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Thread: Salinity probe: Stable & Calibrated, but wrong!

  1. #1
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    Salinity probe: Stable & Calibrated, but wrong!

    Hi Folks,

    I know what you're thinking, another salinity probe going nuts...

    Well, not exactly. My probe is actually very consistent. It's just it's wrong! Salinity is a stable 35ppt according to properly calibrated refractometer and verified with a Milwalkee.

    The probe has been calibrated several times, but still shows this level. There are no air bubbles and turning upside down, shaking etc has had no effect.

    The salinity probe is connected to the PM2 module, mounted in the sump next to the temperature sensor. The changes shown are the ATU, which tops up into the same chamber as the probe.

    How should I best adjust the probe to show the actual salinity?

    Sg.jpg

    Thanks in advance!

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    Was the calibration fluid packet in contact with the tank water the entire time you were calibrating? The salinity reading is affected by temperature. If the calibration fluid was a different temp than the tank, the reading will be off.

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    Quote Originally Posted by nicholb View Post
    Was the calibration fluid packet in contact with the tank water the entire time you were calibrating? The salinity reading is affected by temperature. If the calibration fluid was a different temp than the tank, the reading will be off.
    I put the temp probe and the salinity probe into the pouch containing the calibration solution. There should not be an issue caused by temperature differential

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    Unless you have temperature compensation turned on, doing that would not help. You would be calibrating for the temperature of the fluid not the tank temp. For best results you should have the calibration packet in contact with tank water during the calibration.

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    Quote Originally Posted by tastyfish View Post
    I put the temp probe and the salinity probe into the pouch containing the calibration solution. There should not be an issue caused by temperature differential
    That isnt effective. If you didn't use temp compensation, then that can skew your readings by about 8ppt. If you did use temp compensation and it was incorrect by even 0.1, the value would be skewed by at least 0.5 ppt and even more if the temp compensation is more than 0.1 off (it's not always 2.2)

    Heat the packet to the tank temp, don't bring the temp probe to the packet.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Quote Originally Posted by nicholb View Post
    Unless you have temperature compensation turned on, doing that would not help. You would be calibrating for the temperature of the fluid not the tank temp. For best results you should have the calibration packet in contact with tank water during the calibration.
    Quote Originally Posted by zombie View Post
    That isnt effective. If you didn't use temp compensation, then that can skew your readings by about 8ppt. If you did use temp compensation and it was incorrect by even 0.1, the value would be skewed by at least 0.5 ppt and even more if the temp compensation is more than 0.1 off (it's not always 2.2)

    Heat the packet to the tank temp, don't bring the temp probe to the packet.

    You might be an engineer if...You have no life and can prove it mathematically.
    Ok, thanks guys, I will try this. Does this therefore suggest that even with temperature compensation enabled and the tank temperature changes, then the salinity detection will be compromised?

    Surely if tank temperature is 26C, and calibration solution is 25.5C (with temp probe in the solution, reading the temperature), then the probe will be calibrated correctly?

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    If you have the correct temp comp set it could work. But why calibrate that way? You introduce unnecessary variables, like was the temp prob at the fluid's temp or was it still adjusting.

    Best practice is to have the fluid packet in contact with the tank water and at the same temp as the tank. This is known to work. If your probe is off and you did not do this, then you should probably try this next.

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    Yeah, I followed the BRS video: https://www.youtube.com/watch?v=NyC8x57E5DA

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    Quote Originally Posted by tastyfish View Post
    Ok, thanks guys, I will try this. Does this therefore suggest that even with temperature compensation enabled and the tank temperature changes, then the salinity detection will be compromised?

    Surely if tank temperature is 26C, and calibration solution is 25.5C (with temp probe in the solution, reading the temperature), then the probe will be calibrated correctly?
    Temp compensation compensates for changes in tank temp. During calibration the two must match exactly or it will not be accurate. If for example tank temp is 26 and the packet is 25.5, the reading is skewed by 1.1 ppt. If they were both calibrated at 26 exactly then if the tank temp later dropped to 25.5, the raw reading that you don't see would be skewed by 1.1ppt from the physics of the probe, but temp compensation would correct it back.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Calibration seems to be a big issue on many forums. I've developed a twitch myself. I set up my new Apex 2016 and calibrated the conductivity probe and it was always spot on. The problem is I haven't calibrated the probe for 8 months, so I decided I better after seeing a shift in accuracy by .5 PPT. I dried the probe making sure it was dry, placed the calibration fluid in a probe storage bottle and placed it in my tank for 30 minutes. When prompted, I inserted the probe into the bottle which allowed me to turn the bottle upside down to expel any trapped air. Back in the tank with the bottle for 20 min and calibrated. My PPT is now reading 24 PPT. I have a Milwaukee unit along with a refractometer which both match,but not even close to the Apex probe. I have re-calibrated 6 times even with a new probe I purchased with the same results. I have heard that unplugging the probe and temp probes and manually calibrating will cause a failure resulting in a reset to factory defaults. The probes read very low weather I add temp compensation or not. 1 used probe and 1 new probe read the same. Tank temp is 78 degrees ,no temp compensation. Reading 24 PPT steady. Milwaukee reads 34PPT ,refractometer reads 34 PPT. Any suggestions on the above re-set? Ive ordered 9 more packets of the 53000 just to try and satisfy this twitch I've developed. My 2nd tanks Apex reads correctly at 34 PPT with the same probes.

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