Working with GHL Conductivity Calibration Solution

[BeanAnimal]

For those reading along:
A conductivity probe is an electrical measurement device. The GHL probe is calibrated using a conductive solution. The units are mS/cm, a measure of how well the liquid conducts electrical current.

Fine — that is what the P4 and probe "read" from your tank water.

That number does NOT directly give you density or salinity. The P4 has to convert the measured conductivity and temperature into salinity, and from salinity and temperature it can calculate density. There is a lot of math behind those conversions.

So the P4 measures conductivity and then converts that value to your desired display unit: mS/cm, PSU, or density in kg/L.


Now here is the issue that I think I have identified:
@Matthias Gross can this get some attention or correct me if I am wrong please.


The P4 also has a setting called "Density Offset" that allows you to tweak the calcualted density reading to account for error or to match another measurement probe in your system. But the catch is that "Density" is not what the probe reads, it reads mS/cm and then shoves those values through a set of complex equations to get Density.

The glaring problem? As implemented, it only adjusts the kg/L value that was read in mS/cm, converted to Salinity and THEN to density. It does not affect (adjust) the mS/cm or PSU readings. It is "tweaking" the end of the chain of math, not the input side.

That makes absolutely no sense to me.

All three values are representations derived from the same conductivity measurement. If I apply an offset because the calculated density is wrong, that leaves the conductivity and PSU values unchanged meaning the three displayed values no longer agree with each other.

The adjustment either needs to be applied to the underlying conductivity measurement so that it flows through the conversions to PSU and kg/L, or the equivalent correction needs to be propagated through all three displayed values.

Otherwise I can adjust the P4 so that its density agrees with my reference, switch the display to PSU, and immediately have it disagree again.

IMHO this is a issue that needs to be addressed. This is an "instrument" not a toy and things like this matter.

 

[Dr. Jim]

It's been 6 years since I started this thread, so I'm 6 years older, maybe wiser, and certainly lazier....
so, I'm now content just using the GHL probe reading as an indicator of salinity drifting. But I'm a fanatic at periodically calibrating my ($100) refractometer based on readings from two Tropic Marine High Precision hydrometers (which disagree by 0.0003). I then write down whatever GHL reading corresponds and use that for monitoring for the next several weeks.

 

[BeanAnimal]

I don’t care so much what my salinity is or where it drifts. I just want to be able to set my probe baseline to match whatever standard I decide to compare it against.

This almost comes down to principle. Unless I am missing something, this is a design flaw and given the number of threads it generates, should be addressed.