Working with GHL Conductivity Calibration Solution

[Dr. Jim]

It seems like I can't get the Conductivity calibration to come out like it should. I want a salinity of 35ppt which would equal a conductivity of 53. I calibrated my refractometer with 3 standards (BRS, Naslow and a DIY). There may be a range of difference of 1.001 SG amongst the 3. I calibrated the refractometer using an average of the 3, so the reading may be off by about +/- 1.0005 to 1.0010 SG). My tank is adjusted to 35 ppt using the calibrated refractometer, but the GHL Conductivity reads only 49.8 (which is equivalent to about 1.024 SG). I ran the calibration 3 times and am getting consistent numbers.

(DIY Solution: 79.3 g Morton salt in 2104.4 ml RO/DI per Randy Holmes-Farley)

My questions:
1) How often would you replace the GHL Calibration Solution? (Mine was opened for the first time about 6 months ago. I was careful not to contaminate it and closed the cap tightly, immediately).
2) Regarding technique, I have been rinsing the probe in RO/DI and drying it before calibration. I then place it directly into the bottle. Is this technique acceptable? I also wonder how many times this can be done before discarding the bottle. (I've probably run 8-12 calibrations with the bottle).

Thanks for helping!

 

[Member No 1]

Mine reads low, but very consistent. I don't use it for control, only monitoring, so I don't worry about it.
If your 100% sure that your refractometer is correct, and you "need" the probe to read correctly, then follow the calibration procedure but instead of using the calibration solution, calibrate the probe by placing it in your sump.
Then check it often until you are comfy that it's reading the same as your refractometer.

 

[madweazl]

While you're in the calibration menu, are you adjusting the calibration solution ms to 53 (default is 50)?

 

[Member No 1]

While you're in the calibration menu, are you adjusting the calibration solution ms to 53 (default is 50)?

If he's using the GHL solution, it should be set to 50mS

 

[madweazl]

If he's using the GHL solution, it should be set to 50mS

I think he's using the Randy's DIY solution for the probe calibration now.

 

[Member No 1]

I think he's using the Randy's DIY solution for the probe calibration now.

Possibly, but I took it as he was using Randy's for his refractometer. Let's see what the OP comes back with.

 

[Dr. Jim]

I am using the DIY for the refractometer and the GHL calibration solution for the probe calibration.

That's an interesting idea to use the tank water to calibrate but I agree, it is probably good enough just to be consistent and just know what number it should stay at.

I'm not sure if you can do this.... but I just put the GHL 50 mS calibration solution on the refractometer and it read 1.018 when it should be 1.0247. I wonder how it can be so far off. If anything, you would think evaporation over time would raise the SG. But I'm wondering if there is something about a conductivity solution for measuring a conductivity probe that is different from a solution to calibrate a refractometer. Maybe @Randy Holmes-Farley might be kind enough to help?

Thank you Pete and madweazl for helping!

 

[Member No 1]

Randy will clear it up, but my take, just because a liquid has a conductivity of 50mS, it doesn't necessarily have to have a specific gravity of 1.0247. Some solutions can be used for both, some can't.

 

[Member No 1]

One other thing, do you have temp compensation on or off when calibrating, and are you matching the calibration fluid temp to the tank temp? I always float the bottle in the sump for a little bit.

5.9.7 Conductivity
The conductivity of a liquid is strongly temperature-dependent. For the exact determination of the conductivity by ProfiLux 4, this results in a so-called Temperature compensation for each measurement.
During calibration, you will be asked how ProfiLux 4 is to perform the temperature compensation. ProfiLux 4 offers you two options:
 Aquarium temperature manual: The temperature compensation is carried out by means of a fixed set temperature (Aquarium temperature manual). Then enter the temperature.
 Aquarium temperature by using a continuously measured value: Select the desired Temperature sensor for temperature determination.
You will be prompted to enter the temperature of the calibration liquid (Calibration liquid temperature).
The actual calibration is then started.
Please note the information on the display. After each step, press the RETURN button and wait for the signal tone before you continue.
 Connect the Conductivity sensor.
 Set the Calibration tolerance ->RETURN
 Determine the Calibration value 2 and confirm with RETURN
Tip
The most accurate values for the conductivity value determination are obtained by means of a continuously measured value.
Tip
The easiest and fastest way is to equalize the fluid temp is to place the closed container of the calibration liquid in the aquarium until the temperature has adjusted. This is only useful if the aquarium temperature is known.
ProfiLux 4 Programming Guide
EN 2017-02-06 55
 Select Temperature sensor or Aquarium temperature manual for temperature compensation ->RETURN.
 If you selected the Aquarium temperature manual enter the basin temperature ->RETURN
 Specify the temperature of the calibration liquid ->RETURN
 Hold the sensor in the air (Hold the sensor in the air!) until a signal sounds
 Then immerse the electrode in the calibration liquid corresponding to the Measuring range ->RETURN
 Wait for the signal sound
 If no errors have occurred, save the calibrated values by selecting Yes and confirm with RETURN
 Check the calibration as described below
The Conductivity sensor is now ready for use. Based on your calibration, ProfiLux 4 can determine the Conductivity.

