How accurate are refractometers?,

[chipmunkofdoom2]

Oh interesting.

I commonly get fish in around 1.015 and raise about 0.001 every few days through evap and concentrated salt water top off. I check the salinity daily!

Im really stuck between these two. How accurate have you found the pinpoint? How quickly does it temp stabilize?

The accuracy is great. I haven't needed to touch the calibration once in the 3 years I've owned it.

Temp stabilization depends. It could take up to 5 minutes or so, but it depends on the temperature difference between the probe and the sample. My tanks are in my basement and the probe is relatively cold (68* or so). When I put the probe in the tank, it takes a few minute to stabilize. I usually drop the probe in the tank, turn on the meter, then clean my skimmer or watch the fish/corals for a minute or two. It's usually stable by then.

 

[richarddeweerd]

I recently purchased a veegee refractometer for improved accuracy, it has my head spinning. I have been in contact with support to get a better understanding maybe someone here can help.

The VeeGee Auto Temp Comp. (ATC) is set to 20C. Almost all (if not all) calibration fluids are standardized at 35ppt @25C. That means using a calibration fluid will inaccurately calibrate the Veegee. Make sense, so apparetnly you are supposed to use distilled or RODI water to calibrate this. At 20C pure water has a RI of 1.333 or sg of 1.000. This instrument is accurate enough to calibrate at 1.000 and be accurate across the board. Cheaper refracts cannot say this and that is why people say to use calibration fluid.

Ok so I have the VeeGee standardized at 20C, now when I read my aquarium water it is ATC to 20C so the reading is not the salinity at roughly 25C but at 20C. Is there a way to extrapolate to 25C? Is the difference that noticeable? The temperature variable for reading the refractive index of solutions is confusing me greatly.

Like @chipmunkofdoom2 posted above, I am tempted to switch to using a salinity montor like pinpoint that still relies on temperature but compensates. I do not like the idea of contaminating the probe when testing QT tanks which is where I test salinity the most. Ugh

Any thought on this?

The test water will get the same temperature as your refractometer. If you use only a few drops of water, the temperature of the water film on your refractometer will equalize with the meter it self. So water temperature is not important, the temperature of your refractometer that is what is most important. And that is also how ATC works, it is a bi-metal that is bending depending on the temperature of the refractometer and therefor compensating for its temperature.

This is also the main difference between PPM and specific gravity. the SG will get lower when the temperature rises, ppt will stay the same.

 

[minus9]

I have the D&D refractometer and I like it. I never used RO for calibrating though, just calibration fluid. If I do have to recalibrate it, it's barely out and never a full point. I use aqua craft solution btw, Brightwells is probably the worse solution that I've tried. The fact that you have to shake it seems very wrong to me and it was way out compared to my standard solution. That being said, I wish I would've kept my glass hydrometer from the 80's. I'll probably buy the Tropic Marin one soon, then there's no guessing.

 

[piranhaman00]

The test water will get the same temperature as your refractometer. If you use only a few drops of water, the temperature of the water film on your refractometer will equalize with the meter it self. So water temperature is not important, the temperature of your refractometer that is what is most important. And that is also how ATC works, it is a bi-metal that is bending depending on the temperature of the refractometer and therefor compensating for its temperature.

This is also the main difference between PPM and specific gravity. the SG will get lower when the temperature rises, ppt will stay the same.

Yes so if the instrument ATC to 20C then the reading I am getting will not be what the sg is at 25C correct? The instrument is not correcting to what my sample actually is.

 

[Reef.]

Yes so if the instrument ATC to 20C then the reading I am getting will not be what the sg is at 25C correct? The instrument is not correcting to what my sample actually is.

I believe the only time the refractometer and fluid, has to be at 20c is when you are actually calibrating it, not testing the calibration, if you need to change the setting then you need the refractometer and fluid at 20c...I think that’s it?

 

[piranhaman00]

I believe the only time the refractometer and fluid, has to be at 20c is when you are actually calibrating it, not testing the calibration, if you need to change the setting then you need the refractometer and fluid at 20c...I think that’s it?

