Refractometers and ATC

[Reef.]

I’m am a little confused why my refractometers show different results dependent on how long the water sits on the prism?

I thought you had to wait a few minutes for the refractometer to adjust to the temp of the testing water, and this seems to be the case in practice as I get what seems to be the correct reading after waiting a few minutes but having just read the instructions from a couple of different manufacturers of refractometers, they say the ATC is not for that reason, it is to adjust to room temperature and that the water sample will adjust to the refractometer within seconds so no wait time is needed but as said if I don’t wait 3-4 minutes my reading is way off?

Anyone help me understand what is going on.

 

[Timfish]

As I read your post the initial reading is wrong but if you let it sit a few minutes then it's correct? And this is happening when you use calibration fluid? This doesn't make sense to me as I would expect dried salt left on the refractometer or evaporation around the perimeter of the cover to skew the numbers higher.

 

[exnisstech]

I've never thought about waiting to read mine. Drip on water flip closed and read.

 

[Reef.]

I've never thought about waiting to read mine. Drip on water flip closed and read.

Try it, just realised my salinity was 1.0285.

 

[Reef.]

As I read your post the initial reading is wrong but if you let it sit a few minutes then it's correct? And this is happening when you use calibration fluid? This doesn't make sense to me as I would expect dried salt left on the refractometer or evaporation around the perimeter of the cover to skew the numbers higher.

Talking about tank salt water.

No dried salt, we are only talking a couple of mins.

After a couple of minutes the refractometer matches the hydrometer, if I don’t wait a couple of minutes the reading on the refractometers both read low.

The refractometers are both well thought off, D-D and Vee Gee.

 

[JNalley]

Hey @Reef. something I forgot to mention (though I did sort of mention by way of saying to calibrate via mfctr instructions) is that calibration needs to be done at the calibration temperature of the refractometer. For most (not all) this temp is 20C (68F), so for instance, on a SeaWater refractometer, which calibrates with RO, you need to be in an ambient temperature of 68F, and allow the device itself to reach 68F (typically letting it sit out, not in its case, for 20-30 minutes will let it reach equilibrium with the ambient temp). Then, you drop the RO water onto the prism and close the lid/flap. Wait about 15 seconds or so for the water to reach equilibrium with the device, then calibrate to 0.

At this point, it's safe to use in any ambient temp environment that is not below 10C or above 30C, and it will always give you the SG and PPT of the Seawater you're measuring at 20C (ppt doesn't change with temp but SG does, this is why they're calibrated to room temp (20C) since it only takes a few seconds for a couple of drops of water to equalize with the air around it).

 

[Reef.]

Hey @Reef. something I forgot to mention (though I did sort of mention by way of saying to calibrate via mfctr instructions) is that calibration needs to be done at the calibration temperature of the refractometer. For most (not all) this temp is 20C (68F), so for instance, on a SeaWater refractometer, which calibrates with RO, you need to be in an ambient temperature of 68F, and allow the device itself to reach 68F (typically letting it sit out, not in its case, for 20-30 minutes will let it reach equilibrium with the ambient temp). Then, you drop the RO water onto the prism and close the lid/flap. Wait about 15 seconds or so for the water to reach equilibrium with the device, then calibrate to 0.

At this point, it's safe to use in any ambient temp environment that is not below 10C or above 30C, and it will always give you the SG and PPT of the Seawater you're measuring at 20C (ppt doesn't change with temp but SG does, this is why they're calibrated to room temp (20C) since it only takes a few seconds for a couple of drops of water to equalize with the air around it).

Thanks, yeah I did read that in the instructions but also remember reading Randy say he can’t see why calibration needs to be done at a set temp (hope I remember that correctly). Either way the room the refractometer is in is near enough 20c.

Going to try again now, see what results I get.

 

[JNalley]

Thanks, yeah I did read that in the instructions but also remember reading Randy say he can’t see why calibration needs to be don't at a set temp (hope I remember that correctly). Either way the room the refractometer is in is near enough 20c.