 

[Dr. Jim]

Randy will clear it up, but my take, just because a liquid has a conductivity of 50mS, it doesn't necessarily have to have a specific gravity of 1.0247. Some solutions can be used for both, some can't.

I was thinking something along those lines although I can't explain the reason.
I guess that is why I only got part of my chemistry exam questions right when in school !

 

[Dr. Jim]

One other thing, do you have temp compensation on or off when calibrating, and are you matching the calibration fluid temp to the tank temp? I always float the bottle in the sump for a little bit.

5.9.7 Conductivity
The conductivity of a liquid is strongly temperature-dependent. For the exact determination of the conductivity by ProfiLux 4, this results in a so-called Temperature compensation for each measurement.
During calibration, you will be asked how ProfiLux 4 is to perform the temperature compensation. ProfiLux 4 offers you two options:
 Aquarium temperature manual: The temperature compensation is carried out by means of a fixed set temperature (Aquarium temperature manual). Then enter the temperature.
 Aquarium temperature by using a continuously measured value: Select the desired Temperature sensor for temperature determination.
You will be prompted to enter the temperature of the calibration liquid (Calibration liquid temperature).
The actual calibration is then started.
Please note the information on the display. After each step, press the RETURN button and wait for the signal tone before you continue.
 Connect the Conductivity sensor.
 Set the Calibration tolerance ->RETURN
 Determine the Calibration value 2 and confirm with RETURN
Tip
The most accurate values for the conductivity value determination are obtained by means of a continuously measured value.
Tip
The easiest and fastest way is to equalize the fluid temp is to place the closed container of the calibration liquid in the aquarium until the temperature has adjusted. This is only useful if the aquarium temperature is known.
ProfiLux 4 Programming Guide
EN 2017-02-06 55
 Select Temperature sensor or Aquarium temperature manual for temperature compensation ->RETURN.
 If you selected the Aquarium temperature manual enter the basin temperature ->RETURN
 Specify the temperature of the calibration liquid ->RETURN
 Hold the sensor in the air (Hold the sensor in the air!) until a signal sounds
 Then immerse the electrode in the calibration liquid corresponding to the Measuring range ->RETURN
 Wait for the signal sound
 If no errors have occurred, save the calibrated values by selecting Yes and confirm with RETURN
 Check the calibration as described below
The Conductivity sensor is now ready for use. Based on your calibration, ProfiLux 4 can determine the Conductivity.

Yes...I'm doing all that. Thanks again!

 

[ingchr1]

In regards to having temperature compensation on, I don't think it works as it should.

Conductivity Sensor Settings

Yesterday was a nice warm day here and our house temperature rose to ~80F. As expected, the temperature in our tank went up slightly above setpoint. What I noticed when this happened is that the conductivity reading dropped (unexpected). I had temperature compensation set on Temperature 1...

www.reef2reef.com

 

[Aquagen]

I got my tank where I wanted it using a refractometer calibrated with 3 different solutions. Then I used the tank itself as my cal for the probe.
Reads spot on

 

[Dr. Jim]

I got my tank where I wanted it using a refractometer calibrated with 3 different solutions. Then I used the tank itself as my cal for the probe.
Reads spot on

Hey...that's cheating!

I understand that the probe can be calibrated that way, but it seems to defeat the purpose of the GHL Calibration Solution. The calibration solution is supposed to be relied upon to give you the "true" reading. If it can't do that, why even bother with the calibration solution?? And then, you have to wonder if your refractometer calibrating solutions are accurate. (Of course, if 3 of them agree with each other, they PROBABLY are "true", but it's a shame we have to accept any degree of uncertainty and choose to disregard what a high-tech instrument like the GHL probe, along with it's calibration solution, is showing us).

I can't remember for sure, but I think when my instrument and calibrating solution was brand new (7 months ago), I did get an "accurate" measurement. So, I wonder if the calibration solution quickly loses accuracy over time. I will buy a new bottle the next time I order something else from GHL (to save on combined shipping). Hmmm....maybe I could use another Power Bar.....

All in all.....I'm still happy with all my GHL equipment!!

 

[ingchr1]

I don't think conductivity solution would change its value, unless contaminated.

I'd order another solution and see what it reads without calibration. If off, calibrate.

I last calibrated mine with Neptune solution. I was concerned I may have contaminated the GHL solution over time.

After I calibrated the probe I used it to verify my homemade solution was correct.