Originally Posted by
ChrgerOnDavins
http:///forum/post/2488562
actually, the blue six hydrometer is always 1.5-2 readings lower than the instant ocean hydrometer.
Likely because neither one is properly calibrated. There's no way to tell which one is correct. It doesn't go by model: there's no reason to think that one brand is more accurate. Swing arms all work the same, and all have the same problems/noise/inaccuracy. You are trying to tell the difference between two over a range that is smaller than the average calibration error of swing arm hydrometers. It's a pointless question.
Problem is I don't know which one to believe.
What you don't seem to want to hear is that you can't believe either one.
If all you are doing is trying to match your salinity in tank to mix up water for a water change, then you don't really need to know which to believe: just use the same one to measure where your tank is at, and then to gauge the water change water getting it to the exact same level. That will be good enough.
If you are topping off properly (i.e. when you do a water change, you mark the precise level you filled it to, and then top off always back to that line) then the fact that you don't know the true salinity at the very least won't make your water changes hurt. The main issue is simply whether or not your tank is at the right salinity to begin with.
But you can't know the exact SG with a swing arm hydrometer, or two of them, or ten of them. If you haven't had the hydrometer calibrated recently, then the best it can do is compare two sources of water, like I described. But never ever seriously trust it beyond that.
I didn't really need to be told to buy a refractometer, I thought I made it clear that i intended to get one but needed help as of now. Wouldn't it make more sense to not post at all, rather than to post the obvious?
I didn't post anything about a refractometer, so who are you talking to, exactly?
You don't need to buy a refractometer. Go to a good LFS with a sample of your water, and have them test it with their refractometer or probe (most will do this for free). That will tell you where you are at so that you can make slight corrections (your hydrometer will be good enough to handle the corrections: it may be miscalibrated, but the gradations will at least be relatively useful since you now know what it should be measuring and how far it is off) to your system, obviously very very slowly and gradually.
And again, once you are at the right SG in your tank, it's relatively easy to maintain it as long as you top off precisely and match the tank SG to the water change mix SG. Your hydrometer won't necessarily stay at the same amount off the true measurement of course, so before long you are then back to using it just to run comparisons until you find out how far off it is again.
A refractometer is great to have around, especially if you need to perform a delicate operation like hypo or are mixing up a new tank, or if your skimmer or salt creep are bad enough to remove lots of salt... etc. but by and large you can make do without one.
But you can't answer the question "which hydrometer is correct" just by looking at the brands. There's no reason to trust either one, unless one has just been calibrated and the other hasn't (and even then, they fall out of calibration unpredictable, rather than in a set amount of time). If you want to go and have one calibrated (which would require a solution of known salinity and someone who knows how to calibrate them) then you could know which one is more likely to be correct. If you go and have your water tested, then you could know by how far off each one of them is. But knowing what the "true" answer is is always going to involve an extra step beyond just using a non-recently calibrated hydrometer by itself.
