In order to be used for official compaction and bearing capacity verification, the device must be calibrated annually at a calibration laboratory approved by the German Federal Highway Research Institute (BASt). Calibration must be carried out in accordance with the valid calibration regulation TP BF-StB Part B 8.4.
Only devices with a valid calibration certificate
may be used as an official test method on construction sites.
Measurement results from devices with expired or invalid calibration must not be used to assess soil bearing capacity.
“Most
of our customers have their devices calibrated during the winter months
from December to March,” says Frank G. Schulz, CEO of TERRATEST GmbH.
“We
are very well prepared for this seasonal peak in order to keep downtime
on construction sites as short as possible. During the winter months,
TERRATEST operates with a vacation freeze, and the first VIP
calibrations already start between Christmas and New Year, so that many
customers can use their devices again from the second calendar week of
the new year. We even receive devices from Australia or Peru for
calibration — of course, we also handle the complete air-freight
logistics for our customers. Every device is thoroughly checked and
automatically updated with the latest firmware and newest functions.”
TERRATEST is the only company worldwide operating five BASt-approved in-house calibration rigs for Light Weight and Medium Weight Deflectometers. These calibration rigs were developed and built by TERRATEST itself.
1. Mechanical and electronic functional check
After arrival at TERRATEST, each device is first tested for proper mechanical function. Then the electronics department checks all electronic components, including:
- sensor
- cable connections
- printer
- high-performance rechargeable battery
In addition, the latest firmware
is installed automatically. The rechargeable power battery has a
typical service life of at least four years. During calibration, its
remaining capacity is checked to ensure reliable operation on
construction sites.
If any components are defective or worn, repairs are carried out only after customer approval, based on a detailed cost estimate.
2. Force calibration of the loading device
After the technical inspection, the actual calibration is performed in the calibration department.
First, the impact force of the loading device (rod with falling weight) is adjusted to 7,070 N. This is done by adjusting the drop height via the release mechanism.
The setting is then verified by 30 consecutive drops. The permissible deviation of the impact force within this series must be no more than ±1%.
For Medium Weight Deflectometers with double impact force, the force is set to 14,140 N. According to test regulation TP-Gestein 8.2.1, these devices can also be used to test heavy crushed-stone base layers.
Next, the settlement measurement of the sensor is calibrated. This is carried out using three rubber pads of different thicknesses, simulating hard, medium, and soft soil conditions.
During loading, three high-precision displacement transducers
measure the actual settlement of the load plate. These reference values
are compared with the readings from the TERRATEST sensor.
The calibration of the Light weight Deflectometer is only considered successful if the deviation between reference measurement and sensor reading is less than 20 µm (micrometers).
For each rubber pad, ten consecutive drops are evaluated. For comparison: a human hair has a diameter of about 30 µm.
Precision for reliable construction site results
The entire calibration process is a technically demanding and precisely defined procedure. It ensures that every Light Weight Deflectometer for the dynamic load plate test delivers highly accurate and reproducible measurement results on site — an essential requirement for safe construction decisions and standards-compliant documentation.
