Hydrometers and Specific Gravity: Choose the Scale and Range Before Accuracy
Buying a hydrometer for specific gravity at LabSupplies.com goes wrong most often in one place: people choose on stated accuracy first. Accuracy follows from two earlier choices — the scale the hydrometer reads in, and how narrow its range is — so pick those first and the subdivision, length and cylinder fall into place.
Step One: The Scale Your Method Reports In
A hydrometer floats at a depth set by the density of the liquid, and the scale on its stem converts that depth into a unit. Buy the scale your method or customer reports, not one you will convert from.
| Scale | Used for | Part numbers |
|---|---|---|
| Specific gravity | General chemistry, solutions, QC | 6603 Series |
| Specific gravity and Baumé (dual) | Acids, brines, heavy syrups | 6603DS-1, 6605-1 |
| Brix (calibrated at 20°C) | Wine, brewing, beverage and syrup | 6601-1, 6601-10 |
| API gravity with thermometer | Crude and petroleum products | 6752HTS to 6757HTS |
| Alcohol proof (IRS specification) | Distilled spirits gauging | 6613 Series |
| Soil (ASTM 151H / 152H) | Particle-size analysis by sedimentation | 67151H, 67152H |
The API scale is defined from specific gravity at 60°F as °API = 141.5 / SG − 131.5, so an API hydrometer and an SG hydrometer measure the same property — but a petroleum method written in °API expects the reading in °API, and converting by hand invites transcription errors.
Step Two: Range Sets the Resolution
A hydrometer stem has a fixed length, so the wider the range printed on it, the coarser each subdivision. That trade-off is the whole decision, and the Heathrow Scientific line shows it clearly:
| Series | Length | Span per hydrometer | Subdivision | Best for |
|---|---|---|---|---|
| 6602 | 330 mm | 0.07 (e.g. 1.000 to 1.070) | 0.0005 | Tight QC tolerances |
| 6603 | 305 mm | 0.11 to 1.00 by model | 0.001 to 0.010 | General lab work |
| 6602-7S (165 mm short) | 165 mm | 0.07 | 0.001 | Small sample volumes |
| 6604 | 165 mm | 0.25 (e.g. 1.000 to 1.250) | 0.005 | Screening unknowns |
The practical method: measure a few typical samples with a wide-range hydrometer first, then buy the narrow-range instrument whose span brackets your values with room on both ends. A product that normally reads 1.045 belongs on a 1.000 to 1.070 hydrometer (6602-7), not one that puts your reading at the very end of the scale.
Where a test method names an ASTM hydrometer number, buy that number. The ASTM 111H to 141H specific gravity hydrometers come in two resolutions across each 0.050 band: 13 in models at 0.0005 (67111H for 1.000 to 1.050) and 10.2 in models at 0.001 (67125H for the same band). ASTM E100 is the specification those numbers refer to.
Temperature: Read at the Calibration Temperature or Correct
Density changes with temperature, so every hydrometer is calibrated at a reference temperature. Most of the specific gravity, Baumé and API hydrometers here are calibrated at 15°C/60°F; the Brix hydrometers and the ASTM soil hydrometers at 20°C. A sample at a different temperature needs either conditioning to the reference or a correction from your method.
Thermohydrometers remove the guesswork by putting the thermometer in the body of the instrument, so density and temperature are read from the same sample at the same moment: the 6602TS-7 and 6602TS-8 for specific gravity, and the API thermohydrometers above for petroleum work.
Reading It Right: Cylinder, Meniscus and Bubbles
A hydrometer must float freely, clear of the walls and the bottom. Match the jar to the instrument length:
| Hydrometer length | Hydrometer jar |
|---|---|
| 165 mm (6602-7S, 6604) | 21050-100 (100 mL, fits up to 185 mm) |
| 305 to 330 mm (6603, 6602) | 21050-500 (500 mL, fits up to 360 mm) |
| About 385 mm (6602TS thermohydrometers) | 21050-600 (600 mL, fits up to 390 mm) |
All are in the 21050 Series borosilicate hydrometer jars. Lower the hydrometer gently, give it a light spin to shed air bubbles, and let it settle away from the wall. For clear liquids, read where the main liquid surface cuts the scale, not the top of the meniscus climbing the stem — ASTM D1298 sets out this reading and the correction needed when an opaque sample forces you to read at the top.
Store Them So They Last
Most hydrometers are lost to a roll off the bench. Keep them upright in a 10-place hydrometer rack (121002, 121003) with a drip tray, or in a PVC storage case sized to the instrument — 66CS-8F for 165 mm, 66CS-13 for 343 mm, 66CS-16 for 420 mm.
For a lab running the same test daily, a sensible stock is one working hydrometer per range in use, one spare of the range you read most often, and matching jars. The full line is in Heathrow Scientific hydrometers and accessories.
Frequently Asked Questions
How do I choose the right hydrometer range?
Measure a few typical samples with a wide-range hydrometer first, then buy the narrow-range hydrometer whose span brackets those values with margin at both ends. A narrower range gives finer subdivisions on the same stem length, so a sample that reads 1.045 belongs on a 1.000 to 1.070 instrument rather than a wide-range one.
What temperature are hydrometers calibrated at?
Each hydrometer is calibrated at a stated reference temperature. Most specific gravity, Baumé and API hydrometers are calibrated at 15°C/60°F, while Brix hydrometers and ASTM 151H/152H soil hydrometers are calibrated at 20°C. Condition the sample to that temperature or apply the correction your method specifies.
What is the difference between a hydrometer and a thermohydrometer?
A thermohydrometer has a thermometer built into its body, so density and temperature are read from the same sample at the same moment. That removes the need for a separate probe and makes temperature correction more reliable, which is why petroleum API methods commonly use thermohydrometers.
What size hydrometer jar do I need?
The jar must let the hydrometer float freely without touching the walls or bottom. As a guide, a 100 mL jar fits hydrometers up to 185 mm, a 500 mL jar fits up to 360 mm, and a 600 mL jar fits up to 390 mm, which covers 165 mm, 305 to 330 mm and thermohydrometer lengths respectively.
When should I buy an ASTM-numbered hydrometer?
Whenever your test method names one. ASTM numbers such as 111H or 125H define the range, length and subdivision of the instrument, so buying the named number keeps your results compliant with the method. The 13 in models subdivide at 0.0005 and the 10.2 in models at 0.001.
Setting up a new lab? Our LaunchLab program gives you 15% off everything for 12 months, hydrometers and glassware included. See if you qualify →
Building out a density-testing station? Hydrometers, thermohydrometers, jars, racks and cases in one place. Browse hydrometers →
Send your liquid, its usual reading and the method you test to support@labsupplies.com and we will match the scale, range and jar.
Related Guides
- Lab Thermometers: Immersion Depth, Armored vs FEP
- Handheld Refractometers: Brix, Salinity and ATC
- Lab Glassware 101: Types, Uses and Care
- Borosilicate vs Soda-Lime Glass
- Chemical Storage in the Lab
Sources and Technical References
- ASTM E100 — Standard Specification for ASTM Hydrometers, ASTM International — store.astm.org
- ASTM D1298 — Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method, ASTM International — store.astm.org
— By the LabSupplies.com Technical Team