Why do my Salinity measurements vary by 0.002 sg between refractometer and conductivity testers?

[Tim Olson]

Did you just average them all? My problem with this is if you take 5 wrong readings the average is still wrong.

I trust the floating glass hydrometer more than the others. Fwiw my salinity was 28ppt with icp. I used marin labs. I asked how they measured it, with a refractometer. Really? I of course didnt believe it was 28ppt when 3 devices said 1.026 ish

I agree with you about averaging wrong readings. Especially since I just re-tested and the conductivity testers are low again, compared to the refractometer. So now the range of readings for the tank water is 1.0236 (Hanna) to 1.0268 (Hydrometer).

Also, I hope ATI's test is more accurate than Marin Labs.

 

[Hedgedrew]

Guys. Its the calibration fluid !! For whatever reason randys tends to read 1 ppt lower than acurasea or brightwell. And pinpoints solution in middle !! Ro di not great to use.

 

[Hedgedrew]

In fact Randys reads almost 2 ppt lower than brightwell or Tlf

 

[Cory]

In fact Randys reads almost 2 ppt lower than brightwell or Tlf

I put much more faith in randys diy standard than a huge batch made by a factory and distributed into little vials.

 

[Hedgedrew]

I put much more faith in randys diy standard than a huge batch made by a factory and distributed into little vials.

Ive spoken to julian sprung on his product and hes equally convinced his is right !!

 

[Cory]

Ive spoken to julian sprung on his product and hes equally convinced his is right !!

Having a belief is one thing but being an actual doctor in chemistry is something else. Id tend to have faith in Randys due to his credentials. In the end 2ppt isnt a huge difference.

 

[Hedgedrew]

Having a belief is one thing but being an actual doctor in chemistry is something else. Id tend to have faith in Randys due to his credentials. In the end 2ppt isnt a huge difference.

Its just amazing that they all read differently.

 

[Hedgedrew]

For me now in hypo that 2 ppt swing is difference between effective and waste of time. In that instance we calibrate to center pin point as opposed to others.

 

[Tim Olson]

@Hedgedrew , @Cory ... Thanks. Just to clarify, here's the specific calibration fluids I used for each device.

- Pinpoint Salinity Monitor - calibrated with Randy's Conductivity Standard
- Hanna Salinity Tester HI98319 - calibrated with Hanna's 35.00 Salinity Calibration Solution (HI70024)
- Vee Gee STX-3 refractometer - calibrated with Brightwell Refractometer Calibration Standard
- ATC Refractometer - Brightwell Refractometer Calibration Standard
- LaMotte NIST Hydrometer- not calibrated, but reading was adjusted for temperature

What I'm going to do next is make Randy's Refractometer Standard and see how it matches up to Brightwell's and probably an Hydrometer standard as well.

 

[GK3]

I have 3 devices I use to measure salinity, a refractometer, a Hanna HI98319 salinity tester and a Pinpoint Salinity Monitor. I have acquired these 3 devices in my quest to find the most accurate salinity measurement.

My problem is that the Hanna and Pinpoint testers measure 0.02 sg less than my refractometer. For example, the Hanna/Pinpoint will measure 1.025 and the refractometer measures 1.027 sg.

All three devices have been calibrated multiple times, over the years, with their respective manufacturer calibration fluids. So I'm baffled as to why they're so far off from each other.

Any insights would be greatly appreciated.

refractometer is a direct measurement based on - you guessed it the refraction of light. The one issue here is anything in you water that changes the density will show in the refractometer. We assume that our aquarium water density is changed by the salt only so when we read it, we correlate it to how much salt is in the water.

conductivity measurement is not a direct measurement of salt. These meters are measuring the free ions-in the water and ultimately how conductive it is. We then take a conductivity number and convert it to SG. One thing that causes swings in conductivity measurements is temp. Take a look at this site which does the conversion:

Salinity calculator

A unit converter for calculating salinity in marine aquariums. The calculator supports conductivity (mS/cm), salinity (PSU og ppt), density (g/cm³) og specific gravity.

reefapp.net

You have to put in the water temp to get the conversion. Even a small temp difference will change your SG for the same conductivity.

hydrometers are historically inaccurate and I would trust the other two before I trusted one of these.

IMO - conductivity is much quicker to measure but has some variation. I only use this to make sure nothing has really changed. Refractometer is the way to go if you want a reading to 3 decimal places or if you are mixing new water.

 

[Hedgedrew]

refractometer is a direct measurement based on - you guessed it the refraction of light. The one issue here is anything in you water that changes the density will show in the refractometer. We assume that our aquarium water density is changed by the salt only so when we read it, we correlate it to how much salt is in the water.

conductivity measurement is not a direct measurement of salt. These meters are measuring the free ions-in the water and ultimately how conductive it is. We then take a conductivity number and convert it to SG. One thing that causes swings in conductivity measurements is temp. Take a look at this site which does the conversion:

Salinity calculator

A unit converter for calculating salinity in marine aquariums. The calculator supports conductivity (mS/cm), salinity (PSU og ppt), density (g/cm³) og specific gravity.

reefapp.net

You have to put in the water temp to get the conversion. Even a small temp difference will change your SG for the same conductivity.

hydrometers are historically inaccurate and I would trust the other two before I trusted one of these.

