Laboratory manual for the evaluation of non-automatic weighing devices
LG-3.14 Creep and creep-return-to-zero
Table of contents
Sections 17 and 18 of the Specifications Relating to Non Automatic Weighing Devices (1998)
This test is performed on any complete mechanical or electronic weighing device and on any mechanical or electronic weighing element tested separately.
- The AZT should be disabled for this test. If it cannot be disabled it must be set to the lowest value; if zero is the lowest selectable value, it will be set to zero. The amount that the AZT may track at once can not exceed 0.6e which is the maximum value prescribed by the Specifications.
- If a device is provided with an IZSM, the tests will be performed once with the IZSM set to the maximum of its range.
- The device must be set for the maximum capacity and smallest verification scale interval for which the approval is sought.
- If so equipped, the "enhance resolution feature" of the device will be used during the tests. If this feature is not available, use the small weight method to determine the device errors before rounding.
- The creep and creep-return to zero tests are performed at ambient (≈20°C) temperature only.
- Note that these tests are combined with the test performed to assess the temperature effect on linearity and hysteresis.
- The temperature of the DUT must have stabilized to within ±1°C.
Note: The temperature of the DUT can be monitored by attaching a thermocouple with adhesive tape inside the DUT onto a component of relatively large mass. However, the thermocouple must not be attached to any live component(s) of the DUT. The temperature will be deemed to be stable when the temperature obtained from the thermocouple will have stabilized within ±1°C. Once stabilized, the temperature obtained from the thermocouple will likely be different than the overall temperature of the environmental chamber. The requirement is only that the temperature of the DUT stabilizes to within ±1°C.
- Moisture content in the environmental chamber must not exceed ≈50% RH at test temperatures.
- The test equipment such as load cell simulators, high precision reference indicators, must be isolated from the test conditions and maintained at constant ambient temperature and humidity.
- The increasing and decreasing load tests must be performed before the creep test and creep-return-to-zero test; a recovery time period equal to the accuracy (increasing and decreasing) test time is permitted before conducting the creep test; the scale should be exercised 3 times up to at least 90% Max before conducting the creep and creep-return-to zero test if an extended period of time has passed since the scale was last tested.
Single range or multi-interval device
- Following the increasing and decreasing load test, allow the device to "recover" for a period of time equal to the increasing and decreasing test time. Exercise the scale 3 times up to at least 90% Max.
- Place on the weighing element a load equal to at least 90% of Max, and take a reading 20 seconds after the indication has stabilized. Observe the indication for 30 minutes, taking a reading at 15 minutes and 30 minutes. Do not remove the load
Creep—Return to zero test
- Remove the load and take a reading as soon as the indication has stabilized (wait no more than 20 seconds). If the AZT could not be disabled leave a small weight on the platter to keep the indication above the AZT range.
Multiple range devices
- Complete the creep test described for Single Range or Multi-Interval Devices for each range, using 90% of the Max of the range being tested.
Creep—Return to zero test
- After the creep test for the lowest range has been completed remove the load and take a reading as soon as the indication has stabilized (wait no more than 20 seconds). If the AZT could not be disabled leave a small weight on the platter to keep the indication above the AZT range.
- After the creep test for the highest range (using 90% of the capacity of the device) Remove the load and take a reading as soon as the indication has stabilized (Wait no more than 20 seconds). If the AZT could not be disabled leave a small weight on the platter to keep the indication above the AZT range.
- Immediately, switch the device to the lowest range, take a reading of the weight indication. Observe the weight indication for 5 minutes. Record any changes in the weight indication.
Interpretation of results (creep test)
The device meets the requirements if the difference between the indication obtained 20 seconds after placing the load on the device and the indication observed during the following 2 an hour does not exceed the absolute value of the applicable limit of error for that load.
Interpretation of results (creep—return-to-zero)
- The deviation on returning to zero as soon as the indication has stabilized shall not exceed 0.5e.
- On multi-interval device, the deviation shall not exceed 0.5e1
- On a multiple range device, the deviation on returning to zero from Maxi shall not exceed 0.5ei; moreover, after returning to zero from any load greater than Max1 and immediately after switching to the lowest weighing range, the indication near zero shall not vary by more than e1 during the following 5 minutes.
- Rev. 1 (September 2009)
- Removed requirement for testing at temperatures other than nominal
- Rev. 2 (March 2010)
- AZSM renamed to AZT
- Formatting changes
- Procedures adjusted for 30 minute creep test length
- Date modified: