Discussion on Calibration Method Without Weight

[Summary] This article mainly introduces our knowledge and practice of weightless calibration of weighing instruments;

[Keywords] Analog sensor, digital weighing system, weight-free calibration;

0:Preface

The calibration of weighing instruments #weighingindicator with unconventional shapes of load-bearing devices has always been a major problem that plagued metrology technical institutions. Especially in the metallurgical industry, the reaction is more obvious, and there are many weighing instruments that are difficult to calibrate with weights. For example, for tank scales on platforms with a height of tens of meters to tens of meters, not only the weights are difficult to transport in place, but also weights cannot be placed on the hopper and tank-type carrier. To the point where implementation is impossible, weight-free is produced. Problems with calibrating weighing instruments.

In the late 1990s, we were inspired by the calibration of the thermal instrument temperature measurement system, imitating the calibration method, and calibrating the hopper scale under the high-position silo without weights. In order to meet the urgent need of production.

In recent years, the digital weighing system with digital sensors as the core has made the weight-free calibration method simpler and more accurate than before. But unfortunately, it is still difficult to get universal recognition from users (non-metering personnel). To this end, write down our knowledge and practice of the weight-free calibration method. As a brick for inducing jade, we hope to arouse colleagues' discussion and enlightenment, promote the development of the weight-free calibration method, and play its due role.

1. Feasibility analysis of weight-free calibration

Metrological verification staff understand that the measurement signal process of a temperature measuring instrument is that the temperature sensor converts the heat in the temperature field into an electrical signal (mV level) and sends it to the temperature measuring instrument to display the output temperature value (see Figure 1). The weighing signal process of the weighing instrument is that the load cell transforms the gravity of the material into an electric signal (mV level or pulse sequence), and sends it to the weighing instrument to display and output the weighing value (see Figure 2).

Figure 2 Weighing signal flow

 The calibration method of thermal temperature measurement system is usually to take the temperature sensor offline, send it to the laboratory for test verification, and attach a formal certificate (report) of the test data. For temperature measurement and display instruments, go to the production site and use the potentiometer (UJ36 or UJ31) to input the electrical signal corresponding to the temperature to the temperature measurement instrument according to the configured temperature sensor data that has been tested and verified to complete the calibration of the temperature measurement system .

Weighing and temperature measurement mainly use two different sensors to convert two different physical quantities (weight, heat) into electricity, and then perform a series of processing to display and output the corresponding weight and heat value. Comparing the signal flow of weighing and temperature measurement, the two are basically the same, and there is no essential difference. After the non-electricity is converted into electric quantity, the electric quantity is measured.

The load cell is also manufactured after passing the standard press of the corresponding level and passing the test. Therefore, for weighing instruments that cannot be calibrated with weights, it is feasible to imitate the calibration method of the temperature measurement system and perform weight-free calibration.

2. Necessary conditions for weight-free calibration

The method of calibrating weighing instruments without weights proposed by us is mainly aimed at non-commercial weighing instruments used in the steel production process, such as ladle scales, hopper scales, and tank scales that are difficult or impossible to calibrate with weights. But the following conditions must be met:

① Calibration without weights is a "helpless move" when it is difficult or impossible for a weighing instrument to calibrate with weights, but calibration is necessary. Generally implemented after the user takes the initiative to request. It is also necessary to inform both the supplier and the buyer in advance about the pros and cons of weight-free calibration. It is appropriate to proceed only after obtaining approval from both the supplier and the demander, especially the demander. In this way, the risk of the inspection unit itself can be avoided.

②As the weight-free calibration is mainly carried out based on the factory calibration data of the load cell. Therefore, the weighing sensor (weighing module) required for the weighing instrument must not only have a sufficiently high accuracy (the number of divisions must be greater than or equal to the number of divisions of the weighing instrument, that is, nLC ≥n), but also must have good stability and reliability . In particular, a formal calibration test report with complete technical parameters must be attached.

③Because of the weight-free calibration, the error of the mechanical force transmission structure system involved in the case of directly loading the calibration with the weight is ignored. Therefore, the force transmission structure of the weighing instrument is required to be simple, adopt standardized connecting components, and the load carrier for loading materials is directly installed on the weighing module to ensure that the weight of the materials is completely and accurately transmitted to the load cell.

④The no-load zero point of the weighing instrument is stable. Before calibration, at least three on-site inspections of loading and venting tests are required to confirm.

3. Introduction to weight-free calibration method

Whether it is an analog weighing system or a digital weighing system. Weight-free calibration should be carried out on the premise that the scale itself and the installation quality are good.

①Analog sensor

It mainly refers to the calibration method adopted by thermal instruments, and uses digital voltmeter and potentiometer to complete the calibration of the scale on site. For the analog load cell system, try to choose high-precision measuring instruments and instruments that generate mV signals.