IMO - conductivity is much quicker to measure but has some variation. I only use this to make sure nothing has really changed. Refractometer is the way to go if you want a reading to 3 decimal places or if you are mixing new water.

Still i think you will be surprised at the variance. Lets see results.

 

[Hedgedrew]

I bet randys is near 2 ppt lower than brightwell. !!

 

[Tim Olson]

OK, here's the results from today's tests, done to the best of my ability:
- Used accurate scales to measure RODI water and Morton's salt.
- The temperature of all the standards and samples were normalized, by sitting on the table for a few hours.
- The Pinpoint Salinity Monitor has temperature compensation built in, plus I used their table to convert from ms/cm to specific gravity. I've used the reefapp.net in the past to convert the Pinpoint's ms/cm to Specific Gravity, but over time found the results to vary significantly from Pinpoint's conversion table. I think it was because it ended up compensating for temperature twice.

• Comparisons to Randy's standards:
- The Hydrometer read 1.0275 sg against Randy's hydrometer 1.0264 sg standard. Therefore, I should subtract (1.0275-1.02654) 0.0011 sg from the reading on the Hydrometer.
- The Vee Gee Refractometer read 1.0305 against Randy's refractometer standard, so it reads 0.0041 higher than the Brightwell Refractometer Standard of 1.0264.
- The ATC Refractometer read 1.0310 against Randy's refractometer standard, so it reads 0.0046 higher than the Brightwell Refractometer Standard of 1.0264.
* BTW, I didn't have a commercial conductivity standard to compare to. So the Pinpoint was calibrated to Randy's Conductivity Standard. I also decided to use Hanna's calibration fluid, since it's supposedly meant for the Hanna tester and it hopefully is be a good proxy for a commercial calibration standard,

•Revised tank water measurements:
- Pinpoint Salinity Monitor - 1.0228 or 47.5 ms/cm (calibrated using Randy's Conductivity Standard and Pinpoint's conversion table)
- Hanna Salinity Tester - 1.0236 or 31.3 ppt (calibrated with Hanna's standard)
- Vee Gee Refractometer - 1.0209 (read 1.0250 sg, so adjusted down 0.0041 too Randy's standard, compared to Brightwell
- ATC Refractometer - 1.0204 (read 1.0250 sg, so adjusted down 0.0046 too Randy's standard, compared to Brightwell)
- LaMotte Hydrometer - 1.0234 (calibrated with Randy's Specific Gravity Standard)

So once again, I'm frustrated. I want to keep my tank at 1.0250, but I still really don't know what device to use. I'm leaning toward using my Vee Gee refractometer, but with Randy's standards reading 0.0041 sg higher than Brightwell's standard, I don't know what to use.

So I may have raised more questions than answers today and the Randy vs. Brightwell debate continues.

 

[Hedgedrew]

Your reading randys higher than brightwell ??? Thats not what we got

 

[Hedgedrew]

Get yourself tlf accurasea and compare to pinpoint and brightwell. They are all close. By 1

 

[Tim Olson]

Your reading randys higher than brightwell ??? Thats not what we got

Me too! I was very careful mixing the standards, but obviously there is still room for error. Although, the difference is so high, it seems like it's in the right direction.

 

[Tim Olson]

Get yourself tlf accurasea and compare to pinpoint and brightwell. They are all close. By 1

That's a great idea. I haven't heard of that. I'll try to order some today.

Also, today I used the same tank water as samples for the ATI test I'm sending in. We'll see how that compares too.

 

[Cory]

Do you use instant ocean salt?

You can try this so you dont get too crazy:

Approximately 154.5 grams per gallon for 1.025 specific gravity. Results will vary based on moisture content but it should be pretty close. Mix the salt in a 5 gallon bucket.

 

[Tim Olson]

Thanks for the suggestion ... I use Tropic Marin Pro-Reef salt, so I'll make up a batch today with their instructions. Also, I did order a bottle of the TLF AccuraSea yesterday, which should be here on Wednesday. Plus, I'm going to mail the ATI ICP samples this morning. So that should give 3 more data points.

 

[Rybren]

I have a Hanna HI98319 and a no-name refrac.

I use pinpoint calibration fluid for the refrac and the Hanna calibration fluid for the digital meter.

After calibrating both devices, I measured the salinity of TLF AccuraSea. The hanna read 34.9ppt and the refrac was 35 +/- 0.1 (my eyes aren't good enough to see whether the line was on, just above , or just below the 35ppt line)

I also used the refrac to check a sample of Fauna Marin Multi-Reference solution (supposed to be 33.3 ppt) Again, it was +/- 0.1 of the ref solution. I didn't use the Hanna on this solution as the bottle is only 100ml.