How to Select Digital Weighing Indicator

Digital weighing display is an instrument that provides DC excitation voltage for weighing sensor, amplifies millivolt signal and converts it into digital output. In addition, in order to meet the needs of modern technology, digital weighing display can be attached with many other functions.

Most digital weighing displays are designed to provide 10-15v DC voltage to excite up to eight 350 Ω weighing sensors. However, some displays are limited to four 350 Ω load cells. This paper will discuss what kind of display should be selected when the truck scale with six load cells and the tank truck scale or bucket scale with four load cells must be used.

Usually, when users get used to one type of display, they want to use it for all scales. Other types of displays are often ignored, unless a standard display fails to meet certain requirements.

The following technical parameters should be considered when selecting the best or most suitable display.

1. Accuracy

Some manufacturers have specified the nonlinearity of their displays as ± 0.01%. Others do not mention accuracy at all. Generally, the internal division range of the display is from 50000 to 130000. Generally, the external display division is limited to 10000. This is the result of the new ntep regulation. Except for some automatic control processes, most displays do not require more than 10000 divisions (see Figure 2). In order to meet the needs of automatic control process, a display with 50.000 divisions is developed.

2. Calibration procedure

Some monitors first use the internal dip switch for coarse adjustment of zero and full scale, and then use the key on the front panel for fine adjustment. The traditional calibration method of the old monitor is to use dip switch to adjust roughly, and then use potentiometer to adjust carefully. The latest type of display does not need to use dip switch and potentiometer calibration, just use the key on the front panel of the instrument for full digital calibration.

Usually, there is mutual influence between zero setting and range adjustment. This will result in many times of loading and unloading before the final calibration of the system. For the tank truck scale and funnel scale, it is very troublesome and expensive. Therefore, it is suggested to select the display which does not affect each other between the zero point and the measuring range.

3. Temperature coefficient

When the display is in the room and the temperature changes little, this factor can often be ignored. However, when the monitor is outdoors or the temperature fluctuates greatly, this factor must be seriously considered.

Generally, the coefficient of digital display is 5-7 ppm / ℃. However, some displays are not specified, while others are ± 0.002% F.S. / ° F, that is, 36ppm / ℃. Therefore, if this kind of display is used outdoors, such as truck scale, in the scale shed with heating equipment, the temperature may change from - 40 ° f in winter to + 70 ° f in summer, and there may be 132 pounds of display deviation for 60000 pounds of load. Under the same conditions, the deviation of other displays is only 18.3 pounds.

4. Interface

When the monitor is connected with other devices (such as printer, remote monitor, computer, recorder, etc.), this performance should be considered. Some monitors have standard serial interface with RS232C or 20mA current loop, while others have no standard interface, but have field installation position suitable for serial or BCD interface. Because of the small printed circuit board, this kind of installation becomes very easy. Its advantage is that the cost of basic unit is very low. One monitor has a standard RS232C interface and six other options including an extended serial line. This adds another bidirectional RS232C port and a 20mA current loop serial output. With the two-way RS232C port connected to the computer, a series of commands can simulate the key of the front panel, transmit the weight value and change the set point remotely. 20mA current loop can be set actively or passively to provide long-distance communication. In addition, with other functions in the display, there can be 4-20mA analog output, BCD output and up to 8 set points.

5. Number of data updates per second

Some monitors update data four times a second, which gives a steady reading to a typical scale, but it is not suitable for automatic processes. Other displays that update data 20 times per second can be used for process control. There is a display that uses a tab to select three or ten updates per second. According to the manufacturer, 10 times for less than 3000 display graduations, and three times for 3000 to 10000 display graduations.

6. Choose the right monitor

This paper attempts to describe the technical conditions related to the selection of accurate displays in practical work. It's important to pay attention to the other types of displays available when you have the right one.

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