LimsPulse
Lubricants & oils · Lubricating oils, base oils, hydraulic oils; also diesel and fuel oils

Kinematic Viscosity by Glass Capillary Viscometer

The time taken for a fixed volume of oil to flow under gravity through a calibrated glass capillary at a closely controlled temperature is measured. Kinematic viscosity = viscometer constant × flow time.

  • Petroleum, oil & gas
  • Glass capillary viscometer
  • Viscosity bath
  • Thermometer / logger

Reference: Typical method based on ASTM D445, ISO 3104 and IS 1448 Part 25, with viscometers to ASTM D446. Follow the current edition.

PRINCIPLE

Why this method works

In a capillary viscometer, the liquid flows under its own weight. For a Newtonian liquid and a flow time long enough that kinetic-energy effects are negligible, the flow time is directly proportional to the kinematic viscosity. The proportionality constant C is found for each viscometer by calibration with certified viscosity standards. Because viscosity changes strongly with temperature, the bath is held within ±0.02 °C.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Rinse solventPetroleum spirit, heptane or toluene to remove oil
Drying solventAcetone or another volatile solvent, followed by dry filtered air
Viscosity check standardCertified viscosity standard for periodic checks
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Mix

    Shake or invert the sample to make it uniform.

  2. 02

    Filter

    Remove particles through a fine screen or sintered glass filter.

  3. 03

    Select viscometer

    Choose a size giving a flow time of at least 200 s.

  4. 04

    Charge and equilibrate

    Fill, hang vertically and wait about 30 min.

  5. 05

    Time twice

    Measure two flow times and calculate.

Procedure

  1. 1Mix the sample well. Warm waxy or very viscous samples as the method allows, then let them cool to near test temperature.
  2. 2Filter through a fine screen (for example 75 µm) if particles may be present.
  3. 3Choose a calibrated viscometer whose range gives a flow time of at least 200 s at the test temperature.
  4. 4Charge the viscometer as its type requires and hang it vertically in the bath at 40.00 °C (or 100.00 °C).
  5. 5Allow about 30 minutes to reach temperature.
  6. 6Draw the sample above the upper timing mark, release, and time the meniscus from the upper to the lower mark.
  7. 7Repeat to obtain a second flow time.
  8. 8Calculate each viscosity and check that the two agree within the determinability for the product type; report the mean.

Standard preparation

  1. 1Run a certified viscosity standard in a routine viscometer at the SOP frequency; the result should be within the certificate tolerance.

Notes for analysts

  • 1 mm²/s = 1 centistoke (cSt).
  • Opaque samples (used engine oils, fuel oils) need reverse-flow viscometers.
  • Viscosities at 40 °C and 100 °C are used to calculate the viscosity index (ASTM D2270).
  • Safety: bath oil at 100 °C causes burns; cleaning solvents are flammable and must be used in a fume hood away from the heater.
TEST CONDITIONS

Method parameters (typical)

Test temperature40.00 °C and/or 100.00 °C for lubricants (40 °C for diesel)
Bath control±0.02 °C (15–100 °C)
ThermometerCalibrated, graduated to 0.02 °C or digital equivalent
Minimum flow time200 s
EquilibrationAbout 30 min (longer for viscous samples)
TimerResolution 0.1 s; verified against a reference time signal
ReportingMean of two determinations, to four significant figures, with temperature
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
DeterminabilityTwo values from one filling agree within the limit for the product type in the current D445 (about 0.1–0.2% for many oils)
Check standardWithin certificate tolerance at the SOP frequency

Typical sequence

  1. 1Bath temperature check
  2. 2Check standard (per SOP)
  3. 3Samples
  4. 4Clean and dry viscometers
CALCULATION

Calculation with a worked example

Formula
ν = C × t; η = ν × ρ (dynamic viscosity, if density is known)

Where

ν
Kinematic viscosity (mm²/s)
C
Viscometer constant at the test temperature (mm²/s²)
t
Flow time (s)
η
Dynamic viscosity (mPa·s)
ρ
Density at the test temperature (g/cm³)

Given

Sample
Hydraulic oil labelled ISO VG 46, at 40.00 °C
Viscometer constant C
0.1035 mm²/s²
Flow times t1 / t2
412.8 s / 413.2 s
Density at 40 °C
0.861 g/cm³

Working

  1. 1

    ν1 = 0.1035 × 412.8 = 42.72 mm²/s

  2. 2

    ν2 = 0.1035 × 413.2 = 42.77 mm²/s

  3. 3
    Difference = (42.766 − 42.725) ÷ 42.746 × 100 ≈ 0.10% (unrounded values) — within determinability
  4. 4

    Mean ν = 0.1035 × 413.0 = 42.75 mm²/s

  5. 5

    Dynamic viscosity η = 42.75 × 0.861 = 36.81 mPa·s

Kinematic viscosity at 40 °C = 42.75 mm²/s (dynamic viscosity 36.81 mPa·s)

ACCEPTANCE CRITERIA

What result passes

  • ISO VG grades allow ±10% around the mid-point at 40 °C: ISO VG 46 = 41.4 to 50.6 mm²/s, so 42.75 mm²/s conforms (ISO 3448).
  • For in-service oils, a change of more than about ±10% from the new-oil value is a common alert level (as set in the condition-monitoring programme).
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
TruenessCertified viscosity standards within tolerance
PrecisionDeterminability, repeatability and reproducibility from the current D445 for the product type
TemperatureBath verified within ±0.02 °C
In LimsPulse · Sample Management

Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.

See how