There is a major difference between the additional alarm function of the instrument and the insurance function, because the circuit of the additional alarm function part has many common points with the display, power supply and sensor of the instrument, and the failure of a common place may directly lead to It is necessary to emphasize the loss of alarm function. Therefore, for a system that will cause major consequences in case of loss of control, the additional alarm function and insurance function of the instrument must be set up in accordance with the regulations. There is a major difference, because the circuit of the additional alarm function part and the display, power supply and sensor of the instrument have many Common areas, failure of a common area, may directly lead to the loss of the alarm function, this point must be emphasized. Therefore, an independent protection device must be installed in accordance with regulations for a system that will cause major consequences if it loses control. If you want to achieve a more reliable, insurance-like alarm function, you should use bimetal or pressure instruments for protection, or use a complete set of electronic alarms from the sensor to the actuator. The instruments dedicated to electronic alarms have relatively simple functions and structures. Relatively speaking, their reliability will not be lower than that of more complicated display adjustment instruments. Most display adjustment instruments can be equipped with additional alarm functions. The additional alarm function is mainly realized in the following ways: 1. Unsettable upper limit follow-up alarm function. This function refers to “make an alarm when the input value reaches the set value + full-scale direction-fixed value”. Take XMTA 2311 0～300℃ meter as an example. The standard stipulates that when the input value reaches the set value + (4%～6%)×300, an alarm action should be made. If the setting value of the meter is 200℃, then the meter should be The input value +12℃～+18℃+the allowable setting error of the instrument is (212～218)±3℃, the alarm action will be made. The default alarm action is that the normally open contact of the alarm output relay is closed. 2. Non-settable lower limit follow-up alarm function. This function refers to 'alarm action when the input value reaches the set value—start range direction—fixed value.' Take the XMTA 2311 0～300℃ instrument as an example. The standard stipulates that when the input value reaches the set value—(4%～6%)×300, an alarm action should be made. If the set value of the instrument is 200℃, then the instrument should be When the input value is -12℃～-18℃+the allowable setting error of the instrument is (182～188)±3℃, the alarm action will be made. The default alarm action is that the normally open contact of the alarm output relay is closed. 3. The alarm function that can set the alarm value This function refers to the alarm action when the input value reaches the value that people deliberately set the input on the letter panel, and it is the upper limit, upper limit, lower limit, and lower limit alarm. The default alarm action is to close the normally open contact of the alarm output relay (such as a smart meter). 4. The meter can also use a buzzer as a sound alarm output (special order). In order to improve the anti-interference ability as much as possible, the position difference of the alarm position is chosen to be larger. Generally, it is about 1% to 5% of the full scale. 5. The three-position instrument is used for alarming. Under normal circumstances, it is not advisable to use one of the three-position instruments as an alarm. If the upper limit control of a three-position instrument is used as an upper limit alarm, the action during the alarm is that the normally closed contact is closed again, that is, the instrument is in the alarm state when it is powered on, and the default alarm action will be reversed. This is different from the instrument specially designed for alarm. If the upper limit alarm of the three-digit instrument is used as the lower limit alarm or the lower limit alarm is used as the upper limit alarm, it is possible that the alarm action will be made by mistake when the instrument is powered on.
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