Indentor and spring force verification
Checks the indentor extension and shape, and the spring force at several points across the scale against the force the standard sets for each reading.
Why this is checked
A durometer reading is only meaningful if the indentor has the right geometry and the spring pushes with the right force at each depth. Wear, damage or a weakening spring give readings that look normal but are biased.
Typically every 12 months, after a drop or repair, and when daily test-block checks fail, as defined in your SOP.
Learn how the Durometer worksStandards & tools
- Durometer calibrating device: a calibrated balance or force gauge with a fixture that holds the durometer and pushes the indentor to set readings
- Gauge for indentor extension (for example a dial gauge or gauge blocks reading to 0.01 mm)
- Optical comparator or profile projector for indentor geometry
- Reference rubber test blocks for a final check
Prerequisites
- Durometer clean, with no visible tip damage
- Calibration equipment within its calibration date
- Test at the standard laboratory temperature (typically 23 °C)
Step by step
- 1Check that the dial or display reads 0 with the indentor fully extended.
- 2Measure indentor extension from the presser foot surface (nominally 2.5 mm).
- 3Inspect the indentor tip shape and diameter (Shore A truncated cone; Shore D cone with rounded tip) with an optical comparator.
- 4Mount the durometer in the calibrating device and push the indentor to readings of about 0, 25, 50, 75 and 100 (or as the standard requires).
- 5Record the force at each reading and compare with the nominal force from the standard's formula.
- 6Finish with readings on reference test blocks.
Example observation table
| Scale reading (Shore A) | Nominal force (N) | Measured force (N) | Error (N) | Error (Shore units) |
|---|---|---|---|---|
| 0 | 0.550 | 0.56 | +0.010 | +0.13 |
| 25 | 2.425 | 2.44 | +0.015 | +0.20 |
| 50 | 4.300 | 4.33 | +0.030 | +0.40 |
| 75 | 6.175 | 6.15 | −0.025 | −0.33 |
| 100 | 8.050 | 8.08 | +0.030 | +0.40 |
How to calculate the result
- 1
At 50 Shore A: F nominal = 0.55 + 0.075 × 50 = 4.30 N
- 2
Measured 4.33 N → error = 4.33 − 4.30 = +0.03 N
- 3Error in units = 0.03 ÷ 0.075 = +0.4 Shore A
- 4
Shore D at 50: F nominal = 0.4445 × 50 = 22.225 N
Pass if
- Indentor extension typically 2.46–2.54 mm (nominal 2.5 mm)
- Indentor geometry within the dimensions in ISO 868 / ISO 48-4 / ASTM D2240
- Spring force at each point typically within the force tolerance in the standard (about ±0.075 N for Shore A, equivalent to ±1 unit; about ±0.4445 N for Shore D)
- Reference blocks read within their stated tolerance
What to do
- Remove the durometer from use and label it.
- Replace a worn or damaged indentor or spring, or send it for repair.
- Review results since the last good check, especially near specification limits.
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.