SP or PPM

[hkgar]

I have heard that Specific Gravity can fluctuate based on temp of the water. If that is true, wouldn't we better to try to maintain a specific PPm (35?)?

[disc1]

Yes, you're technically right. But it is important to think about what you are actually measuring and how you measured it. For example, if you use a refractometer then you aren't actually measuring SG. You are measuring RI. The RI also changes with temp, but the refractometer most likely compensated automatically. Then it gives you a number on the SG scale like 1.026 which is actually a surrogate scale for ppt since SG and RI aren't directly related quantities. So the temp effect on SG gets lost in that case because you actually measured RI, converted to ppt, and then reported as equivalent SG at some temperature which may not be what you actually measured.

In reality, unless you are keeping your tank at some extreme temperature, the difference is negligible.

[fishflorist]

David, I found one problem in using refractometer, since I had a thread about pinpoint salinity monitor.

yes we keep the water temperature at a very small range about 24-30 max, but most people live in a place where winter and summer air temperature so different, take winter for example, many placed will have a room temp at about 5 for a long time , so is the refractometer. you have to calibrate it . suppose you calibrated it accurate enough.
Then you put a drop of 25 degree water on to the refractometer, then , the refractometer temp start to change. how much it will change , that is a variable number. depend on how long much time takes you aiming the refractometer to a light source; or the size of the drop of water. That, IME, make the refractometer terribly unreliable. At least a 0.002 or 0.003 drift.
One may say that even a 0.003 off will not cause major damage, but this is a drifting , so you dont know this time it reads higher than real, or next time it maybe lower than real. That makes impossilbe to adjust salinity in winter by using refractometer.
It;s only my experience and have never figured it out.

btw since I did not calibrate my pinpoint good enough I think, the real salinity in my tank now became a myth ...... at least 10 corals died after I post that thread and start to raise my salinity slowly according to Pinpoint readings. (before that , my refractometer tells me 1.027)

[fishflorist]

I got go sleep now .

I dont know if there is any big mistake in my language , hope you guys could understand what I'm saying. :hmm4:

[hkgar]

Louise

I think that the small drop of water is more likely to rise (lower) to the temperature of the refractomotor so as not have any significant effect on SP,

Your 1.027 was mostly very close to actual SG

[Randy Holmes-Farley]

The change in ACTUAL specific gravity is almost trivially small over the range that hobbyists would be concerned.

HOWEVER, some measurement methods may be temperature dependent and they may not accurately correct for temperature changes.

Many refractometers automatically correct for temperature changes in refractive index. :)

[bertoni]

My guess has been that the reading on a refractometer probably doesn't depend on the temperature of the water sample, because I think that the thermal mass of a drop or two of water is too low to change the temperature of the device much. The room temperature can be an issue, though, which is why temperature compensation can be useful.

The conductivity varies with the temperature, too, although the conductivity meters I've seen have some method of computing an approximate compensation.

[fishflorist]

the little plastic is small so one drop may raise the temperature quite a lot.
and in the meantime, the water temperature must have been lower.

I found this happen when I do 3 time measurement continously, I do it this way all the time. The differenet between 3 readings can be quite different.

[Utah Fatman]

My refractometer calls for it to be used at 68 degrees F. and to let the sample set for 30 seconds to come up to the refractometer's temp. That should compensate. Some informal comparisons show it to be very close at temperatures from 65-70 F.