True. But all calibration fluid sold for reefers is 35ppt @25C. So if you calibrate with this solution at 20C you are not calibrating correctly as that fluid is not 35ppt at 25C.

 

[Reef.]

True. But all calibration fluid sold for reefers is 35ppt @25C. So if you calibrate with this solution at 20C you are not calibrating correctly as that fluid is not 35ppt at 25C.

That’s what I think too but you can use it to check the calibration, only if it needs adjusting is there an issue.

 

[Reef.]

This is also the main difference between PPM and specific gravity. the SG will get lower when the temperature rises, ppt will stay the same.

if ppt doesn’t change with temp, wouldn’t that mean your calibration fluid would still, read correctly at 20c?

 

[ReefLab]

if ppt doesn’t change with temp, wouldn’t that mean your calibration fluid would still, read correctly at 20c?

No because you’re not directly reading ppt with a refractometer. You’re reading the refractive index, or how light bends through the water. That property is dependent on temperature

 

[piranhaman00]

No because you’re not directly reading ppt with a refractometer. You’re reading the refractive index, or how light bends through the water. That property is dependent on temperature

Ok I am getting there. Help me, my brain hurts.

I calibrate my refract with 20C distilled water, sg 1.000 (RI=1.3330). I then take a sample from the aquarium (25C) and read on refract at room temp (20C) (In any case the ATC with correct to 20C). Is this reading accurate? The instrument reads the RI at 20C and displays the ppt. But my aquarium is not at 20C? The RI at 25C would be different than at 20C, how does this affect the ppt result?


Does that make sense?

 

[ReefLab]

Ok I am getting there. Help me, my brain hurts.

I calibrate my refract with 20C distilled water, sg 1.000 (RI=1.3330). I then take a sample from the aquarium (25C) and read on refract at room temp (20C) (In any case the ATC with correct to 20C). Is this reading accurate? The instrument reads the RI at 20C and displays the ppt. But my aquarium is not at 20C? The RI at 25C would be different than at 20C, how does this affect the ppt result?


Does that make sense?

Well when you put a drop of water (half a mL) on the refractometer, it will quickly become equal to the temp of the refractometer itself. So it should be accurate enough.

personally I’m thinking of switching to using conductivity to measure salinity but also concerned about cross contamination as others have mentioned.

 

[VR28man]

I've was frustrated enough by my RS refractometer's tiny numbers and lack of accuracy (i.e. repeatable results; i got all kinds of different numbers using the same procedure) that I dumped it for a swing arm hydrometer, which at least has proved pretty repeatable.

I moved to an American Marine Pinpoint Salinity monitor. This monitor and the KP Aquatics aquaculture rock I ordered were the two best ways I've spent money in this hobby yet.

How well does this work? The only salinity probe I've read reviews of are neptune's apex salinity probes, which seem to have lots of problems.

 

[ca1ore]

yeah I check calibration every use, it’s a d-d refractometer I believe it’s designed to be calibrated with rodi water, would it be useful to buy 35ppt calibration fluid?

D-D refractometer is a good quality unit .... not that they actually make it .... just rebadged. Keeps calibration pretty well. Frankly, I rarely calibrate mine unless I get a wonky reading.

 

[Reef.]

D-D refractometer is a good quality unit .... not that they actually make it .... just rebadged. Keeps calibration pretty well. Frankly, I rarely calibrate mine unless I get a wonky reading.

yeah I find it very stable never needed to calibrate it yet, I’ve seen Milwaukee salinity checker mentioned a few times, looks a good bit of kit, finding it hard to justify one though as the D-D refractometer is working fine.

 

[minus9]

D-D refractometer is a good quality unit .... not that they actually make it .... just rebadged. Keeps calibration pretty well. Frankly, I rarely calibrate mine unless I get a wonky reading.

This has been my experience with the DD refractometer. In the five years that I've owned mine, I've only had to adjust it a handful of times and it was only out a fraction of a point, so nothing serious or that would be of concern.

