Aspect
Valuable Member
- Joined
- Apr 8, 2022
- Messages
- 1,580
- Reaction score
- 847
- Location
- USA
- What state or country do you live in
- Ohio
Since you're struggling with understanding how refractometers that are "non electrical" are drifting I researched it for you:They should confuse you IMO....to me those posts don't make (physics) sense.
The advantage of the refractometer is that they are non-electrical....thus should never require "routine" recalibration. Why? Because something would physically have to move inside the refractometer in order for it to become "off calibration". What force will cause the set screw, with a clamping ring locked in place, to move?
I have no idea what law of physics is being violated to allow all these folks to have "drifting refractometers".
Drifting with ordinary usage is, however, a trait of electronic sensors.
Unless you treat your refractometer violently (enough to move the stationary prism inside) or change its calibration via the screw, there's no reason I know to explain it moving out of calibration.
Seeing LOTS of people have good experiences with them online was the reason I bought mine....and mine has never (pushing 20 years now) gone out of calibration unless the calibration screw was fiddled with by a child who didn't know better (literally).
The only rationale I can conceive to explain those posts is user error.
Some refractometers do not have ATC (automatic temperature compensation) and some do....my guess is that some folks who have ATC don't use it properly (takes time). And some folks who don't have it, don't realize it. Temperature changes from reading to reading can generate differing results even with the same actual salinity level.
The refractometer alternative I like is the old-fashioned swing-arm hydrometer....its issues, such as they are, are well known and easy to work with to still get accurate readings.
Floating glass hydrometers would be nicer if they weren't glass....nothing wrong with their ability, they're just too fragile IMO – they would be better suited for the protected environment of the lab.
For what it's worth, Spotte (Captive Seawater Fishes, 1992) only talks about conductivity meters, refractometers and hydrometers (giving excellent breakdowns of each).....noting that hydrometers are the easiest as well as sufficient for husbandry purposes.
The supposed issue with refractometers is a recent fad as far as I'm concerned. Maybe related to the $10 units that seem to be for sale now and which weigh almost nothing...are they made out of paper?!!? In these Amazon years, some folks may need a lesson on what "too cheap" is. But I didn't pay a lot (maybe $30-50?) for my refractometer from Marine Depot and it's been many, many years now. BRS (who bought Marine Depot) still sells reasonably priced models for around $40 to $70....perhaps buying those would make all the difference.
A good link that explores the ACTUAL pros and cons of refractometers while not relying on hobby fads or vendors selling calibration fluid for the facts like AI and most online posts do:
https://www.researchgate.net/profil...tions-Limitations-And-Future-Perspectives.pdf
Even without electrical sensors, microchips, or batteries, a purely optical manual handheld refractometer can experience measurement drift. Because it relies entirely on physical alignment and the geometry of light refraction, any subtle change to its mechanical parts or environment directly shifts the reading scale.
The primary causes of drift in a purely mechanical refractometer include:
1. Temperature Fluctuation & Thermal Lag
• How it causes drift: The refractive index of a liquid changes drastically based on its temperature.
• The "ATC" Trap: Even if your manual refractometer has Automatic Temperature Compensation (ATC), this mechanism relies on a small internal bi-metallic strip that physically shifts the scale or prism assembly as it expands or contracts with temperature.
• Thermal Lag: If you place a sample on the glass that is significantly warmer or colder than the body of the refractometer, the temperature will actively change while you look through the eyepiece. This causes the boundary line to slowly drift upward or downward until the liquid, glass, and internal bimetallic strip all reach thermal equilibrium.
2. Sample Evaporation
• How it causes drift: If the plastic cover plate is not seated perfectly flat, or if you wait too long to read a tiny droplet, volatile compounds or water from your sample will rapidly evaporate into the air.
• The Result: As the liquid dries out, the concentration of dissolved solids (such as salt or sugar) rapidly increases. This causes the sharp boundary line to drift upward right before your eyes.
3. A Slipping Calibration Screw
• How it causes drift: Manual refractometers rely on a small, spring-loaded adjustment screw to physically zero out the internal scale.
• The Result: On cheaper or heavily used units, this screw thread can become loose or stripped. Simple acts like setting the device down too hard, dropping it, storing it in a vibrating area, or the simple friction of wiping the prism clean can micro-turn the screw and permanently drift the baseline calibration.
4. Expansion and Contraction of Internal Optics
• How it causes drift: The internal chassis of a manual refractometer is typically built out of brass, aluminum, or plastic, while the lenses and prisms are made of glass.
• The Result: Repeatedly moving the device between severe temperature zones (e.g., from an air-conditioned room to a hot garage, or under direct sunlight) causes these mismatched materials to expand and contract at different rates. Over time, this mechanical "stress cycling" slightly misaligns the internal mirrors or lenses, pushing the waterline permanently out of spec.
5. Ambient Light Angle and Spectrum
• How it causes drift: Manual units require you to point the device toward an external light source.
• The Result: If you calibrate the unit under soft, natural sunlight but take measurements under a harsh fluorescent bulb or a colorful LED reef light, the wavelength composition changes. This alters how light bends through the prism, frequently causing the line to look blurry, form color fringes, or appear slightly shifted from your original baseline reference.
