DIY Salinity probe calibration solutions

[Bramzor]

Today I received a fancy Salinity probe meter that can measure salinity at 0.1 ppt resolution. https://nl.aliexpress.com/i/33042655191.html
Problem is it measures 24salinity while the tank is 33, so I have to calibrate it using a 7ppt solution and 40ppt solution or close to these values. These are hard to find so I was wondering if I couldn't make them myself using RODI water and Sodium Chloride or even NaCl?

Or can I create a 34ppt solution and just dilute it to get the 7ppt solution?

@Randy Holmes-Farley you figured out a way to DIY but was not able to figure out if I could use it to create a ref solution with a different salinity.

 

[Randy Holmes-Farley Verified Community Expert]

I fail to see why a 35 ppt DIY isn't a perfect way to check the meter.

 

[Bramzor]

I fail to see why a 35 ppt DIY isn't a perfect way to check the meter.

It does. But the goal is a calibration fluid.

 

[Rick Krejci]

Or just calibrate to your tank pretending it's 35 ppt and just know it'll be 2 off. I never could get a good calibration from calibration packets. I think the probe takes a long time to stabilize. Since I want my tank to be 35 ppt anyway, I get the tank to that level measured using a calibrated refractometer and then just calibrate the probe to the tank...works great.

 

[Randy Holmes-Farley Verified Community Expert]

It does. But the goal is a calibration fluid.

Are you saying the device absolutely requires specific concentrations for calibration? I’m still not understanding unless that is the case.
Regardless, you can make 35 ppt and see what it reads.

 

[Randy Holmes-Farley Verified Community Expert]

I can devise a different fluid if needed, I’m just not convinced it is needed.

 

[Difrano]

Today I received a fancy Salinity probe meter that can measure salinity at 0.1 ppt resolution. https://nl.aliexpress.com/i/33042655191.html
Problem is it measures 24salinity while the tank is 33, so I have to calibrate it using a 7ppt solution and 40ppt solution or close to these values. These are hard to find so I was wondering if I couldn't make them myself using RODI water and Sodium Chloride or even NaCl?

Or can I create a 34ppt solution and just dilute it to get the 7ppt solution?

@Randy Holmes-Farley you figured out a way to DIY but was not able to figure out if I could use it to create a ref solution with a different salinity.

I would ask my LFS to mix a couple of cups of salt one at 40 and one at 7 (pay them for it), they should have good salinity meters to be able to do it.

 

[Randy Holmes-Farley Verified Community Expert]

I would ask my LFS to mix a couple of cups of salt one at 40 and one at 7 (pay them for it), they should have good salinity meters to be able to do it.

why do you think such fluids are needed?

I would nut trust a second hand lfs standard.

 

[Bramzor]

Are you saying the device absolutely requires specific concentrations for calibration? I’m still not understanding unless that is the case.
Regardless, you can make 35 ppt and see what it reads.

Yes, the calibration of this device requires specific concentrations. In my case, a solution of 40 ppt and 7ppt. So I need to figure out a way to create those concentrations.

 

[Bramzor]

I would ask my LFS to mix a couple of cups of salt one at 40 and one at 7 (pay them for it), they should have good salinity meters to be able to do it.

That's a way to do it. Problem is that if the purpose is to calibrate my own device, it needs to be spot on. And I think it might be better to just measure a calculated amount of salt for a larger volume of water. This should allow me to create a reference of 7ppt and 40 ppt which could be used for the calibration. Problem is, I'm not sure about how to calculate the amount of salt needed. (And which salt would be the best to do this. I have pharmaceutical grade of NaCl which I could use.

 

[Difrano]

why do you think such fluids are needed?

I would nut trust a second hand lfs standard.

OP is requesting how to make those fluids I am just giving him an idea. Well it depends where you live and how much trust you have on your LFS, in my case is easy because TSA or WWC are 20 minutes away and I trust them.

 

[Randy Holmes-Farley Verified Community Expert]

For the record, I recommend people not buy this device.

Yes, the calibration of this device requires specific concentrations. In my case, a solution of 40 ppt and 7ppt. So I need to figure out a way to create those concentrations.

This sounds peculiar and problematic. Is that seawater ppt, sodium chloride ppt? Dioes the instruction manual specify what units it is measuring? Almost certainly it is not seawater ppt.

A conductivity meter should only be calibrated with 1 solution since they are pegged at zero: Conductivity is single point calibration. The 40 is likely used for the higher range and the 7 for the lower range. You only want the higher range. If you really need both we can make both, but I'd try with just the 40.

My recommendation to try to salvage a useful measurement from this device is to make a 40 ppt seawater equivalent standard using sodium chloride. Calibrate the 0-50 ppt scale with it, and use it to measure seawater.

My DIY standard for conductivity meters is here:

http://reefkeeping.com/issues/2004-06/rhf/index.htm

To make a 35 ppt seawater standard for conductivity, you make a 3.29 weight percent sodium chloride solution.

To make a 3.29 weight percent sodium chloride solution, dissolve 6.20 grams (1 teaspoon) of Morton's Iodized Salt in 182 mL (182 g) of fresh water (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.

