Refractometer Calibration

[Aspect]

When calibrating a refractometer with 35 ppt solution, should the refractometer then ideally be at exactly the 0 line with RODI water or below the 0 line?

 

[KrisReef]

My understanding is to calibrate the handheld refractometer to 35ppt so that it is reading accurately at the target range. (Use a TDS meter for RO) The refractometer scale is too wide to be accurate across the range from RO to Marine water. If it's set to marine water 35ppt the RO will not be at the zero line, i would expect it to read higher, but whatever direction it will not likely be zero.

 

[Aspect]

My understanding is to calibrate the handheld refractometer to 35ppt so that it is reading accurately at the target range. (Use a TDS meter for RO) The refractometer scale is too wide to be accurate across the range from RO to Marine water. If it's set to marine water 35ppt the RO will not be at the zero line, i would expect it to read higher, but whatever direction it will not likely be zero.

It reads below 0 for me

 

[penfold2]

I agree with everything the previous poster said. Expect a refractometer to be accurate only at or near the value it's calibrated at. If you want to measure salinity near 0ppt, calibrate with distilled or RODI, measure, then recalibrate with 35ppt sample for future readings near seawater level.

 

[exnisstech]

I calibrate using Randy's calibration solution or saltwater of a known salinty. I've only heard if a couple refractometers that recomend using RO water calibrate.

 

[SixlineRyan]

What ever you do, it's worth it to make extra sure you've got it right. I borked mine and ended up with salinity that was waaay to high.

 

[KrisReef]

It reads below 0 for me

Yeah, light refraction measurements use physics principals and light bending between two mediums to calculate the results. I am too lazy to look up the physics and the results, I just remember that calibration for hobby grade equipment is best done at the target level.

Side bar: I just purchased a precision glass floating hydrometer that is miles away from being accurate. I have a plastic swingarm that appears to be more accurate. Getting a good tool that repeats measurement at the target level is what we need for the hobby. I bought the expensive floating hydrometer to see how accurate my hydrometer is and was shocked how poorly the hydrometer is.

I would recommend that you rinse with RO and dry a refractometer after you use it and put it away to reduce rusting of the metal parts.

 

[Aspect]

Yeah, light refraction measurements use physics principals and light bending between two mediums to calculate the results. I am too lazy to look up the physics and the results, I just remember that calibration for hobby grade equipment is best done at the target level.

Side bar: I just purchased a precision glass floating hydrometer that is miles away from being accurate. I have a plastic swingarm that appears to be more accurate. Getting a good tool that repeats measurement at the target level is what we need for the hobby. I bought the expensive floating hydrometer to see how accurate my hydrometer is and was shocked how poorly the hydrometer is.

I would recommend that you rinse with RO and dry a refractometer after you use it and put it away to reduce rusting of the metal parts.

Yeah I am never really sure what my actual salinity is, I have 3 pre calibrated solutions that all give me different readings. I am most confident about the brightwell solution which I just try to keep my salinity always stable referring to the calibration with that solution. I just wanted to see if it would measure accurately at rodi levels which looks like it doesn't. I'm waiting back on an icp to see my true salinity level.

 

[KrisReef]

In case you have not seen this?
https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
Randy provides instruction on how to use table salt to make calibration solutions for different refractometer, hydrometer, electronic meters, in this article. Very helpful to know that table salt mixtures of different concentrations are used to calibrate different meters. Turns out "calibration solutions" are not universally useful for all instruments that need to be calibrated. HTH.

 

[NanoSteam]

Yeah, light refraction measurements use physics principals and light bending between two mediums to calculate the results. I am too lazy to look up the physics and the results, I just remember that calibration for hobby grade equipment is best done at the target level.

Side bar: I just purchased a precision glass floating hydrometer that is miles away from being accurate. I have a plastic swingarm that appears to be more accurate. Getting a good tool that repeats measurement at the target level is what we need for the hobby. I bought the expensive floating hydrometer to see how accurate my hydrometer is and was shocked how poorly the hydrometer is.

I would recommend that you rinse with RO and dry a refractometer after you use it and put it away to reduce rusting of the metal parts.

Curious which glass Hydrometer did you pick up? The TM one is supposed to be the gold standard. I also have a more fancy refractometer designed for seawater (Vee-Gee STX-3), those are supposed to be calibrated with RODI according to the manufacture.