Going to try again now, see what results I get.

I, like Randy, question why too (because it doesn't make sense to my feeble little brain), but I figure labs creating them have their reasons for this, so I just perform like a good little soldier, lol. It doesn't take much time to get the house from 70 down to 68 once a month while I calibrate.

 

[exnisstech]

Try it, just realised my salinity was 1.0285.

I'm no help. I just checked mine. 1.025 and 4 minutes later 1.025. it's just a cheap generic atc from Amazon probably 7 or 8 years old. I calibrate with Randy's recipe and check with a TM hydrometer on occasion. Not that the last part matters for your issues.

EDIT. 15 minutes and still 1.025

 

[ReefHunter006]

If I take calibration fluid and set the refractometer to 35ppt, why does the temp matter? The sample when I test it will quickly adjust to the devices temp which is what the 35ppt was set at.

 

[capted]

I calibrate or check calibration without waiting more than 1 minute. When I check my tank water I do the same (under 1 minute). I figure if it is calibrated to 35ppt within that time and the water is tested the same way it shows the correct reading. I use a Vee Gee also. I will let it sit for longer when I test again to see if there is a difference in the readings just to double check though.

 

[Reef.]

Ok this is my excuse and I’m sticking to it…think this is what happened, first of all I’m stupid as I went against my own advice in using a bought reference solution to check my salinity, my excuse for that was I bought it to check other parameters so thought as it also had a reference for salinity why not use it, turns out it was reading around 3ppt high, add in a cheaper (D-D) and a decent refractometer (Vee Gee) and throw in the hydrometer I think things got a little confusing.

Sticking to home made reference from now on, the TM Hydrometer and the Vee Gee….and wait time bar a few seconds doesn’t affect reading today, last night I promise it did!!!

 

[Reef.]

Just thought of something, I bet the light played a part as I was using artificial light last night and day light today!

 

[JNalley]

If I take calibration fluid and set the refractometer to 35ppt, why does the temp matter? The sample when I test it will quickly adjust to the devices temp which is what the 35ppt was set at.

Well, temperature matters overall when you understand what's going on, especially in an ATC Refractometer where you have a bi-metal strip adjusting the prism due to temperature fluctuations. Plus, your calibration fluid is likely a brine fluid (table salt + water) and not an NSW (Natural Sea Water) fluid. Brine will read 1.5-1.7 ppt lower than NSW because the calcium, magnesium, and other ions in the water give the solution a different refractive index, and I think this increases as you move up and down in temperature, but don't quote me on that.

So you have 3 things to ponder over

Is my Refractometer meant for measuring brine or NSW?
Is my calibration/test solution brine or sea water?
What is the ppt variance between them at all given temperatures?

It's definitely not a straightforward topic.

Sticking to home made reference from now on, the TM Hydrometer and the Vee Gee….and wait time bar a few seconds doesn’t affect reading today, last night I promise it did!!!

Your VeeGee is a seawater refractometer, right? It calibrates with RODI? You can make a reference solution to test after calibration. Make your reference solution by adding 14 grams of your own salt mix to 386 grams of RODI water. This should make 400mL of testing solution. Keep it in an airtight dropper bottle. Then you have a known 35ppt solution for your refractometer you can always use to test accuracy.

 

[Reef.]

Well, temperature matters overall when you understand what's going on, especially in an ATC Refractometer where you have a bi-metal strip adjusting the prism due to temperature fluctuations. Plus, your calibration fluid is likely a brine fluid (table salt + water) and not an NSW (Natural Sea Water) fluid. Brine will read 1.5-1.7 ppt lower than NSW because the calcium, magnesium, and other ions in the water give the solution a different refractive index, and I think this increases as you move up and down in temperature, but don't quote me on that.

So you have 3 things to ponder over

Is my Refractometer meant for measuring brine or NSW?
Is my calibration/test solution brine or sea water?
What is the ppt variance between them at all given temperatures?