 

[chipmunkofdoom2]

How well does this work? The only salinity probe I've read reviews of are neptune's apex salinity probes, which seem to have lots of problems.
[/QUOTE

They're both conductivity meters, so they both can have some of the same problems. Air bubbles and equipment interference can cause unstable readings. I think this is more of an issue with the Neptune probe since it's in your tank 24/7. If you're using a conductivity meter to spot-test salinity, it's easier to work around these problems. In my display tank the reading jumps all over, so I just put some water in a cup, drop the probe in, and give it a minute to stabilize. Works great.

I don't have any of these issues with my saltwater mixing station and my QT tank. I can drop the probe right in and get stable readings directly from the tank. This is especially handy when adjusting salinity. When I'm making saltwater, I usually add more salt than I need to. Then, I just adjust the salinity down by adding freshwater. I drop the probe in and add a little fresh water. If the salinity's still too high, I pump in a bit more until it reads where I want. Same with QT when I get a new fish. I drop the probe in the bag, get a salinity reading, then move the probe to the tank. I add freshwater with real-time feedback until the tank matches the bag. Then the fish goes right in.

Many hobbyists seem to be concerned with conductivity meters needing time to temperature compensate or being affected by interference from pumps and other equipment. I've never really understood this. It takes 2 minutes at most for the probe to temp compensate and it's easy enough to scoop up some water in a cup if there's interference. I know that wasn't really your question. I just really think that a lot of hobbyists miss out on a great way to test salinity because of some characteristics of conductivity probes that don't really end up being a problem in the real world.

 

[Daniel92481]

A refractometer is a very useful tool, however, it is meant to be a museum piece, or used as strictly a backup.;Pompus Get a Hanna digital salinity/temp checker and never look back. You’ll be happy you did.

 

[piranhaman00]

They're both conductivity meters, so they both can have some of the same problems. Air bubbles and equipment interference can cause unstable readings. I think this is more of an issue with the Neptune probe since it's in your tank 24/7. If you're using a conductivity meter to spot-test salinity, it's easier to work around these problems. In my display tank the reading jumps all over, so I just put some water in a cup, drop the probe in, and give it a minute to stabilize. Works great.

I don't have any of these issues with my saltwater mixing station and my QT tank. I can drop the probe right in and get stable readings directly from the tank. This is especially handy when adjusting salinity. When I'm making saltwater, I usually add more salt than I need to. Then, I just adjust the salinity down by adding freshwater. I drop the probe in and add a little fresh water. If the salinity's still too high, I pump in a bit more until it reads where I want. Same with QT when I get a new fish. I drop the probe in the bag, get a salinity reading, then move the probe to the tank. I add freshwater with real-time feedback until the tank matches the bag. Then the fish goes right in.

Many hobbyists seem to be concerned with conductivity meters needing time to temperature compensate or being affected by interference from pumps and other equipment. I've never really understood this. It takes 2 minutes at most for the probe to temp compensate and it's easy enough to scoop up some water in a cup if there's interference. I know that wasn't really your question. I just really think that a lot of hobbyists miss out on a great way to test salinity because of some characteristics of conductivity probes that don't really end up being a problem in the real world.

I thought pulling water from the tanks and putting into cups, does the water cooling to room temp affect the reading? Salinity does not change with temp but refractive index does, what about conductivity?

 

[chipmunkofdoom2]

I thought pulling water from the tanks and putting into cups, does the water cooling to room temp affect the reading? Salinity does not change with temp but refractive index does, what about conductivity?

Conductivity does change with temperature, but most conductivity meters, including the American Marine, compensate for temperature. In the case you mention, the water will cool slowly, and the probe will cool right along with it, compensating as it goes.

 

[piranhaman00]

Conductivity does change with temperature, but most conductivity meters, including the American Marine, compensate for temperature. In the case you mention, the water will cool slowly, and the probe will cool right along with it, compensating as it goes.

Interesting, ok thanks for your input. I may try both. I do like the VeeGee now that I got my head around refractive index and salinity haha!