Specific Gravity vs Density Question

[arking_mark]

I have three salinity testers all calibrated with their references. Here are my tank readings:
1. Shows 1.025 kg/l, 35.5 psu, 54.1ms/cm
2. Shows 1.027 sg, 36.8 ppt
3. Shows 1.028 sg

Water temp is 77

Per this calculator:

Water Density Calculator

Aquarium calculator; Calculate the density of water at a given temperature and salinity.

www.hamzasreef.com

1.025 kg/l = 1.028 sg, so we have agreement between 1 and 3

36.8 = 1.025 kg/l, so we have agreement between 1 and 2

However #1, 35.5 psu, 54.1ms, and 1.025 kg/l does not agree with itself!

What is going on here?

P.S. My tank was set to be at 1.025 kg/l which I thought was Specific Gravity. So I've been running high for 5 months!

 

[MnFish1]

I have three salinity testers all calibrated with their references. Here are my tank readings:
1. Shows 1.025 kg/l, 35.5 psu, 54.1ms/cm
2. Shows 1.027 sg, 36.8 ppt
3. Shows 1.028 sg

Water temp is 77

Per this calculator:

Water Density Calculator

Aquarium calculator; Calculate the density of water at a given temperature and salinity.

www.hamzasreef.com

1.025 kg/l = 1.028 sg, so we have agreement between 1 and 3

36.8 = 1.025 kg/l, so we have agreement between 1 and 2

However #1, 35.5 psu, 54.1ms, and 1.025 kg/l does not agree with itself!

What is going on here?

P.S. My tank was set to be at 1.025 kg/l which I thought was Specific Gravity. So I've been running high for 5 months!

There is no such thing as a 'perfect measurement', any piece of equipment is going to have measurement 'error'. If you look just at the Specific gravity, 1, 2, and 3 are nearly 'the same' - 1.027 vs 1.028. I.e. is 1.028 really 1.0275 and is 1.027 really 1.0274? There is no way to know 'for sure'.

As to this question - Shows 1.025 kg/l, 35.5 psu, 54.1ms/cm. Are you using a measuring device that gives you all of those measurements - or did you calculate them? FYI - the 1.025 kg/l = approx 1.028. This calculates to 55.9 ms/cm. so they basically match (i.e. my guess is that they are in the margin of error).?

 

[arking_mark]

There is no such thing as a 'perfect measurement', any piece of equipment is going to have measurement 'error'. If you look just at the Specific gravity, 1, 2, and 3 are nearly 'the same' - 1.027 vs 1.028. I.e. is 1.028 really 1.0275 and is 1.027 really 1.0274? There is no way to know 'for sure'.

As to this question - Shows 1.025 kg/l, 35.5 psu, 54.1ms/cm. Are you using a measuring device that gives you all of those measurements - or did you calculate them? FYI - the 1.025 kg/l = approx 1.028. This calculates to 55.9 ms/cm. so they basically match (i.e. my guess is that they are in the margin of error).?

Nope. #1 is a GHL salinity probe. So it's probably just measuring conductivity and calculating the other numbers. That's what's confusing none of those #s match.

 

[MnFish1]

Nope. #1 is a GHL salinity probe. So it's probably just measuring conductivity and calculating the other numbers. That's what's confusing none of those #s match.

OK - just to point out, the 1.025 = 1.028 specific gravity. If you put 1.028 into your calculator you get numbers that are very close. At least thats how I read it

 

[arking_mark]

OK - just to point out, the 1.025 = 1.028 specific gravity. If you put 1.028 into your calculator you get numbers that are very close. At least thats how I read it

Yes. I think that the 3 testers get similar results...but #1 isn't consistent with itself. 54.1ms/cm does not seem to jive with 1.025 or 25.5 psu.

 

[MnFish1]

Yes. I think that the 3 testers get similar results...but #1 isn't consistent with itself. 54.1ms/cm does not seem to jive with 1.025 or 25.5 psu.

Oh - I thought I posted it - 1.025 - is density - right? that = 1.028 specific Gravity. 1.028 specific gravity = 55.9. which to me is close enough to 54.1. lets say they just measure specific gravity and it measures 1.0276. They probably only average it to 1.028. But the ms/cm could be 54.1. Or vice versa.

 

[arking_mark]

Oh - I thought I posted it - 1.025 - is density - right? that = 1.028 specific Gravity. 1.028 specific gravity = 55.9. which to me is close enough to 54.1. lets say they just measure specific gravity and it measures 1.0276. They probably only average it to 1.028. But the ms/cm could be 54.1. Or vice versa.

Ok. That makes more sense.

 

[MnFish1]

Ok. That makes more sense.

I have to admit - I didnt check that probe - and how they measure things - but I agree with you they measure one thing - and calculate the rest - and there are rounding errors

 

[Randy Holmes-Farley]

FWIW, specific gravity has no units to to, density does.

