(Almost) Everything You Need to Know About Apex Salinity Calibration and TC Factor

[Saltyanimals]

Great guide!
I followed it thoroughly with my new (and first) salinity probe, yet these are the results after a week
From 37 ppt down to 30.6. My real salinity is stable at 35.

Very disappointing, and not sure what to do..?

Since this is your "new and first" salinity probe, I would just give it time to settle down and not reaction or take action just yet. I would manually check the salinity i.e refract for now and give your probe several weeks to settle in.

It has been said that probes may develop basic biofilm which "breaks it in" so to speak. I've seen this with pH probes and lots of readings here where folks allow their new probes to sit in tank for weeks before trying to calibrate. I would assume it may apply here as well.

However on the flip side, my salt probe is 5-8 years old and still kicking. I noticed it will get an occasional finicky where I see a drastic drop in readings with no changes to the tank. This is clearly a probe issue and it seems to resolve itself in days. Sometimes I give it a few good taps to make sure I don't have air pockets somewhere on it that may affect the reading.

 

[keithIHS]

Like Saltyanimas, if I see mine drifting down, I give a couple of taps to make sure there aren't any bubbles inside (it's mounted pointing upward at a 45-80 degree angle). I also mounted it in a location that is away from the fuge light in an area that doesn't have many bubbles. If the taps don't do it, I'll give it a couple of quick dunks in and out of the water to dislodge anything that's inside followed by taps to dislodge bubbles. Every few months I'll carefully run a flat toothpick around inside to dislodge anything that might be inside, followed by dunks, followed by taps (while pointing upward).

 

[All_talk]

I don't seem to have an issue with bubbles. My probe is in the same section of my sump as my drain and skimmer, lots of bubbles large and small, but also lots of flow.

My readings really tuned in well after I calibrated it to the tank, no offset and less variation (graph 1). I recently removed some water while adding coral which the ATO backfilled, and then later I corrected the salinity which the probe tracked well (graph 3). It has agreed with my refractometer which I calibrate with a precision hydrometer. I pretty happy with it at this point.

 

[Saltyanimals]

I simply moved my probe holder about 10 inches over as part of my sump cleaning and it immediately reflected a bad reading. This is of course impossible give it was "correct" just a few minutes earlier. I gave it the usual taps to dislodge bubbles to no avail. I ran out of time so decided to fix it later. Several more taps and movements the next day and finally it's back to my expected range. Went from 34.x down to 30.x back up to 45.x + all over the last 2 days.
Funny graph: Notice the far left and far right. Right back to where I was on the rollercoaster.

Summary? Imagine if this was a new probe into an unknown tank salinity. I would be banging my head against a wall trying to understand why my new probe is wrong spending countless calibration bags trying to get it right. This is a good example of knowing where your salt really is before installing a new probe which may lead you to take unnecessary and/or lethal action to your tank.

 

[All_talk]

I simply moved my probe holder about 10 inches over as part of my sump cleaning and it immediately reflected a bad reading. This is of course impossible give it was "correct" just a few minutes earlier. I gave it the usual taps to dislodge bubbles to no avail. I ran out of time so decided to fix it later. Several more taps and movements the next day and finally it's back to my expected range. Went from 34.x down to 30.x back up to 45.x + all over the last 2 days.
Funny graph: Notice the far left and far right. Right back to where I was on the rollercoaster.

Summary? Imagine if this was a new probe into an unknown tank salinity. I would be banging my head against a wall trying to understand why my new probe is wrong spending countless calibration bags trying to get it right. This is a good example of knowing where your salt really is before installing a new probe which may lead you to take unnecessary and/or lethal action to your tank.

Agreed, I like seeing the graph in Fusion but if it goes screwy it doesn't mean take action, it just mean to check things out.

 

[DeputyDog95]

Not sure what's going on... My old probe was falling apart, so I just replaced it about a month or so ago. Can't get the darn thing to calibrate. It's usually pretty easy. Take it out, wipe it off, calibrate dry, stick it in the tank, calibrate wet, 35ppt at my typical tank temp of 79. I have the TC factor set to only point one so it barely does anything. Now I can't get it to read over 34.3, despite telling it that it's in a 35ppt solution (my tank water).

Any suggestions to force it to read 35ppt as the current baseline?

