R76-1 International Recommendation for Non-Automatic Weighing Instruments makes zero point and zero setting a very important issue, and not only sets out the measurement requirements, but also the technical requirements, because the stability of the zero point of any weighing instrument is the basic guarantee of its measurement performance. The following terms are closely related to the zero point, we explain, analyze in turn.
(1) Expression error: The difference between the indicated value of a scale and the true value of the corresponding mass (convention).
(2) Maximum Allowable Error: For a scale that is in the reference position and has been set to zero with no load, the maximum positive or negative difference between its indicated value and the corresponding true value determined by a reference standard mass or standard weight is recommended to be allowed.
(3) Zeroing Device: A device that sets the indicated value to zero when there is no load on the carrier. For electronic scales, including: semi-automatic zeroing device, automatic zeroing device, initial zeroing device, zero tracking device.
(4) Zeroing Accuracy: After the scale is zeroed, the effect of zero error on the weighing result is within ±0.25e.
(5) Zero point error: after unloading, the zero point of the scale shows the value error, the maximum permissible error in the range of ±0.5e at the first calibration.
(6) Zero tracking device: a device that automatically maintains the zero indicating value within a certain range. Zero tracking device is an automatic zero device.
Zero tracking device can have four states: no, not running, running, out of the operating range.
The zero tracking device is allowed to operate when:
– The indicated value is zero, or the equivalent of a negative net weight value when the gross weight is zero;
– and the balance is in stabilization;
– the correction is not greater than 0.5 e/s.
1. Zero tracking device test
As the vast majority of electronic balance products in China at present, there is a zero tracking device, so the need to test the zero point of error, you must ensure that the zero tracking can not be in operation. Then, the zero tracking device “does not run” the only way is to put a certain weight of load near the zero point, so that the zero tracking beyond its operating range.
(1) determine the correction rate of zero tracking device
Due to the relevant standards and calibration procedures in the zero tracking correction rate is not determined in the method, found that there are some people on this speculation, consciously increase the correction rate, so that the weighing instrument to return to zero faster, in order to show that the quality of the product of individual products is excellent. For this reason, the author summarized in the actual work of a method, you can quickly in the field to check out the zero tracking rate of the scale.
Turn on the power, stabilize for at least 30min, put a load of 10e on the load carrier, so that the scale “zero tracking” is out of the operating range. Gently apply a 0.3e load at intervals of about 2s and observe the value.
After 3 consecutive 0.3e loads, the scale shows a significant increase of one division, indicating that the device is not or will not operate.
If the scale does not visibly change value after 3 loads of 0.3e, the unit is still operating and tracking corrections within 0.5e/s.
Then, gently remove the 3 0.3e loads and the scale should show a significant decrease of one division.
Why are 3 0.3e loads being used?
The 0.3e load is less than the correction rate of 0.5e/s; and the 3 0.3e loads are greater than 0.5e/s and less than the correction rate of 1e/s (because the required correction rate is incremented at 0.5e/s intervals).
(2) Specifically put how much load beyond the zero tracking range
R76, at the time of the test in question, required a load of 10e to be placed beyond the zero tracking range. Why not 5e loads, why not 2e loads?
Although in the international recommendations and our relevant regulations are clearly stipulated that the correction rate of zero tracking device must be “0.5e/s”, but many weighing instrument manufacturers, in the instrument factory did not set the correction rate of zero tracking device at this point. Even some weighing instrument manufacturers, set in the maximum correction rate (currently see the maximum correction rate of 6e/s).
2. Zero accuracy check
If the weighing instrument does not have zero tracking function, or there is a special switch to close the zero tracking device, in the detection of “zero accuracy” and “zero error”, there is no need to put additional load (10e). The problem is that most of the weighing instruments in China are not equipped with a switch that can close the zero tracking device, and they all have the zero tracking function, so in order to get the zero error, we have to put an additional load (10e) to make it go beyond the zero tracking range when the scale is unloaded, so that we can get the accuracy of the zero setting “near zero” and the “zero error”. This results in a “near zero” zeroing accuracy. Sequentially place 0.1e additional weights until the value increases significantly by one division (I+e), and the total of the additional weights is ∆L, so that the zeroing error is: E0=10e+0.5e-∆L-10e=0.5e-∆L≤±0.25e. If the total of the additional weights is 0.4e, then: E0=0.5e-0.4e=0.1e<±0.25e. .
3. Meaning of determining zeroing accuracy
The purpose of determining the accuracy of zero setting is to ensure that the calculation of the “correction error before correction” is completed in the calibration process. When checking the accuracy of a scale, the pre-correction error can be obtained by the formula: E=I+0.5e-∆L-L. In order to know more accurately the error at the specific weighing point of the scale, it is necessary to correct it by the zero point error, i.e.: Ec=E-E0≤MPE.
After correcting the error of the weighing point by the error of the zero point, it is possible to correct the value that slightly exceeds the maximum permissible error as qualified, or to correct the value that seems to be within the qualified range as unqualified. However, regardless of whether the correction is qualified or unqualified, the purpose of using zero point error corrected data is to make the test results closer to the true accuracy of the scale.
4. Zero Error Determination
First of all, the calibration should determine the zero point error of the scale in this way: before removing all the load from the load carrier of the scale, it is necessary to put a load of 10e on the load carrier, then remove the load from the load carrier, and put 0.1e additional weights in order until the value is obviously increased by one division (I+e), and the accumulation of the additional weights is ∆L, then determine the zero point error according to the method of the flashing point, E=10e+0.5 E=10e+0.5e-∆L-10e=0.5e-∆L≤±0.5e. If the additional weight accumulates to 0.8e, then: E0=0.5e-0.8e=-0.3e<±0.5e.
Post time: Aug-14-2023