Dude, you gotta specify what type of meter you are using. A cheap Hanna will have a little screw on the side to calibrate it while using the calibration solution. The Hanna combo pen is what i use and i swear by it. You need to use Hanna's 4.01 and 7.01 calibration solution and like someone said above, you calibrate the 7.01 first and then the 4.01. Or in your case, the pH 4.003 or pH 6.864. With my hanna combo pen, you hold the off button until it says cal. Dip it into the 7.01 until is says OK, quickly dip into just water, give it a lil shake off so as not to water down your 4.01 solution, then dip it into the 4.01 until it says ok. You should be good. Turn the pen off then on to normal ph setting and if you want to test it, dip it into the 7.01 solution and it should read between 6.99 to 7.03 ish.
You can save your old calibration solution, but i would recommend pouring it into a different container so as to not contaminate the fresh calibration solution. This is what i do, but only use the old used shit if your in a bind. The used 7.01 will be more accurate than the 4.01 because the 7.01 stays uncontaminated, whereas the 4.01 ends up a bit watered down from rinsing the meter with fresh water in between solutions. You should calibrate weekly, but it does get expencive, so you can always just stick the meter in the used 7.01 and see how off the meter is. It may not need calibrating for a couple months or it may need it every couple days depending on a lot of factors.
Hope this helps.
Also, go ask the guys at your local hydro store. They will be glad to help you because they will be glad to start selling you 30 dollar bottles of calibration solution haha. You could just have them calibratie for you too. I did that for like 2 months before they started giving me shit for wasting all their calibration solution...haha fuckers deserve to have their shit wasted with the fucking robbery of a markup they throw on all the products.
