Basic knowledge of the hottest instruments and met

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Basic knowledge of instruments

1. Atmospheric pressure: the pressure produced by gravity on the air column on the earth's surface. It is related to altitude, latitude and meteorological conditions

2. Differential pressure (differential pressure): the relative difference between two pressures

3. Absolute pressure: all pressures in the space where the medium (liquid, gas or steam) is located

absolute pressure is the pressure relative to zero pressure

4. Gauge pressure (relative pressure): if the difference between absolute pressure and atmospheric pressure is a positive value, then this positive value is gauge pressure, that is, gauge pressure = absolute pressure - atmospheric pressure 0

5. Negative pressure (vacuum gauge pressure): corresponding to the gauge pressure, if the difference between absolute pressure and atmospheric pressure is a negative value, then this negative value is negative pressure, that is, negative pressure = absolute pressure - atmospheric pressure 0

6. Static pressure: generally understood as the pressure that does not change with time, or the pressure that changes slowly with time, that is, the gauge pressure value measured in the fluid that is not affected by the flow rate

7. Dynamic pressure: corresponding to static pressure, the pressure that changes rapidly with time, that is, dynamic pressure refers to the kinetic energy of a unit volume of fluid. It is usually calculated with 1/2. Where fluid density; V velocity of fluid movement

What are the similarities and differences between HART protocol and fieldbus technology

Both Hart and fieldbus technology can realize remote access to the status and parameters of field devices. At the same time, both technologies support the connection mode of connecting multiple devices on one bus. Both Hart and Fieldbus adopt device description to realize the interoperability and comprehensive application of devices. Therefore, there are certain similarities between them

the differences between them are as follows:

1) the fieldbus adopts real all digital communication, while Hart is superimposed on the original 4 ~ 20mA analog signal in the form of FSK, so it can be directly connected to the existing DCS system without reconfiguration

2) fieldbus mostly adopts multipoint connection, and HART protocol generally adopts multipoint connection only when it is used for monitoring

3) in the control system composed of fieldbus, the equipment can communicate directly without the intervention of the host

4) compared with HART devices, fieldbus devices can provide more diagnostic information

therefore, fieldbus devices are suitable for high-speed network control systems, and the advantages of Hart devices are reflected in their compatibility with existing analog systems

what are the advantages of intelligent pressure/differential pressure transmitter over analog transmitter

the advantages of intelligent instruments mainly include:

simplifying the adjustment process of instrument manufacturing process, compensating sensor defects (such as linearization, environmental factor compensation, etc.), improving instrument performance, reducing manufacturing costs, and forming multi parameter composite instruments

simplify the installation and commissioning process (such as line alignment and zeroing) and reduce the installation and commissioning cost

improve the measurement quality of the instrument operation process, facilitate the soft measurement, facilitate the maintenance and verification of the instrument and asset management (need the support of system and equipment management software)

what are the selection principles of pressure/differential pressure transmitter

the selection of pressure/differential pressure transmitter is mainly based on the property index of the measured medium and the reference of saving money and facilitating installation and maintenance. If the measured medium is high viscosity, easy to crystallize and strong corrosion, the isolated transmitter must be selected

the corrosion of its medium to the capsule metal should be considered when selecting the type, and the capsule material must be selected. The capsule material of the transmitter includes ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum capsule material, etc

the temperature of the measured medium should be considered when selecting the type. If the temperature is high, it is generally 200 ℃ ~ 400 ℃, the high temperature type should be selected, otherwise the silicone oil will produce vaporization and expansion, making the measurement inaccurate

the working pressure grade of the equipment should be considered when selecting the type, and the pressure grade of the transmitter must be consistent with the application. In terms of the measurement range of the transmitter, generally, the transmitter has a certain range that can be adjusted. It is best to set the range used at 1/4 ~ 3/4 of its range, so that the accuracy can be guaranteed, which is more important for micro differential pressure transmitter. In practice, some applications (liquid level measurement) need to migrate the measurement range of the transmitter. The measurement range and migration amount are calculated according to the on-site installation position. Migration can be divided into positive migration and negative migration

why is the transmitter output fixed at 20.8ma? How to solve it

the transmitter output is fixed at 20.8ma, which means that the current main process variable is greater than the upper limit of the sensor's set range, and the instrument is in the output saturation state. The following checks can be carried out:

1) check whether the set upper range of the sensor or the limit range of the sensor is greater than or equal to the current measured signal, and determine the correctness of the selected sensor model and the set range

2) check whether there is leakage or blockage in the impulse pipe. If the impulse valve is used, check whether the valve is fully opened

3) confirm that the introduced measured signal is a stable input; If the measured liquid, confirm that there is no residual gas; If the measured object is dry gas, confirm that there is no liquid

4) check whether there is precipitation on the sensor flange and whether the flange is corroded

5) if it is a remote flange transmitter, check whether there is a potential difference between the two measured signals, and calculate whether the differential pressure caused by the potential difference is greater than the sensor range

6) check whether the power supply is between 12V and 24VDC

7) use the handheld operator to carry out self-test and parameter reading on the instrument to check whether the intelligent electronic components are faulty or uninitialized

what work does the transmitter maintenance include

the maintenance of the transmitter mainly includes the following aspects:

