conductivity probes?
- Thread starter recife111
- Start date
Matthias Gross
.Registered Member
This is not easy to answer. First the conductivity (or conductance = antiprop. resistance) is measured. Then the salinity is calculated based on the conductivity. After that the density is calculated based on salinity and temperature (compensation).
There are some problems with determinig the correct density. First the temperature compensation is very important. Is the measured temperature exactly the temperature on the conductivity probe? Then the salt combination: ProfiLux does not know what salts are in your tank. It assumes the so-called "Kopenhagen water", the mathematical conversions are based on this. And a last very important point: Is the temperature you assumed while calibration for the calibration fluid 100% correct?
We know that the reading and calculated value may have a difference of about +/- 0.005 because of the reasons mentioned above. That's why we will introduce an user-editable offset. Comes in next firmware - a few days.
There are some problems with determinig the correct density. First the temperature compensation is very important. Is the measured temperature exactly the temperature on the conductivity probe? Then the salt combination: ProfiLux does not know what salts are in your tank. It assumes the so-called "Kopenhagen water", the mathematical conversions are based on this. And a last very important point: Is the temperature you assumed while calibration for the calibration fluid 100% correct?
We know that the reading and calculated value may have a difference of about +/- 0.005 because of the reasons mentioned above. That's why we will introduce an user-editable offset. Comes in next firmware - a few days.
AQD_ottawa
New member
Big advise - before trying to prove accurracy against a refractometer please make sure you have calibrated the unit with 50ms solution first!
If you calibrate with zero DI water you will get an offset! hense the main reason people see differences between digital and analogue readings
If you calibrate with zero DI water you will get an offset! hense the main reason people see differences between digital and analogue readings
Matthias Gross
.Registered Member
that's true: 5 min may not be enough. A deviation of some 0.1°C may cause a difference in the reading of app. 3 to 5!
30 mins should be enough.
And as written earlier: A difference could also be caused by your salt-combination. But the solution for that will be: Editable offset in next firmware!
30 mins should be enough.
And as written earlier: A difference could also be caused by your salt-combination. But the solution for that will be: Editable offset in next firmware!
I use a Sybon refractometer calibrated with 35ppt (1.0259) solution. The offset compared with RO/DI calibration is about 0.001 for my unit.
I keep the tank at 1.025. Auto top off is via RO/DI into a kalk stirrer. I evaporate a pretty decent amount of water since there are two 5-fan propeller breeze units running full time on the tank, and there is a 250cfm in-line vent fan for the canopy. I mix replacement water for small, weekly changes a bit hypersaline relative to the tank, to account for losses through the skimmer, salt creep, etc.
The conductivity probe (which I do periodically clean and calibrate) typically reads in the 33 ppt range, but if I select density as display unit it only reads 1.021 for some reason. So the offset in the new firmware will be most welcome.
I'd also like to see an offset feature for the temperature. I have a bunch of cheap thermometers on the system (digital, stick-on, and floating) and they all seem to read at least 0.5-1.0C lower than the Profilux temp probe. I know the Profilux is a high quality thermometer calibrated by the factory, but still I wonder... Unfortunately there is no easy way to establish a temperature reference as we have for pH, redox, conductivity, etc.
I keep the tank at 1.025. Auto top off is via RO/DI into a kalk stirrer. I evaporate a pretty decent amount of water since there are two 5-fan propeller breeze units running full time on the tank, and there is a 250cfm in-line vent fan for the canopy. I mix replacement water for small, weekly changes a bit hypersaline relative to the tank, to account for losses through the skimmer, salt creep, etc.
The conductivity probe (which I do periodically clean and calibrate) typically reads in the 33 ppt range, but if I select density as display unit it only reads 1.021 for some reason. So the offset in the new firmware will be most welcome.
I'd also like to see an offset feature for the temperature. I have a bunch of cheap thermometers on the system (digital, stick-on, and floating) and they all seem to read at least 0.5-1.0C lower than the Profilux temp probe. I know the Profilux is a high quality thermometer calibrated by the factory, but still I wonder... Unfortunately there is no easy way to establish a temperature reference as we have for pH, redox, conductivity, etc.
elzool
New member
I wonder if the probe could handle and the software display a temperature of 0. If so, you could always prepare an ice water solution and check the temperature of that.
This is assuming you prepare a solution that actually is at 0.
This page has a decent explanation on how to prepare a basic solution. It is not laboratory accurate, but might be interesting to see where all of your thermomters stack up.
The other end of the scale would be to use boiling water IF that works for these probes. Again, not scientifically accurate, but very very close I would imagine.
This is assuming you prepare a solution that actually is at 0.
This page has a decent explanation on how to prepare a basic solution. It is not laboratory accurate, but might be interesting to see where all of your thermomters stack up.
The other end of the scale would be to use boiling water IF that works for these probes. Again, not scientifically accurate, but very very close I would imagine.
Phxkeller
Premium Member
I have just found a large difference in my conductivity compared to my ATC refractometer as well.. profilux ready 1.026 which has gone up and refractometer is reading 1.031.. I could just tell something was up by looking at the corals without the meters...
I have a usual range from 1.024-1.025
I have a usual range from 1.024-1.025
elzool
New member
Does the Profilux probe come with a calibration solution?
If it does, maybe calibrate your refractometer with that and try it again.
If not, I have several bottles of the American Marine Pinpoint Salinity Calibration Fluid 53.0 mS standardized fluid and would be happy to give you one if you wanted to meet somewhere in the next couple days. I have to go to AJ's on Sunday and again on Monday so at least I'll be close to you. Let me know.
