glass.
I simply pour the 4.0 Calibration solution in the shot glass, as seen here. I found the little funnel at the local department store.
I got a reading of 4.64, so I used the enclosed micro screw driver and adjusted it to read 4.0, with a turn and a half counter clockwise of the little adjustment screw.
Then I returned the 4.0 Calibration Solution to it's original bottle and did the same thing with the 6.8 Calibration Solution. When I tested the 6.8 Calibration Solution, I got a reading of 7.0 so I simply turned the adjustment screw a half turn counter clock wise. Then I tested the 4.0 solution again, and it read 3.98 and that was close enough to 4.0 for me.
The meter measures the pH within 3 or 4 seconds, and it also measures the temperature of the water. Like any thermometer, it does take 3 minutes to determine the temperature of the water.
I hope that helps you.
