LimsPulse
CBR apparatus · Calibration procedure

Load ring and penetration rate verification

Confirms that the proving ring (or load cell) reads load correctly against a traceable reference, and that the load frame drives the plunger at 1.25 mm/min.

Illustration of CBR apparatus
PURPOSE

Why this is checked

CBR is a ratio of measured load to standard load at fixed penetrations. A wrong ring constant gives a wrong CBR directly, and a wrong penetration rate changes the load the soil carries.

Frequency

Proving ring calibration typically every 12 months (or after overload or a knock); penetration rate typically every 6 to 12 months, as defined in your SOP.

Learn how the CBR apparatus works
YOU WILL NEED

Standards & tools

  • Reference load cell or calibrated testing machine traceable to national standards (ASTM E74 / ISO 376 for the force reference)
  • Calibrated stopwatch
  • Calibrated dial gauge or displacement transducer (0.01 mm)
BEFORE YOU START

Prerequisites

  • Proving ring exercised (pre-loaded) to its capacity three times
  • Ring and reference at room temperature
PROCEDURE

Step by step

  1. 1Load ring: place the ring in series with the reference load cell in a loading frame; zero both.
  2. 2Apply loads at several points across the range (e.g. about 10% to 100% of capacity), recording ring dial divisions and reference load.
  3. 3Repeat for at least two more series; calculate the mean divisions at each load.
  4. 4Calculate the ring constant (load per division) or check the certified constant: ring load = divisions × constant; compare with reference.
  5. 5Penetration rate: mount a dial gauge between the platen and frame; run the motor and time the travel over e.g. 5.00 mm. Repeat three times.
  6. 6Calculate rate = travel ÷ time and compare with 1.25 mm/min.
RECORD

Example observation table

Reference load (kN)Ring reading (div)Ring load = div × 0.0400 kN (kN)Error (%)
250.22.008+0.40
5125.35.012+0.24
10250.410.016+0.16
20499.019.960−0.20
30748.129.924−0.25
CALCULATION

How to calculate the result

Formula
Ring load = Divisions × Ring constant; Error % = (Ring load − Reference) ÷ Reference × 100; Rate = Travel ÷ Time
Worked example
  1. 1
    At 10 kN: 250.4 div × 0.0400 kN/div = 10.016 kN; error = (10.016 − 10) ÷ 10 × 100 = +0.16%
  2. 2
    Penetration rate: 5.00 mm in 242 s (4.033 min) → 5.00 ÷ 4.033 = 1.240 mm/min; 238 s → 1.261 mm/min; 240 s → 1.250 mm/min
  3. 3

    All three are close to 1.25 mm/min.

  4. 4
    Effect on CBR: a load of 82 kg at 2.5 mm gives CBR = 82 ÷ 1370 × 100 = 6.0%; a 1% load error changes this by only about 0.06 percentage points, but a wrong ring constant used for every test biases all results.
ACCEPTANCE CRITERIA

Pass if

  • Ring load within the SOP limit of the reference across the range (commonly ±1% of reading, or the class on its certificate).
  • Penetration rate 1.25 mm/min (IS 2720 Part 16) or 1.27 mm/min (ASTM D1883), within the tolerance in your SOP (e.g. about ±0.1 mm/min).
  • Ring returns to zero after unloading.
IF IT FAILS

What to do

  • Withdraw the ring and have it recalibrated; use the new constant only after review.
  • Service the drive if the rate is out of tolerance.
  • Review CBR results reported since the last good verification.
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