Salinity probe calibration using freshly/tank saltwater instead of calibration fluid

[Bramzor]

I have an Apex salinity probe that drops over time (in the course of one month with new saltwater it dropped from 35 to 31, while salinity of the tank did not change). Not sure why that is (it is not related to temperature TC value) and also not interference since it's running straight to the apex not passing any other cables. But this is not my main question.

I was wondering, why should I bother with buying 35ppm calibration fluid all the time if my tank is on 35ppm anyway (or at least my waterchanges are) and I would actually prefer to calibrate on my saltwater instead of bothering with calibration fluids. I always check it using refractometer so I know it should be stable/close to 35ppm.
So I have 2 questions.
1) Is it technically possible to just calibrate my probe to a baseline (of +-35ppm) using my tank water inside my tank? If I'm correct, the calibration fluids are 35ppm anyway (I know, its 53mS conductivity but I'm not searching for the conductivity of my tank but rather 35ppm or at least the deviation of it compared to my waterchange water, not compared to a calibration fluid). But the calibration fluid is 35 during calibration so technically I could calibrate it using my tankwater right considering that tank water has a salinity of 35? (which is only a few weeks old btw and an empty tank). So if I put it in my tank during the calibration process, it should calibrate my water as being 35ppm correct?
2) Since I'm not interested if my tank is really 35ppm but more about the deviation of my standard 35ppm salinity baseline/tank water, it would make more sense to set the baseline to my tank water instead (after making sure the salinity is close to 35ppm or at least close to my baseline).

Conclusion: I'm not interested how my water is compared to a calibration fluid of 53mS but instead, I want to know if it's higher or lower compared to my normal saltwater (which is 35ppm anyway).

 

[Bramzor]

Did hold it at a 90 degrees angle and saw bubbles coming out so the dropping was probably related to bubbles. Main question remains the same though.
Salinity measurement went from 30 to 40.

 

[Randy Holmes-Farley]

Your new salt water is 35 ppt, not ppm.

If it is perfectly made to 35 ppt, then it is a fine fluid to calibrate any other device you have.

The question is how do you know it is 35 ppt, and each time you extend an original standard to a new standard, additional error can creep in.

 

[Dennis Cartier]

I do this, calibrate my EC probes using the tank water reading that I measure with a Tropic Marin High Precision Hydrometer. I just have to keep my tank at 25C/77F.

I have also found that mounting your EC probes on an angle, stops the air bubble issue. I also have my probes mounted into a manifold that has a dedicated pump to flow water past the probes constantly to try to keep squatters out.

Using these techniques, and many redundant probes, I operate my ATO using EC.

 

[Bramzor]

Using these techniques, and many redundant probes, I operate my ATO using EC.

Nice, I would already be very happy if I could use the salinity/conductivity probe as a safety measurement. So far it has always been a struggle to keep it rock stable. I do have ATO controlled by 3 optical sensors and a water level sensor so salinity should in fact be stable in my tank at the moment. If it was, it would be a perfect 3rd safeguard.

 

[Bramzor]

The question is how do you know it is 35 ppt, and each time you extend an original standard to a new standard, additional error can creep in.

Have 4 methods to measure the salinity so ballpark figure should be correct. I'm not worried that it would drop (creep) a little over time, as long as I have an easy way to calibrate it to my tankwater and measure my tank using the other 4 methods. Although the probe should be good enough to measure drops or increases in salinity over a short period of time to raise alarms.

I would even consider doing a recalibration after every month or waterchange just to reset the baseline. Would also make the calibration process a bit easier and I would know that I need to look out for 35 and not a strange number like 32.5 or 36 which I might get using a calibration fluid.

 

[V A R I A N T]

It’s fair to mention, and help with expectations, that this is a conductivity probe. It doesn’t measure salinity. It’s natural for the reading to fluctuate up and down based on variables in the water. Temperature will have the greatest impact as the density of the water changes as temp fluctuates. I agree it is a great tool to help identify when things aren’t as they should be, like ATO running/not running. And it may not always, or ever, read at 35ppt. I use mine as a tool to know if there are sudden changes, but mine reads consistently at 43.7 and fluctuates 0.5 up or down at any given time. I’ve calibrated mine too many times to count and the reading never changes anywhere near 35 afterwards.

 

[vetteguy53081]

I have an Apex salinity probe that drops over time (in the course of one month with new saltwater it dropped from 35 to 31, while salinity of the tank did not change). Not sure why that is (it is not related to temperature TC value) and also not interference since it's running straight to the apex not passing any other cables. But this is not my main question.

I was wondering, why should I bother with buying 35ppm calibration fluid all the time if my tank is on 35ppm anyway (or at least my waterchanges are) and I would actually prefer to calibrate on my saltwater instead of bothering with calibration fluids. I always check it using refractometer so I know it should be stable/close to 35ppm.
So I have 2 questions.
1) Is it technically possible to just calibrate my probe to a baseline (of +-35ppm) using my tank water inside my tank? If I'm correct, the calibration fluids are 35ppm anyway (I know, its 53mS conductivity but I'm not searching for the conductivity of my tank but rather 35ppm or at least the deviation of it compared to my waterchange water, not compared to a calibration fluid). But the calibration fluid is 35 during calibration so technically I could calibrate it using my tankwater right considering that tank water has a salinity of 35? (which is only a few weeks old btw and an empty tank). So if I put it in my tank during the calibration process, it should calibrate my water as being 35ppm correct?
2) Since I'm not interested if my tank is really 35ppm but more about the deviation of my standard 35ppm salinity baseline/tank water, it would make more sense to set the baseline to my tank water instead (after making sure the salinity is close to 35ppm or at least close to my baseline).

Conclusion: I'm not interested how my water is compared to a calibration fluid of 53mS but instead, I want to know if it's higher or lower compared to my normal saltwater (which is 35ppm anyway).

Too many have found out RO and other fluid does not work. The probe is a conductivity unit and Apex as with mine is less than trustworthy with accuracy. Oten, false readings are caused by temperature, micro-bubbles (apply probe at an angle to allow bubbles to escape and place in an area where the flow is somewhat stagnant) and from a recent seminar with neptune- You can get false readings from electrical interference as its conductive- MAKE SURE THE PROBE WIRES ARE NOT TOUCHING ELECTRICAL WIRES OF ANY SORT WHICH AFFECTS THE MEASUREMENT.

 

[Bramzor]

Just calibrated inside my sump (so with tank water)

Dry calibration point:

Wet calibration point:

Result after calibration as expected: 34.9


I wonder where I will be after a month but if everything goes well, it should still be close to 35.
I also expect that the drop of salinity over time was caused by the bubbles that came out when I put the probe at an angle. After removing the bubbles, value jumped up to 40 so had to recalibrate it anyway. Way easier to do it with tank water though compared to calibration fluid since I can do it in the sump itself.

If it does show the same drop in salinity over time, I can easily recalibrate and check what happened with the above value during the calibration. This calibration value should stay the same anyway unless maybe a small difference because of dirt or bacteria building up inside the probe which would be solved with the recalibration anyway.

 

[Bramzor]

It’s fair to mention, and help with expectations, that this is a conductivity probe. It doesn’t measure salinity. It’s natural for the reading to fluctuate up and down based on variables in the water. Temperature will have the greatest impact as the density of the water changes as temp fluctuates. I agree it is a great tool to help identify when things aren’t as they should be, like ATO running/not running. And it may not always, or ever, read at 35ppt. I use mine as a tool to know if there are sudden changes, but mine reads consistently at 43.7 and fluctuates 0.5 up or down at any given time. I’ve calibrated mine too many times to count and the reading never changes anywhere near 35 afterwards.

Hehe, I had the same with calibration fluids. But if your goal is to measure sudden changes, just calibrate it in your sump. Value will show 35 if you do so and should not go up or down a lot. With Apex you can compare temperature/salinity graphs together and if you see them jumping at the same time, your TC value is incorrect. But in my case, it was set to 2.2 and the value was stable even though temperature jumps on the graphs. So correction value was spot on.

