Hydrometer/Refractometer comparison

gmiller · 02/15/2008, 09:32 AM

Most refractometers are accurate to plus or minus 1 point. Has anyone done a comparison for accuracy between the plastic hydrometer and a refractometer? How much difference was there? Thanks. Gary.

gaspasser1 · 02/15/2008, 10:12 AM

To answer this question you have to talk about accuracy and precision (remember chem 101?).

Accuracy is how close to the actual value the instrument reads on repeated tries. Precision is how close together those measurements are to one another.

IMO, a hydrometer can be a very accurate but it can have poor precision. Repeated measurments will vary because of the way these instruments are made (poorly). Bubbles get trapped, the weight is not wetted to the same degree all the time, ect.

But if you make repeated measurments with a hydrometer you will get a very good idea of the salinity. It's just a pain.

A refractometer is very precise and accurate right out of the box.

IMO the difference in precision (as this is what you pay for) is not significant between a $50 refractometer and one costing $120.

I have a $50 special and it's easy to use and quick.

Randy Holmes-Farley · 02/15/2008, 12:10 PM

Glass hydrometers are often very accurate, but need temperature corrections.

Plastic swing arms are often inaccurate, but need no temperature corrections.

Refractometers can be accurate, but are often salt refractometers incorrectly applied to seawater, or are simply mismanufactured and all should be calibrated with a seawater standard to eliminate most of that concern.

I personally use a conductivity meter to measure salinity.

I discuss various issues relating to hydrometers, including a comparison of accuracy here:

Specific Gravity Measurement
http://www.advancedaquarist.com/2002/1/chemistry

These may also be useful:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

Temperature Corrections for Hydrometers
http://reefkeeping.com/issues/2004-07/rhf/index.htm

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

Using Conductivity to Measure Salinity
http://www.aquariumfish.com/aquarium....aspx?aid=1804

02/15/2008, 09:32 AM	#1
gmiller Registered Member Join Date: May 2007 Posts: 5	Hydrometer/Refractometer comparison Most refractometers are accurate to plus or minus 1 point. Has anyone done a comparison for accuracy between the plastic hydrometer and a refractometer? How much difference was there? Thanks. Gary.

02/15/2008, 12:10 PM	#3
Randy Holmes-Farley Reef Chemist Join Date: Apr 2001 Location: Arlington, Massachusetts Posts: 86,233	Glass hydrometers are often very accurate, but need temperature corrections. Plastic swing arms are often inaccurate, but need no temperature corrections. Refractometers can be accurate, but are often salt refractometers incorrectly applied to seawater, or are simply mismanufactured and all should be calibrated with a seawater standard to eliminate most of that concern. I personally use a conductivity meter to measure salinity. I discuss various issues relating to hydrometers, including a comparison of accuracy here: Specific Gravity Measurement http://www.advancedaquarist.com/2002/1/chemistry These may also be useful: Refractometers and Salinity Measurement http://reefkeeping.com/issues/2006-12/rhf/index.php Temperature Corrections for Hydrometers http://reefkeeping.com/issues/2004-07/rhf/index.htm Reef Aquarium Salinity: Homemade Calibration Standards http://reefkeeping.com/issues/2004-06/rhf/index.htm Using Conductivity to Measure Salinity http://www.aquariumfish.com/aquarium....aspx?aid=1804 __________________ Randy Holmes-Farley Current Tank Info: 120 mixed reef

02/15/2008, 10:12 AM	#2
gaspasser1 Registered Member Join Date: Nov 2003 Location: wisconsin Posts: 69	To answer this question you have to talk about accuracy and precision (remember chem 101?). Accuracy is how close to the actual value the instrument reads on repeated tries. Precision is how close together those measurements are to one another. IMO, a hydrometer can be a very accurate but it can have poor precision. Repeated measurments will vary because of the way these instruments are made (poorly). Bubbles get trapped, the weight is not wetted to the same degree all the time, ect. But if you make repeated measurments with a hydrometer you will get a very good idea of the salinity. It's just a pain. A refractometer is very precise and accurate right out of the box. IMO the difference in precision (as this is what you pay for) is not significant between a $50 refractometer and one costing $120. I have a $50 special and it's easy to use and quick.