Viscometer constant verification
Measures the flow time of a certified viscosity standard in a routine viscometer and compares the calculated constant with the certified constant.
Why this is checked
Each glass capillary viscometer has its own constant C (mm²/s²). Wear, deposits or damage can change it, and a wrong constant gives a proportional error in every result. Running a certified viscosity standard checks the constant, and also checks the bath temperature, timing and technique together.
Typically annually for each viscometer, when a new viscometer is put into service, after a check standard fails, or per SOP.
Learn how the Viscosity bath worksStandards & tools
- Certified kinematic viscosity standard (oil) with a value certified at the test temperature, chosen so the flow time is at least 200 s
- Calibrated bath and reference thermometer
- Timer verified against a reference time signal
Prerequisites
- Bath temperature verified
- Viscometer clean and dry
- Certificate value for the standard at the exact test temperature
Step by step
- 1Charge the viscometer with the standard and equilibrate in the bath for about 30 minutes.
- 2Measure two flow times and check they agree within the determinability.
- 3Calculate the mean flow time.
- 4Calculate the determined constant: C = ν(standard) ÷ mean flow time.
- 5Compare with the certified constant of the viscometer and calculate the percentage difference.
Example observation table
| Item | Value |
|---|---|
| Standard viscosity at 40.00 °C (certificate) | 61.23 mm²/s |
| Flow time 1 | 408.6 s |
| Flow time 2 | 408.2 s |
| Certified viscometer constant | 0.1500 mm²/s² |
How to calculate the result
- 1Mean flow time = (408.6 + 408.2) ÷ 2 = 408.4 s.
- 2C(determined) = 61.23 ÷ 408.4 = 0.14993 mm²/s².
- 3Difference = (0.14993 − 0.1500) ÷ 0.1500 × 100 = −0.05%.
- 4
Within an illustrative SOP limit of ±0.2% — the certified constant is confirmed.
Pass if
- Determined constant agrees with the certified constant within the limit in your SOP (commonly a few tenths of a per cent), or the measured viscosity of the standard is within the certificate tolerance.
- Two flow times agree within the determinability.
What to do
- Clean the viscometer thoroughly (deposits change the constant), re-check verticality and bath temperature, and repeat.
- If it still fails, send the viscometer for recalibration or withdraw it.
- Assess results obtained with that viscometer since the last good check.
Other Viscosity bath calibrations
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.