Also run the probe cable in a way that it's far away from any powerlines or even other probe lines. Mine runs away from anything and I used cable ties to make sure it's stuck. Also calibrating it in the sump at the place it will be doing the readings will help with calibration.

Last thing, when calibrating, hold the probe almost upside down with that small hole to the top while tapping it slightly. Bubbles will come out. Take your time to do so. Probe is waterproof on the top so you can hold it upside down. I used to do this with the calibration fluid too ( by using tape to make sure I can hold it upside down without leaking calibration fluid, and I would push out air from the packet too) but in the sump is way easier as you can easily hold it upside down and see the bubbles and that small hole.

With the above steps, you should get a stable salinity reading. (Just skip the calibration fluid if you just care about the fluctuations like I do).

 

[Bramzor]

Too many have found out RO and other fluid does not work. The probe is a conductivity unit and Apex as with mine is less than trustworthy with accuracy. Oten, false readings are caused by temperature, micro-bubbles (apply probe at an angle to allow bubbles to escape and place in an area where the flow is somewhat stagnant) and from a recent seminar with neptune- You can get false readings from electrical interference as its conductive- MAKE SURE THE PROBE WIRES ARE NOT TOUCHING ELECTRICAL WIRES OF ANY SORT WHICH AFFECTS THE MEASUREMENT.

Correct. These kind of probes measure miliVolts, which is 1/1000s of a Volt. So any power cable or even other probe cables touching the wire can influence it. I also have a titanium heater just to make sure my sump is always connected to ground and there can no be any voltage in the water causing issues.

Temperature should not cause it if you use the TC value. And you can easily confirm TC if you use Apex by comparing temp probe and salinity probe on one graph. If they move in the same way, your TC is incorrect.

In my case it was bubbles so will take your advice and mount it (and all other probes since its the same holder) at an angle. Because I know there are no bubbles now, they can build up over time I assume? Maybe I should tap the probe every week just to make sure.

 

[Dennis Cartier]

Nice, I would already be very happy if I could use the salinity/conductivity probe as a safety measurement. So far it has always been a struggle to keep it rock stable. I do have ATO controlled by 3 optical sensors and a water level sensor so salinity should in fact be stable in my tank at the moment. If it was, it would be a perfect 3rd safeguard.

My experience is that EC probes are difficult to keep stable and accurate. In my case, I wanted to design an ATO that uses EC to deal with a situation that has caused me a couple of crashes in the past. I use dual head peristaltic pumps to do continuous water changes, but have found they always go out of balance, in not matching out, after critters and crud get sucked up and start to impact the flowrate overtime. After having my tank parameters thrown out of whack twice in past (leading to crashes) I decided to do things different.

So I added 3 Atlas Scientific EC probes and 2 temperature probes as the basis of my ATO. My plan was for the ATO to self correct any salinity impacts from mis-matched flowrates. So as long as 2 of the 3 probes were within tolerances I would consider it valid.

That worked OK, but I still had issues with the probes going out of calibration fairly often. As mentioned I mounted them on an angle and in a manifold with a pump flowing water over them.

I then added 2 Thermo Scientific Orion benchtop meters with 4 cell probes. So a total of 5 EC probes operate my ATO. I also use 4 temperature probes (2 Atlas, 2 in the Thermo probes).

The Thermo probes are much better, and are not impacted as easily as the Atlas ones. It takes a pretty beefy amphipod to throw the readings off with them.

The upside of using an ATO based on EC is that it will auto correct salinity if I mess up on new saltwater mixing or if my AWC is out of balance (which it is currently).

 

[Bramzor]

My experience is that EC probes are difficult to keep stable and accurate. In my case, I wanted to design an ATO that uses EC to deal with a situation that has caused me a couple of crashes in the past. I use dual head peristaltic pumps to do continuous water changes, but have found they always go out of balance, in not matching out, after critters and crud get sucked up and start to impact the flowrate overtime. After having my tank parameters thrown out of whack twice in past (leading to crashes) I decided to do things different.