Since you want a 40 ppt seawater equivalent, we need a different recipe. We want a conductivity of 59.7 mS/cm. I would make such a standard with 3.71 weight percent sodium chloride in water. That would come from dissolving the 6.2 grams of salt in 161 mL of water (161 grams).

 

[Randy Holmes-Farley Verified Community Expert]

If you are using your pharma grade NaCl, that's fine, but do not use the teaspoon measure. You need to weigh it, or find out the bulk density of that material from the supplier.

 

[Bramzor]

For the record, I recommend people not buy this device.

This sounds peculiar and problematic. Is that seawater ppt, sodium chloride ppt? Does the instruction manual specify what units it is measuring? Almost certainly it is not seawater ppt.

As it is from a Chinese manufacturer, manual is always too limited. It does not mention what units it's measuring but does it matter since you have to calibrate it anyway? In that case, it will always measure what you calibrate it for right?
It has 2 ranges, one for 0.00ppt - 9.99ppt and one for 10.0 ppt - 50ppt . Probably that is why it requires 3 calibration points in total, 0ppt, 7ppt and 40ppt.

A conductivity meter should only be calibrated with 1 solution since they are pegged at zero: Conductivity is single point calibration. The 40 is likely used for the higher range and the 7 for the lower range. You only want the higher range. If you really need both we can make both, but I'd try with just the 40.

I'm not sure if I can just ignore the 7ppt calibration. Because it is 1 method to do the calibration and it goes through 0ppt, 7ppt and 40ppt. So I guess I would need all 3.

My recommendation to try to salvage a useful measurement from this device is to make a 40 ppt seawater equivalent standard using sodium chloride. Calibrate the 0-50 ppt scale with it, and use it to measure seawater.

My DIY standard for conductivity meters is here:

http://reefkeeping.com/issues/2004-06/rhf/index.htm

To make a 35 ppt seawater standard for conductivity, you make a 3.29 weight percent sodium chloride solution.

To make a 3.29 weight percent sodium chloride solution, dissolve 6.20 grams (1 teaspoon) of Morton's Iodized Salt in 182 mL (182 g) of fresh water (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.

Since you want a 40 ppt seawater equivalent, we need a different recipe. We want a conductivity of 59.7 mS/cm. I would make such a standard with 3.71 weight percent sodium chloride in water. That would come from dissolving the 6.2 grams of salt in 161 mL of water (161 grams).

So that's 6.2 grams of NaCl salt right?
Do you have an idea about the amount that I need to make the 7ppt (12.245 mS/cm)?

 

[Randy Holmes-Farley Verified Community Expert]

Really stupid calibration. Beyond stupid. One point is the preferred method. Adding more is not better.

Id calibrate with air for zero (don’t even put It in water). I’ll come up with 7 ppt solution tomorrow.

 

[Randy Holmes-Farley Verified Community Expert]

As it is from a Chinese manufacturer, manual is always too limited. It does not mention what units it's measuring but does it matter since you have to calibrate it anyway? In that case, it will always measure what you calibrate it for right?
It has 2 ranges, one for 0.00ppt - 9.99ppt and one for 10.0 ppt - 50ppt . Probably that is why it requires 3 calibration points in total, 0ppt, 7ppt and 40ppt.



I'm not sure if I can just ignore the 7ppt calibration. Because it is 1 method to do the calibration and it goes through 0ppt, 7ppt and 40ppt. So I guess I would need all 3.



So that's 6.2 grams of NaCl salt right?
Do you have an idea about the amount that I need to make the 7ppt (12.245 mS/cm)?

yes 6.2 grams NaCl.

 

[Randy Holmes-Farley Verified Community Expert]

I don’t have time to verify it tonight, but a recipe using 7/35ths of the amount of NaCl in my recipe should be very close to 7 ppt seawater equivalent, or 0.2 times the NaCl amount in the same water volume as my 35 ppt standard.

 

[Bramzor]

Really stupid calibration. Beyond stupid. One point is the preferred method. Adding more is not better.

Id calibrate with air for zero (don’t even put It in water). I’ll come up with 7 ppt solution tomorrow.

Will try to ignore the 7ppt step in the calibration and see if that works. If that does not work I'll try to make the 7ppt by diluting the 40ppt one. Can I just go 1 part 40ppt and 4 parts of RODI to get to 8ppt?

 

[Bramzor]

Really stupid calibration. Beyond stupid. One point is the preferred method. Adding more is not better.

Id calibrate with air for zero (don’t even put It in water). I’ll come up with 7 ppt solution tomorrow.

Was able to do the calibration. It did measure the 40ppt and 8ppt stock solutions after calibration correctly so seemed that calibration was ok, but when I measured a reference fluid of 33ppt, it still showed 21.2 ppt, 2.12% and 1.014 for that 33ppt saltwater solution. So I assume it does not measure seawater ppt but some other salinity?

Found a "data sheet" of the product: http://www.tools.in.th/manual/Catalog-AR8012.pdf

 

[Bramzor]

I noticed that I was able to select a calibration point between 25ppt and 40ppt during calibration so I lowered it to 33ppt and calibrated towards a reference solution I had that was 33ppt saltwater. And using that calibration point, I was able to measure different salinities that actually matched my refractometer.
So it seems to work properly now. Although I'm a bit lost why it does not work using the 40ppt reference that I made while it does work for a saltwater reference?