New Refractometer - Now Questions

[falconut]

I've been using an Instant Ocean Hydrometer for years, which I figured giving me accurate numbers for my reef tank.

I finally purchased a refractometer from Premium Aquatics along with a bottle of Sybon Standard Seawater Saline. Out of the box, the meter read the 35ppt Sybon at 40ppt (after letting it sit for 30 seconds), so I adjusted the calibration screw until it was 35ppt.

I'm assuming that this now has my meter reading very close to seawater at 35ppt. So, I tested my display tank and it was reading 1.029, wasn't expecting this. My hydrometer was reading around 1.0245. I then tested my QT tank water with the refractometer and it read 1.0275.

Confused by this I tried calibrating the refractometer with RO/DI water at 0.00. Then tested my DT and it read 1.030. Oh, and the RO/DI water read -0.1. So the adjustment was slight. From my understanding, if I use RO/DI to calibrate, shouldn't my test be 1.7ppt higher?

I want to make sure it's high before I start lowering the salinity.

 

[downbeach]

This will explain it:

http://reefkeeping.com/issues/2006-12/rhf/index.php#13

 

[falconut]

Thanks Jim, but I've read that like 5 times and my take was:

- if I use RO/DI to calibrate it will read 1.7ppt lower than seawater, so I'll need to adjust what I see.
- if I use Sybon 35ppt to calibrate it should be what I see.

Am I understanding this correctly?

If so, this means my DT is really 1.029?

 

[downbeach]

Yes, if you calibrate it to 35ppt it'll be fine, even though it may be off at other levels. It does appear that your SG is a little high. If you're going to the meeting, bring some of your water, and we'll test it. Let me know and I'll bring my refractometer.

 

[falconut]

Alright, then I'll slowly start lowering it. Just wanted to make sure before doing so. Not sure if I can make the meeting yet, very busy weekend. I'll be sure to let you know if I can.

 

[disc1]

- if I use RO/DI to calibrate it will read 1.7ppt lower than seawater, so I'll need to adjust what I see.

That depends a great deal on the particular refractometer that you have.

 

[falconut]

That depends a great deal on the particular refractometer that you have.

Mine is a saltwater refractometer, not a seawater refractometer. That's why I believe I would need the adjustment.

 

[Bpb]

I had the same issue. My swing arm was reading low causing me to run hyper salinity. Pretty typical. Trust your calibrated refractometer over your hydrometer. I'm the only local with a refractometer so I was a nice guy and went around to all the local reefers, cleaned their swing arm hydrometers, and calibrated them with my refractometer, making new markings to reflect the true readings

 

[hedgedrew]

Just make sure the temp of calibration fluid is near 77 and the refract is akso not too cold. Like should be room temp. Not very cold

 

[falconut]

I'm keeping the refractometer in it's case, next to the tank. I also let sample sit on the meter for 30 seconds as instructed. I replaced a few gallons with fresh RO/DI and it's now at 1.028. I figure in a few more days I'll have it down to 1.026. Now I understand why others always say that everybody should have one of these.

 

[hilgert]

The article mentioned in post #2 describes DIY reference solution for refractometer calibration using Morton Iodized Salt and RODI water. I would suggest anyone that has a refractometer does so periodically (I do it every few months). It's easy to do with a gram scale and a good measuring cup.

The formula is 73.1g of Morton Iodized Salt to 2000ml (2 liters) of RODI water, although I make mine at 1/10th of that (7.31 grams of salt and 200ml of RODI water as I have both a scale and a liquid container that happens to be very accurate). If you don't have access to a scale then you can use the "rough method" described in the article to get close enough for our purposes, and will give a good "close enough" indication of calibration adjustments needed. I would advise not worrying too much about the 0ppt calibration (the RODI calibration), as we really only care about being accurate in the 35ppt range (in other words, don't chase the entire scale of accuracy...just worry about accuracy in the "reef zone").

Having used hyrdrometers in the distant past I can say that acquiring a refractometer is one of those no-brainer decisions that any reefer should make. Mine happens to be a Marine Depot branded one I picked up on sale several years ago. That being said, hydrometers can be very accurate if they are calibrated against a [calibrated] refractometer. However, I find there is more "technique" required to use a hydrometer properly in order to get repeatable results, while the use of a refractometer is fairly hard to screw up.

 

[dkeller_nc]

I'm keeping the refractometer in it's case, next to the tank. I also let sample sit on the meter for 30 seconds as instructed. I replaced a few gallons with fresh RO/DI and it's now at 1.028. I figure in a few more days I'll have it down to 1.026. Now I understand why others always say that everybody should have one of these.

Regardless of what the instructions say, I would recommend that you place a drop of your calibration solution on your refractometer and read/adjust immediately. Within 5 degrees Fahrenheit of 77, the interpreted specific gravity of seawater will only vary by +/- 0.001, and the potential problem with waiting for temperature equilibration is evaporation. Whether or not evaporation will significantly affect your reading depends a great deal on the local relative humidity and how well-built your meter is (e.g., how well the thin film on the prism is "sealed").

Digital refractometers don't have as much of an issue with this, because the ratio of sample volume to sample surface area is higher than with the manual, thin-film types of refractometers. But either way, it's better to just ensure that the calibration fluid and instrument are close to 77 deg F (+/- 5 deg F) by keeping both in a closed aquarium cabinet for 20 minutes before your calibrate them than attempting to let the sample cool down to the refractometer's temperature.