50mS/cm calibration solution

[kirucd]

I have a question regarding GHL calibration solution 50mS/cm.
I have 3 different solutions bought in different years from GHL,
2 of them are expired, 1 is still valid until 2016. I had measured
the salinity of this 3 solutions with 2 different digital refractometer
(1 Hanna seawater HI96822 and 1 Milwaukee seawater MA887)
and with both digital refractometers I have the same results for
all 3 calibration solutions: the salinity is 24 ppt (or 23 PSU) and that
means 37,84 mS/cm. That is way too different from 50mS/cm.
The 2 digital refractometers have temperature correction, but in
any case the results were obtained at 24,5 degree C.
Any explanation for this? Are the calibration solution not good?

I mentioned that the valid calibration solution (not expired) was not
used until now.

[Gunther]

Are you sure that it´s useful to use a refractometer (which is a optical device) to check the calibration fluid for an electrical device?
I suggest the calibration fluid has the correct conductivity but different optical characteristics.

[Antipodes]

As Gunther has said, if you want to check a calibration fluid for an electrical probe you will need to use a electrical probe to read it accurately.

What you can do is read it with probe.
Measure an amount of sea water as comparison (by probe after calibration) and see what it reads, then read that sea water with your refractometer (Which I don't believe are very accurate unless preciously calibrated against a known sample which has ben measured by a lab or accurate probe)
Also note that I have searched over and over again and I have not found one correlation chart to be the same as the other when it comes to PSU Vs MS Vs PPT Vs SG.

The samples should not deteriorate if not used (sealed)as they are not organic. They generally get contaminated by use.

[kirucd]

Yeap, you are right. I was comparing the calibration solution using another
method, that is refractometer, and obviosly the solution is having different properties
in terms of refracting the light compared to seawater. Two solutions with the same
conductivity can have different readings to light refraction. I am using digital refractomer
as a backup unit to double check the salinity of the water, especially when I am preparing
water for change.

One more question, maybe you know, what cell constant is having the GHL platinum probe?

[Antipodes]

One more question, maybe you know, what cell constant is having the GHL platinum probe?

Sorry, I am not sure what you are asking here?

The platinum probes are used as they are more resilent to sea water and organic contamination. (But they still can get organic growth on them, so wipe with a cloth or paper towel every week or so)
They do not seem to be adversely affected by calcium buildup etc.
(I had a carbon probe on my old Aquadine and this was a real pain as the calcium affected it all the time)

[kirucd]

Every conductivity probe is having a cell constantant, for probes used to measure saturated
solutions is usually 10 or 5, for brackish water is 1, for low conductivity solutions is 0,1 and so on.
This is the construction cell constant and based on it the probe can measure conductivity in an interval.
When you make the calibration of the probe you practically (the software and hardware is doing that)
calculate the real cell constant of that probe.

Normally the cell constant is K=d/a where d is distance between electrodes and a is area of electrodes
and is measured in 1/cm. The usual probes are having this constant 10, or 1 or 0,1 etc. The probe from
GHL is not having an usual constant and I wanted to know what constant is having.

[Antipodes]

Interesting, I found some info on calculating the constant however it is a little beyond my current knowledge.
Also not really in my to do list at the moment.

GHL will have to answer this one!

[kirucd]

You can find some theory here:

http://www.tau.ac.il/~chemlaba/Files...nductivity.pdf

[Antipodes]

Yeah Ok. That is a bit more readable than the info I found last night.
So to find cell constant you just need to have a feeler gauge, and a vernier caliper.
Not really sure why you would want this as the profilux is programmed with this data and would be irrelevant to the average user.

But anyway, measurements are (Platinum GHL probe)
Width of one electrode .3cm
Height of one electrode .5cm
Distance between electrodes .15cm

Cell constant K=d/a
K = cell constant (cm-1)
a = effective area of the electrodes (cm2)
d = distance between the electrodes (cm)

.5*.3=.15*2=.30 mm2 = .3 cm2

.15/.3=.5

Cell Constant is .5 ?


Or if you take into account the surface area of only 1 electrode this would be a cell constant of 1!



I see page 20 refers to more than the physical geometry determining cell constant.

[kirucd]

I wanted to know cell constant in order to know what are the measuring limits of the probe.
The real cell constant is actually determined by controller in the moment of calibration.
The physical cell constant that you calculated is only a theoretical one. I don't think the
cell constant for GHL platinum probe is .5 because you cannot measure seawater with
a probe that is having this constant. Probes with cell constant around 1 are used for
brackish water, with 0.1 for pure water, and usually with 10 for solution with high conductivity
like seawater.

I was curious also to find out what solution is GHL solution of 50mS/cm because if I am
going to use for calibration a different solution but with the same conductivity at 25 degrees
it is possible to have a bad calibration if I am doing the calibration at a different temperature
then 25 degrees.

Hope somebody from GHL will tell us what type of probe is the one they are selling.