 

[All_talk]

Bumping this thread up because its a very good one, and for a little advice.

Since setting up temp correcting and calibrating to the tank water my salinity probe has been very stable and accurate. But recently I had a issue with one of my heaters which lead to more temp variation than normal and I noticed my salinity probe has a small opposite reaction to temperature, i.e. salinity reads lower when temps go higher (see included graph). I suspect this means my TC factor could be better tuned. Currently its at 1.9, does it need to be increased or decreased? I guess I can experiment a little and figure it out, just figured it would be worth asking to see if anybody had experience.

Certainly, the system is working well enough but I like to fine tune things if I can.

 

[Righteous]

Bumping this thread up because its a very good one, and for a little advice.

Since setting up temp correcting and calibrating to the tank water my salinity probe has been very stable and accurate. But recently I had a issue with one of my heaters which lead to more temp variation than normal and I noticed my salinity probe has a small opposite reaction to temperature, i.e. salinity reads lower when temps go higher (see included graph). I suspect this means my TC factor could be better tuned. Currently its at 1.9, does it need to be increased or decreased? I guess I can experiment a little and figure it out, just figured it would be worth asking to see if anybody had experience.

Certainly, the system is working well enough but I like to fine tune things if I can.

I believe the recommendation for TC is around 2.1-2.2. I have mine set at 2.2 and it works great.

By the way, no need to recalibrate the probe if you change TC, as long as it was calibrated with a TC above 0.

 

[DeputyDog95]

Bumping this thread up because its a very good one, and for a little advice.

Since setting up temp correcting and calibrating to the tank water my salinity probe has been very stable and accurate. But recently I had a issue with one of my heaters which lead to more temp variation than normal and I noticed my salinity probe has a small opposite reaction to temperature, i.e. salinity reads lower when temps go higher (see included graph). I suspect this means my TC factor could be better tuned. Currently its at 1.9, does it need to be increased or decreased? I guess I can experiment a little and figure it out, just figured it would be worth asking to see if anybody had experience.

Certainly, the system is working well enough but I like to fine tune things if I can.

This probably more info than you were asking for, but figured I would share anyway

If I remember correctly, the more you increase the TC, the more it decreases the salinity reading.

That being said, since I started calibrating to tank water using my Hanna conductivity meter as the reference point, and the Neptune salinity probe has been great. I had like 4 different salinity devices and the Hanna, despite being simple, inexpensive, and easy to use/calibrate, was always the closest to my ICP tests. After testing a couple of times, I stopped using all my other salinity devices (manual refractometer, Milwaukee digital refractometer, etc) and have stuck with the Hanna.

I am fastidious about maintenance on it though. I keep it stored in it's case, I rinse with RODI water every time and dry it before putting it back in the case, and I calibrate it pretty frequently (at least every 60 days). It's easy and takes about a minute.

Another bonus for using the Hanna... I mix my water change water to a specific lower salinity to account for salinity creep because I dose around 100ml a day of sodium based alkalinity and want to keep it at 35ppt. Otherwise, the salinity goes up quite a bit (min half a point) even after just a couple of months. I use one of the wizards online to tell me what to mix the salt to based on my current salinity reading, the volume of water in the system, and the amount of the water change water. From there, I add 2000g of salt to my mixing barrel, let it mix clear, measure it, and then slowly add the salt while measure it in real time as it goes up with the Hanna device floating in the water. Makes it super easy to make up custom salinity batches of water. It's basically a real-time probe in your mixing barrel.

For me, my tank temp variance is less than half a degree a day... So I have the TC turned to it's minimum. Which is .1 if I remember correctly? Basically, it's borderline turned off and I only have it on the min so I can play it with it a little without having to recalibrate. That being said, I find I almost never monkey with it anymore and if I start to get a little drift with the Neptune probe, usually sliding it up and down in the probe holder a few times will clear any debris or air and get it back to the correct reading.

See attached screenshot from this morning. My AC is being serviced this morning as it's not working as well as it should, or the tank never usually goes over 79. But the office is getting a hotter than usual now and the GHL cooling fans are struggling to keep up.

So if your tank temp rarely deviates, I would just leave the TC off, or close to off, as it gives wider readings with it on in my experience. Now if you're swinging a few degrees a day, that's probably a different story. If you get your heater straightened out and your temp settles to fairly tight margins, I would run it the way I am.

