Hydrometer Inaccuracies

[CageUK]

More questions I'm afraid...sure sign I'm new at setting up marine tanks.

I have two 'Instant Ocean' hydrometers. One is new and one I got with the tank (used). I thought this would be a good thing as I could corroborate my results to ensure accuracy... WRONG!

Both of them give completely different readings. The new one gives me just under 1.023 sg and the old one gives me a tad under 1.021 sg. If I had only one of these, I would have been in blissful ignorance of it's potential inaccuracy and just accepted the reading it gave me. I'm now in dilemma. Which one do I trust? Well neither in fact I'm going to take these to an lfs to see if I can establish which one is accurate (if any) and if neither are then which is the closest and by how much.

Finally getting around to my questions.

For live rock how critical is the sg reading? It could be either of the figures mentioned (1.021 or 1.023) OR either side of either of these readings.

Assuming I have to add more salt, I know that you should leave a salt mix standing for a day or so but IF I get my live rock today and have to increase the salinity to suit, is this likley to adversely effect the live rock bacteria? If so I'll just have to be more patient for the live rock.

Also, how are hydrometers normally stored, wet (filled with tank water etc) or dry?

I may get a spectrometer at some stage but that's an expense for another time.

Cheers

Mark

 

[woodyofcastle]

More questions I'm afraid...sure sign I'm new at setting up marine tanks.

I have two 'Instant Ocean' hydrometers. One is new and one I got with the tank (used). I thought this would be a good thing as I could corroborate my results to ensure accuracy... WRONG!

Both of them give completely different readings. The new one gives me just under 1.023 sg and the old one gives me a tad under 1.021 sg. If I had only one of these, I would have been in blissful ignorance of it's potential inaccuracy and just accepted the reading it gave me. I'm now in dilemma. Which one do I trust? Well neither in fact I'm going to take these to an lfs to see if I can establish which one is accurate (if any) and if neither are then which is the closest and by how much.

Finally getting around to my questions.

For live rock how critical is the sg reading? It could be either of the figures mentioned (1.021 or 1.023) OR either side of either of these readings.

Assuming I have to add more salt, I know that you should leave a salt mix standing for a day or so but IF I get my live rock today and have to increase the salinity to suit, is this likley to adversely effect the live rock bacteria? If so I'll just have to be more patient for the live rock.

Also, how are hydrometers normally stored, wet (filled with tank water etc) or dry?

I may get a spectrometer at some stage but that's an expense for another time.

Cheers

Mark

Hello

you need a refractometer they cost about £40 i had yo buy one yesterday, i also have a instant ocean hydrometer, and mine is the same, the hydrometer is 1.022 and the refractometer is at 1.024 i trust the refractometer.
to your question, how long has the salt been in the tank?

 

[CageUK]

Hi woodyofcastle

I put the salt in on Wednesday night so has been in a couple of days or so. The tanks has been running with just the water in since then. I had a tankful of microbubbles initially from the additional pump I put in (see my other post) but they have now subsided. My skimmer is off at the moment.

I would be nice to get my LR sorted this weekend but I have to get my better half to agree to that.

Cheers

Mark

 

[Colin_T]

Take both hydrometers to your LFS and test some water with yours and theirs. Then you should be able to see how high or low yours read.
As for storing them, it depends on what type they are. If they are glass floating hydrometers then they can float around the tank. The plastic chambers ones which you fill with water and sea where the needle points should be rinsed with freshwater and put in a cupboard.

 

[andywg]

If you want to see which is more accurate you have to put them in a known salinity solution. A refractometer is only as accurate as its calibration. Most refractometers that we can afford are inaccurate between 0 and 35 ppt salinity. To calibrate them you need to get a known salinity solution rather than calibrating to RO water as 0.

More important than what you use is knowing how it works and how to ensure you get the most accurate readings. It is also worth noting that many cheap hydrometers may be for measuring at 15 degrees C rather than 25 degrees C.

 

[SkiFletch]

This will help, a DIY salinity verification standard . Should be required reading for all aquarists IMO. The stickies are your friend

 

[Lynden]

Swing arm hydrometers may be inaccurate, but apparently floating ones are fine. You do not "need" a refractometer.

 

[Colin_T]

I have seen glass floating hydrometers where the paper with marks on are at different height in the tube. They were all completely inaccurate.
The only truly accurate hydrometer is a calibrated refraction-metre.

 

[SkiFletch]

Lets see if I can get in here before the salinity police come on over

A better statement is, the only truly acurate salinity/sg measuring device is one that is calibrated against a known concentration standard equivalent to that which the aquarist is trying to keep his/her tank at.

 

[CageUK]

Thanks for all the answers guys.

Bottom line is, I took my water to my lfs who tested it with a (hopefully calibrated) refractometer and my new one was the closest and was almost spot on. However I have heeded the advice and have ordered a refractometer to give me the consistency that is required.

Watch this space for advice on how to calibrate them

Cheers

Mark