I am going to have to agree with Tom Wood on this, it really is not that hard to get accustom to using a cheap-o and a rough estimate is all we need anyway, but it's you're money.
I will leave the expensive kit's to the biologist's and colorblind..
We are not building "piano's" but growing weed's underwater, varibles change daily, if one could set the N03 level and keep it that way, maybe it would be different.