 

[GSPClown94]

In case you have not seen this?
https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
Randy provides instruction on how to use table salt to make calibration solutions for different refractometer, hydrometer, electronic meters, in this article. Very helpful to know that table salt mixtures of different concentrations are used to calibrate different meters. Turns out "calibration solutions" are not universally useful for all instruments that need to be calibrated. HTH.

This! I only trust Randy's recipe to make a calibration solution. Too many times i have found those commercially available solutions to be way off.

 

[mcarroll]

When calibrating a refractometer with 35 ppt solution, should the refractometer then ideally be at exactly the 0 line with RODI water or below the 0 line?

It's a minor difference either way if you're following the directions and calibrating properly.

The only situation I know where ultimate precision is required is if you're trying to do hyposalinity for new fish. In that case you're trying to make the water as low-salinity as the fish will tolerate before it causes them damage. So, being accurate is crucial.

But for general tank care, mixing saltwater, etc. I'm not sure it matters at all.

From the RHF article linked in GSPClown's post:

It is widely believed that only pure water is required to calibrate refractometers. That fact is true of some refractometers [....] and it may be acceptable for routine calibration[...]

Edited just for brevity. Emphasis mine. Please read the article if you haven't already.

Again: Pure water (even tap water, in my experience) is totally acceptable for routine calibration.

 

[dedragon]

i use the tm hydrometer to calibrate if i need to test multiple samples. if it only for one or two readings, i just use the tm and no refractometer

 

[dedragon]

when calibrating with 35ppt, i wouldnt worry about what rodi was reading at all

 

[KrisReef]

Curious which glass Hydrometer did you pick up? The TM one is supposed to be the gold standard. I also have a more fancy refractometer designed for seawater (Vee-Gee STX-3), those are supposed to be calibrated with RODI according to the manufacture.

That’s why I bought the TM, but I might have gotten a dud?

It’s too buoyant- seen here floating in Randy’s Morton’s Iodized salt calibration solution. I raised the temperature to 30C and it was still floating too high. 1.026 +

 

[NanoSteam]

Curious which glass Hydrometer did you pick up? The TM one is supposed to be the gold standard. I also have a more fancy refractometer designed for seawater (Vee-Gee STX-3), those are supposed to be calibrated with RODI according to the manufacture.

That’s why I bought the TM, but I might have gotten a dud?

It’s too buoyant- seen here floating in Randy’s Morton’s Iodized salt calibration solution. I raised the temperature to 30C and it was still floating too high. 1.026 +

Interesting this is the first complaint Ive heard of it. Mine matches with my refractometer so I havent questioned it but I've been itching to get a swing arm for old times sake and simplicity sake. Which one are you using at the moment?

 

[KrisReef]

I have an antique Instant Ocean (Seatest) Hydrometer

 

[Randy Holmes-Farley Verified Community Expert]

When calibrating a refractometer with 35 ppt solution, should the refractometer then ideally be at exactly the 0 line with RODI water or below the 0 line?

Many refractometers used in the hobby are brine refractometers repurposed from other industries to sell to reef hobbyists. I suspect many resellers do not even understand what they are selling and why it is not ideal.

A brine refractometer is made to measure salinity of sodium chloride, which is a lot, but not all of what seawater is.

A true seawater refractometer is designed to measure the salinity of seawater.

A perfectly made and perfectly calibrated brine refractometer cannot read both zero ppt and 35 ppt correctly. One must be off, as you have found. If you calibrated with an accurate standard at 35 ppt, then it will read 35 ppt correctly, and will even do so if less than perfectly made.

A perfectly made and perfectly calibrated true seawater refractometer can read both zero ppt and 35 ppt correctly. If you calibrated with an accurate standard at 35 ppt, then it will read 35 ppt correctly, and will even do so if less than perfectly made.

Thus, there are several scenarios where calibration with 0 ppt solutions can lead to errors, and no scenarios where calibration with an accurate 35 ppt (or similar salinity) standard will lead to errors.

That said, commercial salinity standards seem to have a poor track record of accuracy for reasons that I cannot understand. They are cheap and easy to make. Why do they go wrong?

For this reason, I use and recommend a diy using a good scale for both the sodium chloride and the water. Using a salt mix to make a standard is also not a good plan since it can have variable amounts of water in the solids.