If you own a refractometer, read this

[raytse]

Look like there is some interest in getting a standard salt solution to calibrate their refractometer or hydrometer. As a chemist, I believe that, if accuracy is what you want, you need to calibrate your instrument on an ongoing basis, regardless of what it is. NIST certified equipment is only certified as it leaves the factory and during the warranty period. Beyond that, you are on your own. I am sure that anyone involved in QC/QA can give his/her opinions.

I offer that if someone gives me the EXACT formula of the solution they want, I can see whether I can make it. If I can make it, I shall measure the refractive index of the solution against a calibrated analytical grade refractometer. Then, you can a standard solution for calibration.

All services are free!

 

[SaraB]

I read this thread yesterday and it caught my attention as I have the blue RHS-10ATC refractometer as well. I have calibrated it per the instructions of using distilled water, not RO/DI water as others wrote.

I also have a Coralife Deep Six Hydrometer that has been laying around not used since I picked up the refractometer about a year and a half ago.

This morning I tested my salinity with the hydrometer 3 times and the result came up consistently at 1.0205. I state that number as it is between 1.020 and 1.021. My refractometer measures the salinity at 1.025. I also double checked the calibration on my refractometer with the distilled water per the instructions and it recorded as accurate.

#1. Which do I believe?
#2. I need to replace my refractometer as the swing down plate is starting to warp. What should I replace it with?
#3. How do I bring my salinity back up to 1.025 if I belive the hydrometer?

Sara

 

[pk1]

According to the link by Pam Little I would believe the hydrometer, if it one if the ones you see here:

http://www.wetwebmedia.com/ca/volume_3/cav3i4/hydrometers/Impressions.htm

I can bring the pinpoint solution to the November meeting and anyone who would like to check their hydrometers are more than welcome to. RHF said that once calibrated correctly they are fine. Top off with salt water instead of RO/DI to get back in the range you want.

Raytse, thanks for the offer, I drive through oak park every day, maybe I could drop off some solution and you could give it a test, just to make sure. RHF is pretty confident in the solution though, and he's always been a reliable source in this hobby, so I would be happy with his OK also.

PK

 

[Crued]

can someone comment on the need to calibrate hydrometers? i seem to remember that when you buy one of those you are supposed to do something with it or it will read incorrect results. i had one a few years ago and it was *way* off and a local LFS guy mentioned that you had to calibrate it now. i use refractors now and my fish stay alive, so i've never questioned it.

 

[ostrow]

Ok, Chemistry Prof over here mixed 35ppt/1.026 solution for me. My black RHS-10ATC measured it as just below 1.023/33ppt.

So, now I have a bottle of 35ppt and I can calibrate with RO/DI 2.5 lines or so above the zero mark.

I always tried to keep my tank at 1.0255. I guess it was really around 1.023 or a tad lower.

 

[SaraB]

I top off evaporation with Kalk water, so I can't go that route as I will decrease Alk. Can I just slowly raise my water change water's salinity? If I do a 15 gallon water change on a 200 gallon system and use the correct salinity on the water change water? Or is this something I should be asking Randy?

 

[pk1]

Joel, that's not that bad, better than the .005 mine was off at least. Sara, I actually like your idea better, IMO the change should be very gradual.

 

[SaraB]

pk1 - Thanks for the feedback.

I actually have a Pinpoint Salinity Monitor that I had purchased slightly used earlier this year ... guess I should dust it off and get some calibration fluid and hook it up!

 

[raytse]

Ostrow,
Yes, I do teach Chemistry in a local college â€"œ part time.

To answer a few questions.
Calibration is done using known standards. It can be single or multiple standards. Of course, the multiple standard method is preferred. Ideally, the standards should be close in composition as the material you are measuring. For the type of refractometer we are using, having multiple standards is not practical since you cannot adjust the instrument after each standard. However, we can use other standards to check the instrument. For the hydrometers (swing arm or stem), you need to use multiple standards. You mark the position of the measurements using the known standards. The engraved scales on the hydrometer may not be accurate.

In our case, making standards is the major source of error. If you want to make 35 ppt solution, I think you need to mix 35 parts of salt with 1000 parts of water. Am I right here? How accurately can you weight salt and water? Please consider the accuracy and precision of the scale you use. Let’s work in oz. For example, if we get the salt 34 oz of salt in 1010 oz of water. Now you have 33.66 ppt solution. I only assume 1% error on each weighting. 1% error is not much. Typical kitchen scales can be off by 2 to 5%. Therefore, unless you have analytical quality of equipment, making standards on your own is not advised.

The salt we use can be a problem. Different manufacturers use different chemicals, even though they are very similar. I have seen some salt is quite hydroscopic (taking moisture from air). By doing so, the weight of the salt actually goes up and affecting the weight.

Sorry for the long answer. Professor likes to talk (write in this case)!

 

[ostrow]

Raytse: I had no reason to doubt you. Don't quite understand you post, though thanks for the info.

I teach Political Science at Benedictine Univ. Had a chemistry prof mix me the solution, and posted my results.

 

[raytse]

If you have a good solution, you shall be OK. However, some people may get off track using bad standard solutions.

 

[ostrow]

According to RHF, it turns out 35g per 100ml won't have the same refractive properties as natural seawater. Need to mix 36.5 nacl to match that, and calibrate the instrument at 35ppt.

 

[blackdak]

The whole moral of the story in all of this is:

Whatever you use to measure salinity, take the time to check that it is calibrated!

The store bought solution (or homemade) can be used for any device. This goes for swing arm hydrometers and refractometers, conductivity meters etc.

I would put a swing arm hydrometer that is clean and calibrated up against a refractometer any day. Those little devices are surprisingly accurate. Over time the arm can get chipped or damaged and cause an errant reading but that means it just needs to be calibrated so you know that when your hydrometer says 1.025 it is really 1.023 or something like that!

I love gadgets and being a scientist (Ph.D. analytical chemist, to be exact!), I like measurement gadgets more than most people... but a properly cleaned and calibrated hydrometer is as good as anything especially for less than $15.

Thats my story and I am sticking to it!
Brian

 

[Hammercoral]

The real question is what does ROD do?? I think he just tastes the water, but hopefully he will share his secrets.....