Difference between hydrometer and refractometer

[DanATL]

I'm getting different readings from my BRS refractometer and Tropic Marin hydrometer. I used BRS Refracto Juice (Brightwell) to calibrate it at 35ppt/1.025sg. I tested some water using the refractometer that I mixed up 5 days ago and it was spot on at 35ppt. I floated my hydrometer in the same dead still brute can of salt water and it registered 1.0285 or 38ppt. There were no bubbles or crud on either device. What do others think? The salt mix was in the garage at about 65 degrees while the refractometer was calibrated in the house at 74. Any ideas who to trust? I'm leaning to trust the hydrometer.

 

[WisReef]

I'm actually going to side with the calibrated refractometer. Sometimes the lines on the hydrometer aren't perfect and I've seen people draw their "own" lines on the hydrometer based on levels they've confirmed with a separate calibrated device. Many modern refractometers also have ATC which should compensate for the temperature change.

 

[KStatefan]

If the water that you were checking with the hydrometer was not at 77° F were you correcting the number for temperature?

 

[PatW]

I would also trust the calibrated refractometer over the hydrometer. It has been quite some time since I used a hydrometer. If you have doubts, take the refractometer to the local fish store and compare your reading to their reading. If you are a regular customer, I would think they would do that as a courtesy (unless they are really busy).

 

[Snoopy 67]

Test them both @ the same temp according to the hydrometer specs. Then trust the hydrometer.

 

[tharbin]

A quality glass tube hydrometer is almost always far more reliable and accurate than a refractometer. As @KStatefan and @Snoopy 67 said, either test at 77 degrees or use the reference chart to correct but a glass tube hydrometer is a direct reading of the specific gravity of the water a refractometer is not.

 

[EeyoreIsMySpiritAnimal]

If the water that you were checking with the hydrometer was not at 77° F were you correcting the number for temperature?

^^^THIS

 

[KStatefan]

Make a calibration solution and verify the hydrometer
This is what I use

 

[DanATL]

Make a calibration solution and verify the hydrometer
This is what I use

I'm going to try this too. thanks.

 

[DanATL]

Test them both @ the same temp according to the hydrometer specs. Then trust the hydrometer.

gonna bring some of the salt water inside to warm up and redo my tests at the same temp.

 

[DanATL]

A quality glass tube hydrometer is almost always far more reliable and accurate than a refractometer. As @KStatefan and @Snoopy 67 said, either test at 77 degrees or use the reference chart to correct but a glass tube hydrometer is a direct reading of the specific gravity of the water a refractometer is not.

This was my answer. At 77 F everything is in agreement. Clearly user error on my part.