If it does, maybe calibrate your refractometer with that and try it again.
If not, I have several bottles of the American Marine Pinpoint Salinity Calibration Fluid 53.0 mS standardized fluid and would be happy to give you one if you wanted to meet somewhere in the next couple days. I have to go to AJ's on Sunday and again on Monday so at least I'll be close to you. Let me know.
AQD_ottawa
New member
The GHL probes are always supplied with 50ms calibration. if you are trying to achieve the same reading you get with a refractometer then you should use the SAME calibration for both.
Refracts also have a history of innacuracy depending on what type you have, in our store the ones we import we tried various types before we found one to trust and not surprisingly it was not a branded model!!!
I would put my trust in a platinum plated pharmeceutical grade GHL probe any day over a read glass refractometer.
With regards to temperature measurement, ALL standard thermometers have a tolerence range of +/- 1degree so it is extremely hard to get coleration, the GHL is strictly set in the factory on various levels where as standard theermometers go through little factory testing. There is a post on this forum to this effect worth reading I put up a few weeks back
Hope this helps
Refracts also have a history of innacuracy depending on what type you have, in our store the ones we import we tried various types before we found one to trust and not surprisingly it was not a branded model!!!
I would put my trust in a platinum plated pharmeceutical grade GHL probe any day over a read glass refractometer.
With regards to temperature measurement, ALL standard thermometers have a tolerence range of +/- 1degree so it is extremely hard to get coleration, the GHL is strictly set in the factory on various levels where as standard theermometers go through little factory testing. There is a post on this forum to this effect worth reading I put up a few weeks back
Hope this helps
Phxkeller
Premium Member
calibration solutions 50ms LOW
calibration solutions 50ms LOW
Mine did come with 50ms calibration solution. I have attempted 4 times to re calibrate the probe and each time wind up with the same range of measurement 1.022 -1.024. The corals appearance and the refractometer don't agree. 1.030
I did some comparing of other aquarists water with my refractometer and theirs all normal limits 1.023-1.025.
I belive I may have a bad (LOW) calibration solution. It was only opened once to set up the probe initially so it was not messed with or diluted by anything I had done before calibration.
HAS anyone else come across this?? I'll just need to get some other calibration solutions to confirm.
calibration solutions 50ms LOW
Mine did come with 50ms calibration solution. I have attempted 4 times to re calibrate the probe and each time wind up with the same range of measurement 1.022 -1.024. The corals appearance and the refractometer don't agree. 1.030
I did some comparing of other aquarists water with my refractometer and theirs all normal limits 1.023-1.025.
I belive I may have a bad (LOW) calibration solution. It was only opened once to set up the probe initially so it was not messed with or diluted by anything I had done before calibration.
HAS anyone else come across this?? I'll just need to get some other calibration solutions to confirm.
AQD_ottawa
New member
A Conductivity probe needs cleaning thoroughly before calibration and should be done so weekly, this is the BIGGEST reason for low readings, a dirty probe.
2 tests -
1. calibrate the probe in 50ms solution and then rinse the probe in fresh water, now place the probe in 50ms solution again and what does it read?
2. make sure you have calibrated your refractometer with the SAME solution and repeat the same test with your refractometer, what does it read?
This will give you conclusive answers, BUT only if you have properly cleaned the probe element before doing this, otherwise the test is pointless.
I have always found pinpoint 400mV redox calibration an excellent conductivity probe cleaner, or by simply scrubbing it with an old tooth brush in water and wiping dry.
ALL conductivity probes MUST be cleaned regularly.
Phxkeller - If you wish to try new solution please contact your retailer and we will arrange this for you through them.
Best wishes
Michael
2 tests -
1. calibrate the probe in 50ms solution and then rinse the probe in fresh water, now place the probe in 50ms solution again and what does it read?
2. make sure you have calibrated your refractometer with the SAME solution and repeat the same test with your refractometer, what does it read?
This will give you conclusive answers, BUT only if you have properly cleaned the probe element before doing this, otherwise the test is pointless.
I have always found pinpoint 400mV redox calibration an excellent conductivity probe cleaner, or by simply scrubbing it with an old tooth brush in water and wiping dry.
ALL conductivity probes MUST be cleaned regularly.
Phxkeller - If you wish to try new solution please contact your retailer and we will arrange this for you through them.
Best wishes
Michael
Last edited:
Matthias Gross
.Registered Member
In firmware 3.07 the calculation mS -> density will be adjustable.
That means you can enter an offset.
Comes next year...
That means you can enter an offset.
Comes next year...
AQD_ottawa
New member
This update will allow you to adjust your profilux density to match your refractometers.
Nobody has yet said what results they have had by calibrating their refractometers with the GHL calibration solution. This is an important test as mentioned above.
I did this before we left the UK with our refractometer when this question came up before and once calibrated the same way the readings were pretty much within tolerence so i do urge you to do this.
Thanks Matthias for the input, I knew the firmware update was coming but was not sure when so did not want to release the info. Thanks for doing so.
Happy Christmas
Michael
Nobody has yet said what results they have had by calibrating their refractometers with the GHL calibration solution. This is an important test as mentioned above.
I did this before we left the UK with our refractometer when this question came up before and once calibrated the same way the readings were pretty much within tolerence so i do urge you to do this.
Thanks Matthias for the input, I knew the firmware update was coming but was not sure when so did not want to release the info. Thanks for doing so.
Happy Christmas
Michael
