I calibrated both of them mate but the dearer one seems more accurate. There is not much in it but I test 3 water jugs with 2 L's of water and the expensive one gives you a reading more quickly. I then take the ph meters out and test again and the cheaper one gives a slightly different reading. Again there is not much in it, but the expensive one give the exact same reading as the previous test. So it's more accurate. I did read that you should invest in a good PH reader. This is so true I've found. Since purchasing it and taking a reading before I add nutes and all other things I add to the water my PH is around 7.4. I then add nutes, root enhancer, bud candy etc I do another test and my PH is around 6.0. I like it to be at 6.4 or 6.5 no more no less. Since doing this my plants have gone from slow growing and light green in colour to absolutely flying in less than 3 days. It has been a huge game changer. They now grow an inch or two a nite. Sorry about the ramble