What role does a multimeter play in the measurement of orifice plate flow meters?

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A necessary tool for a qualified after-sales technician is a multimeter, which can be used to measure the resistance values of orifice plate flow meters and differential pressure transmitters, as well as to determine whether they are powered on. It can identify any issues that may exist. Below, Shanghai Automation Instrument Co., Ltd. will provide you with a brief introduction to the functions and features of a multimeter, as well as its role in orifice plate flow meters. I. Multimeter usage skills: A multimeter can be used to measure low-level DC voltage, DC current, AC voltage, resistance, etc. It can also measure AC current, decibels, current, capacitance, transistor parameters, and more. Multimeters come in both analog and digital types. Pointer multimeters have a simple structure and low cost. A digital multimeter was developed based on the analog multimeter, and it is easy to use. II. Usage of the pointer-type multimeter: The usage guide should be placed flat on the working surface. Before use, check whether the pointer is at the zero mark on the far left; if not, use a small screwdriver to turn the adjustment screw at the center of the dial in order to bring the pointer to zero. Before making measurements, check the insertion position of the test probes: the red probe should be connected to the red “+” terminal, and the black probe should be connected to the black “–” terminal. Turn the range selector switch to the corresponding range. For a meter with two knob switches, first turn the knob to select the switch for the item to be measured, and then turn the knob to select the switch for the appropriate range. When measuring resistance, turn the range selector switch to the ohm setting corresponding to the value to be measured, short-circuit the two test leads, and the pointer should point to the zero mark at the right end. If it is not at zero, adjust the zero adjustment knob until the pointer indicates zero. If the orifice plate flow meter does not display any readings, it is possible to check whether power is being supplied by looking at the pointers at both ends, as well as between the differential pressure transmitter and the orifice plate flange. If there is no power supply, then other instruments should be checked. 3. Precautions for use: ① It is important to develop a good habit when using a multimeter – namely, to check once again whether the correct setting has been selected before each measurement. By developing such a *habit, the probability of burning out the meter will **decrease**. ②The range should be selected so that the pointer indicates around two-thirds of the full scale, for more accurate readings. If the values of the voltage and current to be measured are unknown, a high-range setting should be selected; then, based on the reading obtained, the range should be adjusted again to ensure accurate readings. ③When measuring resistance, it is necessary to reset the zero setting after changing the range setting in order to obtain accurate readings. If it cannot be adjusted to zero, it indicates that the battery voltage in the device is insufficient, and the battery needs to be replaced. ④When measuring resistance online, the power supply should be turned off before proceeding with the measurement. It is also necessary to check whether there are any other components that form a parallel circuit with the resistor being measured; if needed, one end of the resistor can be soldered off from the circuit before taking the measurement. For circuits with electrolytic capacitors, the capacitor must be fully discharged before measurement. ⑤When using a multimeter, it is important to develop the habit of not touching the metal parts of the test leads to prevent electric shock. Additionally, if the hands touch the metal parts of the test leads while measuring resistance, it will affect the readings. ⑥When measuring DC voltage, connect the red pen to the “+” terminal and the black pen to the “–” terminal, in order to prevent reverse polarity from causing the pointer to deflect in the wrong direction and thus damaging it. When the positive and negative polarities are unknown, start by setting the meter to the high-range setting; then quickly touch the point being tested with the test leads, and determine the correct polarity by observing the direction in which the pointer moves. In particular, the mA or resistance setting must not be used to measure voltage, as this can damage the instrument. ⑦When measuring current, it is absolutely forbidden to connect the two probes across the power supply, to avoid damaging the meter gauge. Generally, a multimeter can only measure direct current and cannot measure alternating current. ⑧After using the meter, set the range switch to the highest voltage setting. For multimeters that have a short-circuit or open-circuit setting, use that appropriate setting as well, to prevent damage to the instrument if someone else uses it carelessly. There are many types of multimeters, each with a different structure and varying methods of use. However, once one understands the basic principles of their use and