Salinity Measurement Troubles...

[arking_mark]

So I've been dealing with a couple of meters that all give me different readings after being calibrated:
GHL = 34.6 psu, 52.7mS, 1.0245kg/l
Hanna = 35.4ppt
Other probe = 37.3ppt

I bought a Tropic Marin Precision Hydrometer to establish ground truth...

Tropic Marin Precision Hydrometer: 1.02775 @ 25c

Per https://reefapp.net/en/salinity-calculator:

I believe the TM High-precision Hydrometer as truth! Which means:
1. GHL is only good for it's density reading in kg/l.The other measurements seem to not align with standards.
2. Hanna and another probe are off and shouldn't be depended on.

I'm glad to finally have confidence in GHL, now that I know which GHL reading to follow.

I hope this helps others.

 

[T-J]

If your Tropic Marin is what you're going by, and it gave you 1.02775, wouldn't the GHL be worse than the Hanna?
GHL is 1.8ppt lower than TM, and Hanna is only 1.4 lower.
Tropic Marin - 1.02775sg - 1.0247g/cm3 - 36.8ppt
GHL - 1.0261 sg - 1.023g/cm3 - 34.6ppt
Hanna - 1.0267 sg - 1.0236g/cm3 - 35.4ppt.

 

[arking_mark]

If your Tropic Marin is what you're going by, and it gave you 1.02775, wouldn't the GHL be worse than the Hanna?
GHL is 1.8ppt lower than TM, and Hanna is only 1.4 lower.
Tropic Marin - 1.02775sg - 1.0247g/cm3 - 36.8ppt
GHL - 1.0261 sg - 1.023g/cm3 - 34.6ppt
Hanna - 1.0267 sg - 1.0236g/cm3 - 35.4ppt.

Using GHL density reading is almost spot-on for the TM SG reading when converted.

 

[arking_mark]

@Randy Holmes-Farley Can you advise here to the original post? What should be trusted?

 

[arking_mark]

So is this conclusive evidence?

1. I lowered my salinity to 1.0235 kg/l per my GHL (50.7ms)
2. I measured my SG via the High-precision Hydrometer
3. It measured 1.02655 SG @ 25.5 degrees celsius or temp corrected to 1.0267 SG
4. 1.0235 kg/l converts to 1.0267 SG, an exact match!

Should I now have confidence that my tank is closer to 1.026 SG?

My Hanna measures 34.1 ppt / 1.025.

My other probe measures 35.9 ppt.

 

[arking_mark]

So is this conclusive evidence?

1. I lowered my salinity to 1.0235 kg/l per my GHL (50.7ms)
2. I measured my SG via the High-precision Hydrometer
3. It measured 1.02655 SG @ 25.5 degrees celsius or temp corrected to 1.0267 SG
4. 1.0235 kg/l converts to 1.0267 SG, an exact match!

Should I now have confidence that my tank is closer to 1.026 SG?

My Hanna measures 34.1 ppt / 1.025.

My other probe measures 35.9 ppt.

I just realized that 35.9 ppt converts to 1.0270 which is also almost exactly 1.0267!

 

[Randy Holmes-Farley]

Yes, that all seems reasonable. No idea why they would give data in density. no one uses that in the hobby.

 

[arking_mark]

Yes, that all seems reasonable. No idea why they would give data in density. no one uses that in the hobby.

Yeah. I didn't even realize that it was displaying density...then the fact that their EC readings don't correlate with expected values for density, ppt, or SG.

 

[Dkeller_nc]

A couple of comments to the OP. The variation you're getting between the GHL, Hanna and "other" conductivity readings seem quite reasonable, particularly if the meters aren't temperature compensated. Specific gravity doesn't vary very much with a change in temperature over the kind of range that we'd experience as reefers - say 18 deg C for ambient room temperature to 25 deg C for a typical reef tank. However, conductivity does change quite a bit with temperature. So if the particular conductivity meters you're using aren't temperature compensated, that in and of itself could explain the variation you're seeing.

One other thing to keep in mind is that if conductivity is analyzed very carefully, it's quite accurate as far as calculating salinity for seawater. However, in addition to the issue with conductivity's temperature variation, typical probes are very sensitive to the presence of air bubbles, and in some cases, particularly with tank water that is supersaturated with atmospheric gases, you can easily have bubbles form on the probe tip that you can't readily see. In fact, there's a company making a device for mounting a conductivity probe into that's designed to take care of just that problem by constantly pushing a small flow of tank water across the conductivity probe:

SPS Kit – The Salinity Probe Stability Kit | Vivid Creative Aquatics

vividcreativeaquatics.com

 

[arking_mark]

A couple of comments to the OP. The variation you're getting between the GHL, Hanna and "other" conductivity readings seem quite reasonable, particularly if the meters aren't temperature compensated. Specific gravity doesn't vary very much with a change in temperature over the kind of range that we'd experience as reefers - say 18 deg C for ambient room temperature to 25 deg C for a typical reef tank. However, conductivity does change quite a bit with temperature. So if the particular conductivity meters you're using aren't temperature compensated, that in and of itself could explain the variation you're seeing.

One other thing to keep in mind is that if conductivity is analyzed very carefully, it's quite accurate as far as calculating salinity for seawater. However, in addition to the issue with conductivity's temperature variation, typical probes are very sensitive to the presence of air bubbles, and in some cases, particularly with tank water that is supersaturated with atmospheric gases, you can easily have bubbles form on the probe tip that you can't readily see. In fact, there's a company making a device for mounting a conductivity probe into that's designed to take care of just that problem by constantly pushing a small flow of tank water across the conductivity probe:

SPS Kit – The Salinity Probe Stability Kit | Vivid Creative Aquatics

vividcreativeaquatics.com

All the probes are temp compensated. I'm not seeing any issues with measurements. Bubbles would make readings inconsistent.