PDA

View Full Version : Calibrating Conductivity probe



xiaan
01.06.2009, 12:32
My Conductivity probes values have been slipping I tried recalibrating my probe but It doesn't seem to reed correctly. I leave the probe in the calibration fluid after and for 50mS it still reads 42mS. I have also found that there is a bit of rust where the glass meets the plastic on the probe and I am wondering if this has compromised the probe. The probe is less than 6 months old.

I have added an Reef Status attachment of my Conductivity from the last few weeks. The sharp spikes to 0 are my attempt of calibrating the probe. Also I have not done anything to to the tank to have it drop that much (my refactometer is still showing SG of ~1.025).

Matthias
02.06.2009, 07:37
if you see a value different to 50mS when the probe is in the calibration fluid then the temperature of the fluid is not equal to the temp. of the temperature probe.
A correct temperature compensation is very important for correct conductivity readings.

-> for a successful calibration the temp. of the fluid must be correct - read more in section "conductivity" of the manual

After a successful calibration (with correct temperatures) you MUST read someting about 50mS, maybe with a deviation of +/-0.3 (assuming temp probe has same temp like cali. fluid)

xiaan
02.06.2009, 16:24
I think I found the problem I was not applying the new settings :rolleyes:. The probe now works correctly (reads 49.9mS in the calibration fluid)

xiaan
03.06.2009, 22:59
So Everything was working after I calibrated the probe but now my conductivity dropped down to 28mS over the last day. (My system has not changed and my calibrated refactometer reads SG of 1.024)

The probe currently reads 33.6mS in 50mS calibration fluid and 5mS in the air.

I will try and recalibrate the probe today and see if it slips again.

xiaan
03.06.2009, 23:27
Just recalabrated now back showing 50mS in the calabration fluid

Calabration Data
0mS = 39 ADC
50mS = 289 ADC

In 50mS solution it reads 49.8
Out of solution it reads 0.4mS

In the tank it reads 51mS

Finally when I was calibrating it I found that there is liquid in the probe!
That and the rust around the seal I think that the probe is not working right.
I did not think that a $200 conductivity probe would die so soon (The probe is less than 6 months old)

Matthias
04.06.2009, 05:28
the probe must be cleaned from time to time

why do you assume the probe would be dead? It reads 49.8mS in the solution and 0.4mS outside - these are perfect values -where is the problem now?

the probe can't rust - the electrodes are made of platinum (if you have the correct probe!)

xiaan
04.06.2009, 15:52
why do you assume the probe would be dead? It reads 49.8mS in the solution and 0.4mS outside - these are perfect values -where is the problem now?
correct probe!)

I think the reason I was thinking the probe was defective is that it dropped from 45mS to 25mS in a day. :confused:

The Probe seems to be stable today. Right now I am getting a reading of 43.9mS from my tank if that reading is correct my SG should be 1.020? but right now my calibrated refactometer reads 1.024. Is that a correct reading from the probe?

How often should I recalibrate it?

Matthias
05.06.2009, 06:32
I recalibrated my conductivity probe not even once in the last 2 years, usually this is not necessary.
It is more important to clean it frequently.

boboxx
05.06.2009, 12:19
Would it be close to your ATO of fresh water output?

C.


I think the reason I was thinking the probe was defective is that it dropped from 45mS to 25mS in a day. :confused:

The Probe seems to be stable today. Right now I am getting a reading of 43.9mS from my tank if that reading is correct my SG should be 1.020? but right now my calibrated refactometer reads 1.024. Is that a correct reading from the probe?

How often should I recalibrate it?

xiaan
05.06.2009, 13:39
Would it be close to your ATO of fresh water output?

C.

Nope It is in a different compartment that my ATO and also I have turned off my ATO the last few days as I am trying to increase my SG to 1.026 from 1.024

keener
21.09.2009, 17:30
I'm having the exact same problem with my Salinity probe. After calibration and or cleaning it works great.. but then drifts during the course of the day and settles at something much much lower than it should be.. right now it is reading 1.0214 when it should be reading 1.026 and I just cleaned and calibrated it yesterday!? Is this normal?? Did you resolve your issue?

Matthias
22.09.2009, 07:46
is the measured value stable after settling?

if yes - then enter a reasonable "offset" and it should be fine

keener
22.09.2009, 17:10
is the measured value stable after settling?

if yes - then enter a reasonable "offset" and it should be fine

It is stable for a time but then fluctuates downward quite quickly and frequently.

For example, yesterday evening I did as you suggested and set an offset of 0.001 to get the correct reading of 1.0265 (where it was reading 1.0255 previously). It was fine and now I just received an email from my profilux that says the salinty is 1.0243. I'm pretty sure the salinity hasn't dropped that much from yesterday evening to this morning. Again, I have an auto top off and the fresh water is not in contact with the probe. Last night I also cleaned and calibrated the probe and placed it in a higher flow area (under the outlet of a very slow flowing powerhead to reduce the chances of junk getting on the contacts). So frustrating.. Is it possible the probe is defective? Or what am I doing wrong? Do I really need to clean this $300 probe once a day ??

edit: this evening I will try changing the offset to 0.002 to bump that value up again but I've done this before..

Matthias
23.09.2009, 07:04
I can't imagine that the probe is defective - was the calibration successful?

I think this is a problem with temp. compensation. Is the temp. probe nearby the cond. probe?
Is the compensation activated? Was everything regarding temp. set correctly during calibration.

Are you using the correct cond. probe (with Platinum)?
What are the both ADC calibration values?

keener
28.09.2009, 16:43
It seems to have stabilized more now that I've put it closer to my return pump. It seems to need quite a bit of flow near it.