I. Overview
In the International Recommendation R76-1 "Non-Automatic Weighing Instruments", zero point and zero setting are regarded as very important issues. Not only are metrological requirements put forward for them, but also technical requirements are stipulated, because the stability of the zero point of any weighing instrument is the basic guarantee of its metrological performance. The following terms are all closely related to the zero point. We will explain and analyze them one by one.
Indication error: The difference between the indicated value of a weighing instrument and the true value of the corresponding mass (agreed).
2. Maximum allowable error: For weighing instruments that are at the reference position and zero under no-load conditions, the recommended maximum positive or negative difference between the indicated value and the corresponding true value determined by the reference standard mass or weights.
3. Zeroing device: A device that sets the reading to zero when there is no load on the carrier. For electronic weighing instruments, they include: semi-automatic zeroing devices, automatic zeroing devices, initial zeroing devices, and zero tracking devices.
4. Zero setting accuracy: After the weighing instrument is set to zero, the influence of zero point error on the weighing result is within ±0.25e.
5. Zero-point error: It refers to the indication error of the zero point of the weighing instrument after unloading. The maximum allowable error during the first verification is within ±0.5e.
6. Zero-point tracking device: A device that automatically maintains the zero-point indication within a certain range. The zero-point tracking device is an automatic zeroing device. The zero-point tracking device can have four states: none, not running, running, and out of working range.
The zero-point tracking device is only allowed to operate under the following circumstances:
-- The indicated value is zero, or equivalent to a negative net weight value when the gross weight is zero;
And the balance is stable;
The correction amount shall not exceed 0.5e/s.
Ii. Test Methods
Zero-point tracking device
As the vast majority of electronic weighing instruments in our country are equipped with zero-point tracking devices at present, when it is necessary to test the zero-point error, it must be ensured that the zero-point tracking is not in operation. Then, the only way for the zero-point tracking device to "not operate" is to place a certain weight of load near the zero point, causing the zero-point tracking to exceed its working range.
2 (1) Determine the correction rate of the zero-point tracking device
As there is no method for determining the zero-point tracking correction rate in relevant standards and verification regulations, it has been found that some manufacturers take shortcuts in this regard, consciously increasing the correction rate to make the zero return speed of the weighing instrument faster, indicating that the product quality is excellent. For this reason, the author has summarized a method in practical work that can quickly check the zero-point tracking rate of weighing instruments on site.
Connect the power supply and let it stabilize for at least 30 minutes. Then, apply a load of 10e on the load-bearing device to make the "zero point tracking" of the weighing instrument exceed the working range. Every 2 seconds or so, gently apply a load of 0.3e and observe the reading.
After continuously adding three 0.3e loads, the reading of the weighing instrument significantly increased by one graduation value, indicating that the device was not in operation or was not functioning.
If the weighing instrument does not show a significant change in its indication after adding three 0.3e loads, it indicates that the device is still in operation and the tracking correction is within 0.5e/s.
Then, gently remove the three 0.3e loads. The reading of the weighing instrument should significantly decrease by one division value.
Why are three 0.3e loads used? The correction rate for a 0.3e load less than 0.5e/s; The three 0.3e loads are greater than 0.5e/s and less than the correction rate of 1e/s (because the correction rate is required to increase at intervals no greater than 0.5e/s).
(2) The specific amount of load added exceeds the zero-point tracking range
When conducting relevant tests on R76, it is required to apply a load of 10e to exceed the zero-point tracking range. Why not a 5e load? Why not a 2e load?
Adding a load of 10e beyond the zero-point tracking range is based on a correction of no more than 0.5e/s, which may be related to the duration of the operation. If the correction amount is 0.5e/s, it will take 20 seconds to achieve a tracking range of 10e, 10 seconds to reach a tracking range of 5e, and 5 seconds to reach a tracking range of 2e. Although both international recommendations and relevant regulations in our country clearly stipulate that the correction rate of the zero-point tracking device must be "no more than 0.5e/s", many manufacturers of weighing instruments do not set the correction rate of the zero-point tracking device at this point when the instruments leave the factory. Even some weighing instrument manufacturing enterprises set it at the maximum correction rate (the maximum correction rate currently seen is 6e/s).
2. Zero setting accuracy verification
If the weighing instrument does not have a zero-point tracking function or there is a dedicated switch to turn off the zero-point tracking device, there is no need to add an additional load (10e) when detecting "zero setting accuracy" and "zero-point error". The problem is that most of the weighing instruments in our country are not equipped with this switch that can turn off the zero-point tracking device, and they all have the zero-point tracking function. Therefore, in order to obtain the error at zero point, it is inevitable to add an additional load (10e) when the weighing instrument is unloaded to make it exceed the zero-point tracking working range. In this way, a zeroing accuracy near zero point is obtained. Add 0.1e additional weights in sequence until the indicated value significantly increases by one division value (I+e), and the cumulative value of the additional weights is ΔL