1. Overview
The "tare removal device" is a device that sets the indication to zero when there is a load on the load carrier. According to the function, it can be divided into: non-automatic tare removal device, semi-automatic tare removal device and automatic tare removal device.
In my country's old weighing instrument product standards and metrological verification regulations, there is no provision for the need to test the "tare removal device" for weighing instrument products. It was not until the formulation of the "stationary electronic scale" metrological verification regulations in 1990 that this requirement was added based on the relevant content of the "tare removal device" international recommendation R3 "Non-automatic weighing instrument metrological regulations" and after separate tests by the manufacturing unit and the verification agency.
The accuracy index of the tare removal device is a relatively demanding assessment requirement. In the currently implemented JJG555-96 "Non-automatic weighing general verification regulations", since the size of the "tare value" is not specified, it only states that "at least 2 different tare weights should be weighed and tested", and the operating range is "the tare removal device shall not be used at or below zero and above the maximum tare removal amount". So if the selected tare value can be 90% of the maximum weighing capacity, that is, within the range from zero to 90% of the maximum weighing capacity, it is also possible; the tare value can also be selected as 10% of the maximum weighing capacity. This brings two problems: First, after each tare, the net weight zero point accuracy must be maintained at ±0.25e;
Second, after each tare, the allowable error system must be established according to the new zero point, as shown in Figure 1.
2. Discussion of Several Issues
In the 2006 version of R76-1 "Non-automatic Weighing Instruments", it is stipulated that the tare device in the electronic weighing instrument is required: the weighing instrument should be subjected to tare weighing test under the following circumstances:
Weighing instrument with deducted tare: use a tare value between 1/3 and 2/3 of the maximum tare; Weighing instrument with added tare: use two tare values of 1/3 and 3/3 of the maximum tare (since most of the non-automatic weighing instruments we are facing now are "deducted tare weighing instruments", so the relevant issues of "added tare weighing instruments" are not involved in this article).
— 1 — 1. Determine the tare value
What tare value should be selected when performing a "tare removal" test? Theoretically, the "tare value" can be arbitrarily selected within the entire weighing range. If this is done, although it should not affect the weighing accuracy of a properly designed, manufactured, and installed weighing instrument, it will inevitably affect the scope of use of the weighing instrument.