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Viscosity bath · Calibration procedure

Bath temperature verification

Checks the viscosity bath temperature with a calibrated reference thermometer at each test temperature, at the viscometer positions and over time.

Illustration of Viscosity bath
PURPOSE

Why this is checked

For many oils viscosity changes by roughly 0.2–0.3% for each 0.1 °C near 40 °C. ASTM D445 therefore requires the bath to be held within ±0.02 °C of the test temperature (in the 15–100 °C range), measured with a thermometer of matching accuracy. This check proves accuracy, uniformity and stability.

Frequency

Typically a daily check at the reference thermometer before use, and a full multi-position verification every 6–12 months or per SOP.

Learn how the Viscosity bath works
YOU WILL NEED

Standards & tools

  • Calibrated reference thermometer: liquid-in-glass with 0.02 °C or finer graduations or a digital contact thermometer of equivalent accuracy, traceable to national standards
  • Certificate corrections for the reference thermometer at the test temperatures
BEFORE YOU START

Prerequisites

  • Bath running at the set temperature for at least 1 h with lid fitted
  • Bath medium at the correct level
  • Reference thermometer in date
PROCEDURE

Step by step

  1. 1Place the reference thermometer at the first viscometer position at the depth of the viscometer bulb.
  2. 2Wait for a stable reading and record it, applying the certificate correction.
  3. 3Move to each other viscometer position and record readings.
  4. 4At one position, record readings every 5 minutes for 30 minutes to check stability.
  5. 5Repeat at each test temperature used (for example 40 °C and 100 °C).
  6. 6Calculate mean deviation, spread across positions and variation over time.
RECORD

Example observation table

PositionSet (°C)Corrected reference reading (°C)Deviation (°C)
140.0040.012+0.012
240.0040.005+0.005
340.0039.996−0.004
440.0040.009+0.009
CALCULATION

How to calculate the result

Formula
Deviation = corrected reading − set temperature; Uniformity = highest − lowest reading
Worked example
  1. 1
    Mean = (40.012 + 40.005 + 39.996 + 40.009) ÷ 4 = 40.0055 ≈ 40.006 °C.
  2. 2

    Largest deviation = +0.012 °C, within ±0.02 °C.

  3. 3

    Uniformity = 40.012 − 39.996 = 0.016 °C.

  4. 4

    All positions within ±0.02 °C of 40.00 °C — pass.

ACCEPTANCE CRITERIA

Pass if

  • Every position within ±0.02 °C of the test temperature (15–100 °C range, per D445); wider limits apply outside this range as stated in the method.
  • No drift beyond these limits over the stability period.
IF IT FAILS

What to do

  • Check stirrer, lid and medium level; adjust controller offset if the SOP allows, then repeat.
  • Do not use positions that fail.
  • Service the heater or controller if stability cannot be achieved.
  • Assess viscosity results since the last good check.
MORE

Other Viscosity bath calibrations

In LimsPulse · Calibration Management

LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.

See how