 

[All_talk]

Follow up...

My tank typically runs with less than 1 degree change, but in the summer it may heat up a little on a hot afternoon (no central AC). I would like the TC to be tuned so that salinity reads correct if the temp is outside normal.

I tried running the TC at 2.2 and its seemed worse, now its 1.7 and seems better. With my heater issue sorted, I have had to force some slight temp swings to test things, but its looking like slightly lower than the 1.9 I was running will tune it in.

 

[Bramzor]

Use Manual calibration and figure out what your dry and wet value is supposed to be. If you know that, you can watch it during next calibration and see what happens. My dry and wet values are 68 and 484.
So I know that when these are off during calibration, it’s probably bubbles or a bug in Apex. Noticed that in some occasions that you click next too quickly during manual calibration, dry value will read 0 or -5 and does not correct. If I go slow, it measures 68 consistently (after RO rinse or x hours dryout). If I do not rinse or wait it to dry out, it will drop from 72 to 68. This will only affect the lower portion of your salinity reading so if you want a bit higher resolution, it’s better to not rinse it or let it dry out. You do need to tap it a little bit. Everything above 80 it’s probably still reading a wet value, even if it’s out of the tank.

 

[SLVRADO]

I was trying to calibrate my new salinity probe a few weeks ago, and ran into all sorts of problems. Calibrations that were way off the mark, big fluctuations in readings that couldn't possibly be correct (and were verifiably not), etc. After searching around the internet, it was clear that a lot of folks have problems with this sensor. Some have written it off completely, and others insist that such problems are the result of calibration error. Information on correct calibration and configuration was present but scattered. Some of it was incomplete or incorrect. After having read many forum threads, spoken to Apex customer service, and experimented with different approaches, I am here to tell you that the sensor does work reliably when correctly configured and calibrated. This post is intended to capture everything I've learned in a single place, hopefully making this easier for someone else in the future.

Apex Configurables
There are only two configurable aspects of the Apex salinity sensor that can affect the reading. The first is, of course, the calibration point. The second is the temperature compensation factor (or TC factor). The salinity probe's readings are significantly affected by temperature - the TC factor setting allows you to compensate for this if you also have a temperature probe configured. The rest of this post captures the concepts and steps necessary to configure these inputs correctly.

TC Factor and Correctness
The TC factor configurable can be found on the advanced tab of the conductivity probe configuration page. It is a percentage, set manually - apparently different tanks land on different values. The percentage controls exactly how much compensation should be applied (i.e. x% compensation per degree).

One way to see if your TC factor is correct is by looking at your salinity graph with your temperature readings overlaid. If the TC factor needs to be adjusted, you will be able to see that the salinity readings track fluctuations in temperature. If the TC factor is correct(-ish), this correlation will not be present.

Here is what this looks like when TC is disabled (i.e. 0%):

Here is the same system with TC configured:

You can see from the first screenshot a clear correlation between temperature and salinity. Compare to the second screenshot with TC applied, and the salinity is relatively stable. I'm still not quite sure about those little bumps though. I continue to play around fine tuning the TC factor to see how they react (if at all - it's possible that the fluctuation is related to some funny business with my overflow).

Configuring the TC Factor
The TC factor defaults to 0. This is confusing for new Apex owners, as the salinity probe is all but worthless without applying temperature compensation. After speaking with customer service, and reading through some documentation, it appears that the default is 0 because setting it to anything other than zero causes problems in systems that don't have a temperature probe installed (the documentation includes a large WARNING section about this). Since Neptune cannot know or guarantee that any given system will have its temperature probe installed, the default is 0 to be safe.

If you have a temperature probe installed, you will definitely want to adjust the default TC factor. It is important to note that the temperature probe must be plugged into the same module as the salinity probe. In other words, if your salinity probe is plugged into your base unit, then you must also have a temperature probe plugged into the base unit. Similarly, if your salinity probe is plugged into a PM2 salinity module, then the temperature probe must be plugged into the same PM2.

TC Factor Value Range
It may take some experimentation to find the correct TC factor for your system. The Apex documentation states that seawater normally has a TC factor between 2.1% and 2.3%, thus recommending 2.2% as a good starting point. By comparison, a Neptune representative told me that in their experience, correct values are often found in the 1.9% to 2.1% range. Your mileage may vary.

