Issue with GHL salinity measurements

arking_mark

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Simply stated, the salinity measurements of this device is inconsistent with itself!

I have a measured GHL salinity at 53ms/cm, 34.7psu, 1.0246 @ 76.9 degrees.

Per reef calculators, 53ms/cm = 35ppt (35psu) and 1.0234 kg/l (1.0264 sg)

Why would this be the case? Which is right? Is this a bug?

Screenshot_20210516-094551_GHL Connect.jpg


Screenshot_20210516-094538_GHL Connect.jpg


Screenshot_20210516-094527_GHL Connect.jpg


Screenshot_20210516-094520_GHL Connect.jpg
 

wyster

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I have the same issue and spent days going backwards and forwards with GHL support, They put it down to some interferance somehow, not sure from where as it runs straight out of the back of my profilux and into the tank not past any return pumps etc. In the end I gave up and do not rely on it.
 

kenchilada

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I have the same issue and spent days going backwards and forwards with GHL support, They put it down to some interferance somehow, not sure from where as it runs straight out of the back of my profilux and into the tank not past any return pumps etc. In the end I gave up and do not rely on it.

Good to know. I sold my Apex to get away from these problems so I won’t bother buying the GHL probe either.
 

Lasse

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It is important that the salinity probe is placed in a high flow area. If not - gas bubbles can build up on the electrode - giving wrong reading. With high velocity - they will be flushed away. In my system - the probe show around 2 psu to low figure (and to low conductivity if I use automatic temp compensation). In order to get a right measurement figure - I have change to manual temp correction and set this manual compensation to the value necessary to use if the probes figures should be the same as my refractometer. I have a stable temperature around 25 degree C.

Sincerely Lasse
 
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It is important that the salinity probe is placed in a high flow area. If not - gas bubbles can build up on the electrode - giving wrong reading. With high velocity - they will be flushed away. In my system - the probe show around 2 psu to low figure (and to low conductivity if I use automatic temp compensation). In order to get a right measurement figure - I have change to manual temp correction and set this manual compensation to the value necessary to use if the probes figures should be the same as my refractometer. I have a stable temperature around 25 degree C.

Sincerely Lasse

The issue is that it shows 53ms/cm, 34.7psu, 1.0246 @ 76.9 degrees.

However, 53ms/cm is NOT 34.7 psu or 1.0246kg/l.

Per reef calculators, 53ms/cm = 35ppt (35psu) and 1.0234 kg/l (1.0264 sg)
 
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You are comparing a real measure with the theoretical - IMO - it close enough

Sincerely Lasse
Is it? If the kg/l is actually 1.0246 then my sg is actually 1.0276 which is running high. No need for me to rn it elevated...I just need to know which GHL number to use since they DON'T agree. Very frustrating!

Salinity calculator (reefapp.net)
 

Lasse

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If you try to chase numbers with salinity and conductivity meters - good luck. I run between 34.5 and 35 psu. I decide with help of my refractometer - properly calibrated with 35 psu solution for refractometers (NOT with RO water) and adjust my salinity meter to show that in my actual temperature. I adjust with manual temp because my real temp differ only around 0.1 degree between day and night. I also do a regular check with an external lab in order to know if I in a range of ± 0.3 psu from the real value.

The difficulty with conductivity meters is that it is not only the conductivity probe´s accuracy you must rely on - you have also the temperature probes accuracy. This is of importance both during calibration and measurements - if the temperature probe is only 0.1 degree C plus or minus from the real figure - it will differ in the conversion.

With mine - it shows 34.8 psu - 53 mS - 1.024 at 25 degree C.

Most here aim at 35 psu but IRL - seawater is normally around 34.6 psu. You should also know that PSU and ppt is not exactly the same.

Sincerely Lasse
 

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Lasse

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@[email protected] thanks for the explanation - I have one question however according to this

If a user is sure that the actual density reading of the ProfiLux is a bit different to reality, he can fine-tune the reading by entering an offset in the conductivity settings page.

If I finetune - it is only affecting the readings of the density - not the readings of the PSU. It would be good to have fine tuning for PSU readings too in the future

Sincerely Lasse
 

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A better answer...

 
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A better answer...


I finished my analysis...

If you are using GHL, you should only be using the density. In GHL world, 50.7ms = 34.1 ppt = 1.0235 kg/l = 1.0267 SG.

In non-GHL world, 53.6ms = 35.4 ppt = 1.0235 kg/l = 1.0267 SG.
 

Lasse

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And according to the article @[email protected] posted - the question is - who is nearest the real value - the GHL or "other" world. There is a bunch of algorithms in order to translate counductivity into PSU and the other way of expression. - my GHL shows at the moment 52,7 ms -> 34,6 PSU and 1,244 kG/L- My refractometer shows 1,26 and/or 34.6 PSU - fair enough for me.

Sincerely Lasse
 
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And according to the article @[email protected] posted - the question is - who is nearest the real value - the GHL or "other" world. There is a bunch of algorithms in order to translate counductivity into PSU and the other way of expression. - my GHL shows at the moment 52,7 ms -> 34,6 PSU and 1,244 kG/L- My refractometer shows 1,26 and/or 34.6 PSU - fair enough for me.

Sincerely Lasse

@Lasse Most people would target 53 ms to get their salinity in line. If you do this with GHL, as I have done, you actually end up with a higher then desired salinity. If you use their density you get were you expected.

I did not appreciate running at a higher salinity.
 

Lasse

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But if I have two equipments saying 34.6 psu (the Oceans average salinity) measured with two different methods - I give a dam what the conductivity says - its of no interest because it could vary a little depended of the actual ion composition. I aim into 34,6 psu and check with my refractometer. I use my salinity probe in order to pump out water if the salinity get to high and I check it with my refractometer. If the both are in the range of ± 0,3 pSU - I´m satisfied with that. I also check with external measurements ( 3 times a year) and I use the ICP test and this website in order to check that I´m in line with what I want


Last ICP give me a Salinity (measured at lab of 35,2, calculated by the web site 34,7. GHL 34,6 and manual refractometer 35,2. This is fair enough for me but for the moment I read 34,6

Sincerely Lasse
 
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But if I have two equipments saying 34.6 psu (the Oceans average salinity) measured with two different methods - I give a dam what the conductivity says - its of no interest because it could vary a little depended of the actual ion composition. I aim into 34,6 psu and check with my refractometer. I use my salinity probe in order to pump out water if the salinity get to high and I check it with my refractometer. If the both are in the range of ± 0,3 pSU - I´m satisfied with that. I also check with external measurements ( 3 times a year) and I use the ICP test and this website in order to check that I´m in line with what I want


Last ICP give me a Salinity (measured at lab of 35,2, calculated by the web site 34,7. GHL 34,6 and manual refractometer 35,2. This is fair enough for me but for the moment I read 34,6

Sincerely Lasse

I'm not in disagreement with you. What your saying is totally consistent and would be perfectly acceptable for my tank as well.

Finding a salinity I believe was my problem. Three meters all different and one inconsistent with expected calculations.

In the end, I was able to find something I believed was accurate enough...and perfectly match a GHL measurement.

So I feel confident in trusting that measurement in GHL.

I've automated stabilizing my salinity with GHL. The setpoint is done in ms. So I had to discover the needed ms setpoint which was also a slight pain cause you can't use 53. I found that 50.7 gets me close enough.
 
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Just a quick follow-up.

Tank is running at steady 1.0233 kg/l (GHL setpoint of 50.7). My high-precision TM Hydrometer puts this almost spot on for 1.0264.

I'm feeling fairly confident in GHL's density number.
 

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