The steps are as follows:

a. Check whether the force transmission mechanism, limit device, and sensor meet the requirements and are in a normal state.

b. Check and confirm the basic parameters of the input scale Max, n, d, etc.

c. Check the stability of the no-load zero point of the scale. After the zero point of the scale is determined, measure and record the mV signal value corresponding to the “0” point of the scale.

d. According to the capacity, quantity, sensitivity (average value of multiple sensors) of the scale, the bridge supply voltage and the maximum weighing capacity of the scale, the mV value corresponding to Max is calculated by the following formula. Taking into account the attenuation of the signal loss caused by the wiring and junction box resistance network, the calculation result is increased by about three ten thousandths.

S×V×Max / C×N

Where:

S: Sensitivity V: Supply bridge voltage Max: Maximum scale

C: Sensor capacity N: Number of sensors

e. Input the sum of the mV value corresponding to Max and the mV value corresponding to the “0” point of the scale to the meter for maximum weighing value calibration.

f. If possible, try to do a comparative verification test. 

②Digital load cell

In the digital weighing system with digital sensors as the core, analog quantities become digital quantities. The connection between the scale's instrument and sensor is no longer an analog signal, but digital communication. The meter receives the digital signal (pulse sequence) sent by the sensor. Each sensor has its own address, and the instrument can directly obtain the original output signal of each sensor through the RS-485 communication interface. The digital sensor has been calibrated with a standard force measuring machine in the manufacturing process, and its output corresponds to the standard value. There is basically no loss in digital signal transmission, and the read value is the actual measured value, which is simple and accurate compared to the analog weighing system.

The meters of the digital weighing system all have the function of weight-free calibration. As long as you enter the state of debugging and calibration, press the calibration function key (calFREE) to enter the weight-free calibration interface. Open the menu, select the weight-free calibration, press the OK key, the meter will automatically read the parameters from the digital sensor. According to the operation manual, you can easily complete the calibration without weights. Of course, the mechanical structure system of the weighing instrument must be good and meet the above necessary conditions to achieve good results.

4. Problems and suggestions for weight-free calibration

For many years, under the condition of conditions, we try our best to do the comparison and verification of the real objects (weights or recognized weights) after calibration without weights. The results are:

Analog weighing system, calibration accuracy without weight is 0.5%～1.0%;

The calibration accuracy of digital weighing system without weight calibration is 0.4%～0.1%;

The recent comparison verification two examples are as follows:

Example 1 A scrap steel scale in Tongling

Max＝60t、d＝e＝20kg, 4 Pcs SLC820-50t digital load cell、

A12 Weighing indicator  

After weight-free calibration, a comparative verification test is performed with a known weight (30t) of scrap steel ingots. The results are:

Because production does not allow more tests, the repeatability error of three measurements is zero.

The relative error is: (30000-29980)÷30000=6.6×10-4, which is about seven ten thousandths, better than one thousandth. The user is very satisfied.

Example 2 Select a truck scale (Max = 100t, e = d = 50kg, 8 SLC820-50t digital load cells, IND880 weighing instrument) that has reached the verification period of Maanshan Iron and Steel, and do the test verification of the effect of weightless calibration . After routine maintenance of truck scales, weight-free calibration was implemented. Then use the check weighing cart and weights to perform follow-up verification in accordance with the requirements of the medium-accuracy scale. The verification results: eccentric load, repeatability, weighing test error are less than or equal to the maximum allowable error.

The above two examples show that as long as the digital weighing instrument is of good quality and installation quality,

The accuracy of weightless calibration can reach the level of 0.1%.

However, the comparative verification after weightless calibration that we have done is only limited to weighing instruments that can easily place weights (fixed loads). However, weighing instruments that can place weights (fixed loads) generally do not use weight-free calibration. However, a weighing instrument that needs to be calibrated without weights cannot be placed with a weight (fixed load) for comparison verification after weightless calibration. Therefore, the current weight-free calibration is not strong enough, and its effectiveness is often questioned, and it is difficult to get the general recognition of users (non-metric verification personnel), which is the biggest problem.

The internal code automatic calibration function commonly used in electronic balances is similar to the weightless calibration of weighing instruments. Why is it easy to be recognized and used? Obviously because of electronic balances, weights can be conveniently used for comparison and verification at any time.

The weighing indicator measures materials, and it is calibrated with physical weights, which is reasonable, legal and reasonable. In order for users to see and touch, they can intuitively see the effectiveness of weightless calibration. It is recommended that the weighing instrument manufacturer provide a "demonstration scale" for demonstration of comparison and verification of weights (a few tons of weights can be borrowed locally) after on-site calibration without weights. Seeing is believing, so that everyone can see with their own eyes how the effect of weightless calibration is, and how big the error is!

5. Conclusion

Work practice over the years has shown that weight-free calibration is a cost-effective calibration method for weighing instruments used in storage tanks, containers, hoppers, and silos where weight calibration cannot be performed. . We should formulate operating instructions for weight-free calibration in a timely manner, standardize the process of weight-free calibration, and ensure the quality of calibration. All parties work together to make it play its due role.