The Apex documentation states that the salinity probe must be re-calibrated when changing the TC factor. According to a Neptune representative with whom I spoke, this is only partially true. When TC factor is non-zero and the salinity probe is configured, the current water temperature is stored inside the salinity calibration point that Apex records. This temperature is used to apply temperature compensation going forward. If the TC factor is zero when the salinity probe is configured, then the current water temperature is not stored, making future temperature compensation impossible. Therefor, in order for TC factor to take effect, it must be configured with a non-zero value at the time that the salinity probe is calibrated.

Fear not, however, in your search for the correct TC factor value. The Neptune representative informed me that once the salinity probe calibration point includes temperature data, the TC factor value can be changed at will without requiring re-calibration. Changes to the TC factor will be applied to all salinity readings going forward - readings that were taken in the past will not change.

How to Configure TC Factor for the First Time

1. Ensure that a temperature probe is plugged into the same module as your salinity probe
2. Navigate to the `Advanced` tab of the salinity probe configuration
3. Change the TC factor value to any non-zero positive number (2.1 is a good starting point)
4. Click "Update Apex" in the top right corner to upload the new configuration
5. Perform a salinity calibration
6. Salinity readings going forward will now have temperature compensation applied accordingly

Calibrating Salinity
There are a number of factors that require attention when calibrating the salinity probe. If any one of them are ignored, the resulting readings will be incorrect. Some of these are described in the documentation, and some are not.

We have learned that salinity readings are affected by temperature. As a result, temperature plays a very important part in the calibration process. When temperature compensation is disabled, and the Apex salinity calibration solution is used (the grey pouch, 53,000 microsiemens), the temperature during calibration is assumed to be 77.0 degrees fahrenheit. This is rarely actually the case, and folks calibrating their salinity probes who aren't aware of this detail will almost certainly experience inaccurate readings.

If however, you have a temperature probe, you do not need to warm the calibration solution to precisely 77.0 degrees. Apex will instead calibrate the salinity using the temperature reading provided by the probe. Note that this still means that the solution must be at precisely the same temperature as the probe! To accomplish this, you can simply float the calibration solution pouch in water near the temperature probe location for 15 minutes. Note that the calibration solution temperature can change rapidly - for instance, holding the pouch in your hand while performing the calibration will affect the accuracy. Be sure to keep the pouch in the water during the entire calibration process to ensure that the solution temperature doesn't deviate.

How to Calibrate Salinity

1. Ensure that your TC factor has already been configured with a non-zero value
2. Float the calibration solution pouch near your temperature probe for 15 minutes
3. Navigate to the `Basic` tab of the salinity probe configuration
4. Click `Automatic Calibration`
5. When asked to dry the salinity probe, remove it from the water and rinse with RO/DI. Then gently shake the probe, and pat the bottom dry with a paper towel
6. When asked to insert the salinity probe into the calibration solution, open the pouch and insert the probe. Be sure to keep the pouch submerged in water during this process so the temperature remains stable
7. Gently shake the probe in the calibration solution to remove any bubbles that might be trapped. This step is VERY important - shake for a little longer than you might otherwise
8. Wait for the readings to stabilize, and complete the calibration. You're all done!

Note that the manual calibration process might work here as well, though I've never done that and so have no idea what it does or doesn't involve

Finding More Information
I spent several days searching for information on this topic. This post is, as far as I know, the most complete source of information on Apex salinity calibration and temperature compensation currently available. As a new Apex owner, I was shocked to find that none of this information was included in any of the documentation I could find. When asking Neptune customer support about this, they referred me to the PM2 documentation. There is definitely more information there than the "regular" Apex docs, however it is still not complete and does not fully apply to the use of the base module for salinity readings.

If you find that any of the information in this post is inaccurate, or if you have information that is not included here but should be, please post a comment on this thread!!

Summary
I have found the Apex salinity sensor to be relatively accurate when correctly configured. Of particular importance is configuring temperature compensation, ensuring stable temperature conditions during calibration, and ensuring the correct order of operations. I hope that others who may be struggling with their Apex salinity sensor are able to use this thread to fix things up. Happy reefing!

Extremely helpful! Thank you!