refers to the instructions provided for that particular multimeter, it becomes possible to use all types of multimeters. IV. How to Use a Digital Multimeter Preparation Before using a digital multimeter, it is necessary to be familiar with the functions of the power switch, range selector, sockets, special sockets (such as jacks, Cx sockets, etc.), and knobs, as well as how to replace batteries and fuses. Understand the instrument overload indicator symbols, polarity indicator symbols, etc. Due to the high input impedance of digital multimeters, when the test leads are open-circuited, external interference signals may cause the meter to display numbers that follow a certain pattern, which is a normal phenomenon. Although digital multimeters have overvoltage and overcurrent protection, it is still necessary to prevent damage to the instrument due to accidental operation. Before each measurement, the position of the range switch should be checked; measurement can only be carried out once it is confirmed to be correct. For digital multimeters with automatic range selection, it is also important to make sure that the function switches and input jacks are placed in the correct positions. V. Precautions for use: ① When it is not possible to predict the magnitude of the voltage or current being measured, start by setting the instrument to the highest range for a measurement, and then gradually reduce the range to an appropriate value as needed. After the measurement is complete, turn the range switch to the highest voltage setting and turn off the power supply. ②Thanks to its function of automatically switching polarity, a digital multimeter allows one to measure DC voltage without having to consider the positive and negative poles. However, if the AC voltage range is used to measure DC voltage, or vice versa, an overflow symbol “000” will be displayed. ③When measuring AC voltage, the black test lead should be connected to the analog ground COM, as well as to the lower potential end of the voltage being measured, such as the common terminal of a signal source or the chassis, in order to reduce measurement errors. ④It is strictly prohibited to turn the range switch while measuring high voltage or large currents, in order to prevent arcing and the burning out of the switch contacts. When measuring higher voltages, it should be done with one hand: first fix the black test lead to the common terminal of the circuit being tested, and then use the red test lead to touch the test point in order to ensure safety. ⑤The lowest voltage range of most multimeters is 200 mV or 400 mV, with a voltage resolution of 0.1 mV. When the voltage under measurement is less than 2 mV, the measurement error increases, and this requires special attention when measuring the thermoelectric potential of thermocouples. ⑥The input resistance of the DC voltage range on a digital multimeter is relatively high, typically 10 MO. When measuring signal voltages with high internal resistance, the measurement error is very small and can be disregarded. However, when measuring signals with an input resistance greater than 10 MQ (such as at the input of a field-effect transistor), the effect of the input resistance must be taken into account. The bypassing effect of the input resistance alters the operating condition of the original circuit, resulting in significant measurement errors. ⑦When measuring current, if both the internal resistance of the power supply and the load resistance are low, it is advisable to choose a larger current range in order to reduce the value of the shunt resistor, minimize the voltage drop across it, and improve the accuracy of the measurement. ⑧On the resistance setting, diode setting, and when testing continuity, the red test lead carries a positive charge while the black test lead carries a negative charge – this is the opposite of what happens with analog multimeters. On the resistance setting of an analog multimeter, the red test lead is connected to the negative pole of the battery inside the meter; therefore, it carries a negative charge ； The black test lead is connected to the positive terminal of the battery, and therefore carries a positive charge. When measuring polar components such as transistors and electrolytic capacitors, it is necessary to pay attention to the polarity of the two test leads. ⑨Since the maximum test current for each resistance range is not the same, the lower the range, the greater the current. It is normal for the resistance values measured using different resistance ranges for the same nonlinear component to vary. The resistance setting of a multimeter can supply only a very small test current, so it cannot be used to test transistors with this setting. ⑩When measuring components on a circuit board, the influence of other components connected in parallel with it must be taken into account. If necessary, one end of the component under test can be soldered off for measurement; for bipolar transistors, two electrodes must be soldered apart in order to conduct testing. When measuring resistance, both hands should hold the insulated handles of the test leads to avoid measurement errors caused by the parallel connection of the human body’s equivalent resistance.