Density is very temperature dependent, specific gravity not so much.


36 PSU seawater has a sg of about 1.0271, a density of 1.0241 g/mL at 25 deg C, and a conductivity of 54.4 mS/cm.

I'm not sure #1 is exactly internally consistent, but all are pretty close.

 

[arking_mark]

FWIW, specific gravity has no units to to, density does.

Density is very temperature dependent, specific gravity not so much.


36 PSU seawater has a sg of about 1.0271, a density of 1.0241 g/mL at 25 deg C, and a conductivity of 54.4 mS/cm.

I'm not sure #1 is exactly internally consistent, but all are pretty close.

So if I'm using kg/l for target Salinity, I should be looking to keep it at 1.023 kg/l?

 

[MnFish1]

So if I'm using kg/l for target Salinity, I should be looking to keep it at 1.023 kg/l?

Curious - why not just use specific gravity?

 

[arking_mark]

Curious - why not just use specific gravity?

Not an option on the GHL. In fact, I feel pretty dumb for not knowing that my 1.025 kg/l was not 1.028 sg and my tank was running at higher salinity.

 

[Randy Holmes-Farley]

So if I'm using kg/l for target Salinity, I should be looking to keep it at 1.023 kg/l?

I am confused. This is a GHL conductivity probe? Assuming so, can't you just use conductivity?

It's quite unusual for anyone (hobbyists or oceanographers) to be focusing on density.

 

[arking_mark]

I am confused. This is a GHL conductivity probe? Assuming so, can't you just use conductivity?

It's quite unusual for anyone (hobbyists or oceanographers) to be focusing on density.

The actual set point and hysteresis is using conductivity. So that makes sense.

However, it's reported conductivity does not match its calculated salinity density. The strange thing is that the calculated density does match my two other salinity testers.

That's what's bothering me.

The GHL 1.025 kg/l matched the 36.8ppt and the 1.028 sg readings of the other probes.

It's 54.1ms/cm calculates to 1.024 kg/l and not the 1.025 kg/l it displays...the displayed value matches my other probes almost to a tee.

 

[Randy Holmes-Farley]

I'd ignore the density and stick to the conductivity, especially since that is presumably how you calibrate it.

Density is temperature dependent, and it may be choosing a temperature (say, 20 deg C, where it seems correct) to report the density rather than using a measured temperature (does it even know the temperature?).

Here's a table for density at 54.1 mS/cm vs temp in deg C:

25 1.0239
20 1.0253
15 1.0265

 

[arking_mark]

I'd ignore the density and stick to the conductivity, especially since that is presumably how you calibrate it.

Density is temperature dependent, and it may be choosing a temperature (say, 20 deg C, where it seems correct) to report the density rather than using a measured temperature (does it even know the temperature?).

Here's a table for density at 54.1 mS/cm vs temp in deg C:

25 1.0239
20 1.0253
15 1.0265

Yes, it knows the temp. You actually need to float the calibration fluid and either provide the temp or use its temp sensor.

I calibrated the temp sensor with an ISO 17025 calibrated thermometer.

 

[Randy Holmes-Farley]

Yes, it knows the temp. You actually need to float the calibration fluid and either provide the temp or use its temp sensor.

I calibrated the temp sensor with an ISO 17025 calibrated thermometer.

OK, but that doesn't necessarily mean it uses it , or uses it properly, for the conversion of conductivity to density. Would be interesting to see what it reads if the same solution is measured at different temperatures.

 

[arking_mark]

OK, but that doesn't necessarily mean it uses it , or uses it properly, for the conversion of conductivity to density. Would be interesting to see what it reads if the same solution is measured at different temperatures.

It does seem that the conductivity of the probe is sensitive to temp as temp is required to calibrate it.

 

[arking_mark]

I'd ignore the density and stick to the conductivity, especially since that is presumably how you calibrate it.

Density is temperature dependent, and it may be choosing a temperature (say, 20 deg C, where it seems correct) to report the density rather than using a measured temperature (does it even know the temperature?).

Here's a table for density at 54.1 mS/cm vs temp in deg C:

25 1.0239
20 1.0253
15 1.0265

The issue I have here is that I was essentially controlling my tank at the wrong salinity. I was targeting 1.025 sg. I was keeping it at 1.025 kg/l. It was measuring 54.1ms/cm or 1.027 sg

I guess it's possible that I miscalibrated the salinity probe with the wrong temp or the temp probe is off...which might explain the 54.1 to 1.025 kg/l issue.

Either way, I'm going to recalibrate both the temp probe and the salinity probe.

 

[arking_mark]

I am confused. This is a GHL conductivity probe? Assuming so, can't you just use conductivity?

It's quite unusual for anyone (hobbyists or oceanographers) to be focusing on density.

So if I use conductivity, I should be targeting roughly 51 for an sg of 1.025...