It's definitely not a straightforward topic.

Your VeeGee is a seawater refractometer, right? It calibrates with RODI? You can make a reference solution to test after calibration. Make your reference solution by adding 14 grams of your own salt mix to 386 grams of RODI water. This should make 400mL of testing solution. Keep it in an airtight dropper bottle. Then you have a known 35ppt solution for your refractometer you can always use to test accuracy.

Do you know why Randy recommends table salt? As opposed to marine salt?

 

[JNalley]

Do you know why Randy recommends table salt? As opposed to marine salt?

I think his solution (If I read his table right) compensates for the disparity between the two. He has instructions for mixing up 3.65% table salt which translates to 35ppt seawater, but in actuality it's 36.5ppt of table salt. I could be wrong though.

That's probably good enough for a home-standard, but ionic composition varies based on salt manufacturer and probably a little even bucket to bucket/scoop to scoop, so if I were trying to get the most accurate solution for my specific needs, I would mix it up in a ratio of 35grams of salt /1000grams of RODI. You can adjust from there to get smaller quantities like my recipe above makes 400mL of solution, which is a lot honestly. I'd bottle up some of it and toss the rest in the tank during my next water change. But the principal is the same.

 

[Reef.]

I think his solution (If I read his table right) compensates for the disparity between the two. He has instructions for mixing up 3.65% table salt which translates to 35ppt seawater, but in actuality it's 36.5ppt of table salt. I could be wrong though.

That's probably good enough for a home-standard, but ionic composition varies based on salt manufacturer and probably a little even bucket to bucket/scoop to scoop, so if I were trying to get the most accurate solution for my specific needs, I would mix it up in a ratio of 35grams of salt /1000grams of RODI. You can adjust from there to get smaller quantities like my recipe above makes 400mL of solution, which is a lot honestly. I'd bottle up some of it and toss the rest in the tank during my next water change. But the principal is the same.

So that recipe should also work for checking my TM hydrometer?

 

[JNalley]

So that recipe should also work for checking my TM hydrometer?

yes, as long as you accurately measure the temperature of the solution it's in (while floating/time of capture) and then factor the temperature against TMs chart. A simple digital thermometer should be accurate enough.

 

[Reef.]

yes, as long as you accurately measure the temperature of the solution it's in (while floating/time of capture) and then factor the temperature against TMs chart. A simple digital thermometer should be accurate enough.

Great thanks, couldn’t get my head around why Randy had different recipes for each device.

 

[ReefHunter006]

Well, temperature matters overall when you understand what's going on, especially in an ATC Refractometer where you have a bi-metal strip adjusting the prism due to temperature fluctuations. Plus, your calibration fluid is likely a brine fluid (table salt + water) and not an NSW (Natural Sea Water) fluid. Brine will read 1.5-1.7 ppt lower than NSW because the calcium, magnesium, and other ions in the water give the solution a different refractive index, and I think this increases as you move up and down in temperature, but don't quote me on that.

So you have 3 things to ponder over

Is my Refractometer meant for measuring brine or NSW?
Is my calibration/test solution brine or sea water?
What is the ppt variance between them at all given temperatures?

It's definitely not a straightforward topic.

Your VeeGee is a seawater refractometer, right? It calibrates with RODI? You can make a reference solution to test after calibration. Make your reference solution by adding 14 grams of your own salt mix to 386 grams of RODI water. This should make 400mL of testing solution. Keep it in an airtight dropper bottle. Then you have a known 35ppt solution for your refractometer you can always use to test accuracy.

I agree there is complexity, but I still don’t understand why not minimize the temp variable.

If I use Randy’s calibration (which I am). Walk into my fish room and calibrate the (saltwater) refractometer to 35ppt. Then test my tanks sample, temps are for our purposes the same and if the tank reads 32ppt then I know I need to work to raise it.

Am I missing some element here? I am not an expert on the subject.