I also used AWC before and will probably do it again on this tank. The way that worked for me was using a single motor with 2 peristaltic pumps on the same shaft. Even with that configuration, I still noticed a small difference in water that goes in and comes out during the AWC. I was able to correct this with an RO valve which got it close enough so it would be almost identical.

I also have a very stable waterlevel in my sump together with 3 infrared waterlevel sensors (Low, Normal, High) which I use for ATO and to make sure the AWC will keeps within a safeguard of I think 0.25 gallons. Low is 0.10gallons under Normal etc. I now also have the new Apex waterlevel sensor that I can also use as a safeguard or maybe later for the AWC. So during the AWC my waterlevel should never change. That waterlevel sensor is also very accurate and stable during my testing. Currently I'm not using it to control my ATO but probably will do so in the future instead of the infrared sensor with the other one as a safeguard.

 

[Randy Holmes-Farley]

It’s fair to mention, and help with expectations, that this is a conductivity probe. It doesn’t measure salinity. It’s natural for the reading to fluctuate up and down based on variables in the water. Temperature will have the greatest impact as the density of the water changes as temp fluctuates. I agree it is a great tool to help identify when things aren’t as they should be, like ATO running/not running. And it may not always, or ever, read at 35ppt. I use mine as a tool to know if there are sudden changes, but mine reads consistently at 43.7 and fluctuates 0.5 up or down at any given time. I’ve calibrated mine too many times to count and the reading never changes anywhere near 35 afterwards.

I want to clarify a few things.

1. There is NO method that reefers use that is any closer to directly measuring salinity than is doing it by conductivity, so it is somewhat misleading to say that "it doesn't measure salinity" as some sort of comment about its utility.

2. Changes in the readings of conductivity meters are NOT because of changes in density. Changes in the readings of floating glass hydrometers are directly due to changes in density with temperature. Changes in conductivity with temperature are due to higher mobility (conductivity) of individual ions as the temperature increases.

3. Nearly all conductivity meters that reefers use automatically correct for temperature changes. In those circumstances where that correction is adjustable, reefers willw ant to be sure it is turned on and is set at an appropriate value. In most meters, such choices are not available and reefers generally need not worry about temperature.

4. Your comment about it not reading 35 ppt or 53 mS/cm (the conductivity of 35 ppt seawater) seem odd and just are not true for any properly functioning conductivity meter. it sounds like you are either misclaibrating yours, or it is malfunctioning.

 

[V A R I A N T]

Thank you for adding clarity and correction to my comments.

 

[Bramzor]

Uch, salinity dropping again over time (a week), temperature is stable.

 

[Dennis Cartier]

Uch, salinity dropping again over time (a week), temperature is stable.

I would suggest picking up a Tropic Marin High Precision Hydrometer to provide a cross reference when you get these sorts of suspect results. The only requirement is for your water to be at 77F/25C, which it looks like you are using, and a tall cylinder to measure in. TM sells a plastic cylinder.

That way you can easily determine if the EC probe is right or just being wonky. Also handy for tracking down out of balance issues with your AWC.

 

[Bramzor]

Problem is not the salinity since the tank is empty and ATO is stable so as a result salinity should also be stable. Which it is because I confirmed it with a refractometer. Not sure what is causing the drop again. Might be bubbles again.

 

[Randy Holmes-Farley]

Problem is not the salinity since the tank is empty and ATO is stable so as a result salinity should also be stable. Which it is because I confirmed it with a refractometer. Not sure what is causing the drop again. Might be bubbles again.

Test error, if what you say is true.

Is a skimmer removing salt water?

Any other way salt water gets removed?

 

[Bramzor]

Test error, if what you say is true.

Is a skimmer removing salt water?

Any other way salt water gets removed?

Currently no skimmer in the tank and I'm not pulling out any saltwater.