1) patrol inspection:

instrument indication and whether the instrument indication is abnormal

whether the air source pressure of the pneumatic transmitter is normal

whether the power supply voltage of the electric transmitter is normal

ambient temperature, humidity and cleanliness; Whether there is leakage and corrosion between instrument and process interface, impulse pipe and valve

2) regular maintenance:

check the zero point regularly and verify it regularly

regularly discharge sewage, condensate and vent

regularly purge the impulse pipe of the easily blocked medium and fill the isolation fluid regularly

3) major equipment inspection:

check the use quality of the instrument to achieve accuracy and sensitivity, the indication error and static pressure error meet the requirements, and the zero position is correct

the instrument parts are intact, free of serious rust and damage, the nameplate is clear and correct, the fasteners must not be loose, the connectors are in good contact, and the terminal wiring is firm; The technical data are complete, accurate and meet the management requirements

what is the concept of the working differential pressure range of the mass flow controller

a gas flow regulating valve is set in the mass flow controller (MFC), which can adjust the flow through the controller from zero to the full range of measurement. During the working process, there will be a pressure drop between the inlet and outlet of the controller, that is, the pressure difference. The working differential pressure range of MFC is usually 0.1 ~ 0.3MPa. If the differential pressure is lower than the lowest value (0.1MPa), it may not reach the full range value; If it is higher than the maximum value (0.3MPa), the flow may not be less than 2% F.S when it is closed. When users use MFC, no matter whether the reaction chamber they work in is vacuum or high pressure, the pressure difference between the inlet and outlet ends of MFC should be kept within the range that the required pressure difference is not negligible, and the pressure should be relatively stable

what are the common faults of electromagnetic flowmeter

common fault phenomena of electromagnetic flowmeter include:

(1) no flow signal; (2) Output shaking; (3) Zero point instability; (4) The measured value of flow is inconsistent with the actual value; (5) There are 5 types of overfilling value of mountain transmission signal

what are the frequently used inspection means or methods and their inspection contents

(1) general routine instrument inspection

(2) substitution method uses the interchangeability between the converter and the sensor and between the components of the converter's internal circuit board to identify the location of the fault by substitution method

(3) signal trace method replaces the sensor with an analog annunciator to provide a flow signal under the condition that the liquid is not flowing, so as to test the electromagnetic flow converter

the inspection starts from whether the display instrument works normally and reverses the direction of flow signal transmission. Test the converter with an analog annunciator to determine whether the fault occurred in the converter and its rear instrument or in the upper sensor of the converter. If the converter fails, the converter or the circuit board in the converter can be easily used for alternative debugging if conditions permit; If the sensor fails and needs to be replaced, it is often difficult to stop the operation and close the pipeline system because it involves a wide range. In particular, the trial replacement of large-diameter flow sensors requires a large amount of work. Usually, it is only after other inspections that the decision is made to remove the pipe to check the internal condition of the sensor measuring pipe or replace it

what should be paid attention to when using ultrasonic flowmeter

(1) select the appropriate flowmeter type according to the different medium, flow and working site

(2) install the transducer in a reasonable way according to different types of ultrasonic flowmeter

(3) maintain regularly, and regularly check the working state of the flowmeter and the connection of the display

(4) calibrate the flowmeter regularly

the electromagnetic flowmeter is used for sewage discharge measurement in our plant. The flowmeter has been verified before installation, and the measurable data has been inconsistent with the quantity indicated by other flowmeters. Why

it is most likely that the installation position is wrong. If the flowmeter is installed at the highest part of the system, the bubbles in the pipeline will seriously affect the measurement accuracy; Or the flowmeter is installed on the vertical pipe where the fluid flows downward, which may produce non full pipe flow. It is recommended to install the flowmeter on the horizontal pipe at the lower position of the system or the vertical pipe flowing upward. It is best to install an degasser or exhaust valve in the system

what is a mass flowmeter? What is a mass flow controller

mass flowmeter, massflowmeter (abbreviated as MFM), is an instrument for accurately measuring gas flow. Its measured value is not inaccurate due to temperature or pressure fluctuations and does not need temperature and pressure compensation

mass flow controller, massflowcontroller (abbreviated as MFC), not only has the function of mass flowmeter, but also can automatically control the gas flow, that is, the user can set the flow according to needs, and MFC will automatically keep the flow at the set value, even if the system pressure fluctuates or the ambient temperature changes, it will not deviate from the set value. In short, the mass flow controller is a flow stabilizing device, which can be manually set or automatically controlled by connecting with the computer

what are the main advantages of mass flowmeter/mass flow controller

(1) the measurement and control of flow shall not be inaccurate due to the fluctuation of temperature or pressure

for most flow measurement and control systems, it is difficult to avoid the pressure fluctuation of the system and the temperature change of the environment and medium. For ordinary flow meters, pressure and temperature fluctuations will lead to large errors; For mass flowmeter/mass flow controller, it is generally negligible

(2) automation of measurement control

mass flowmeter/mass flow controller can output the flow measurement value as an output standard electrical signal. In this way, it is easy to realize digital display of flow, automatic measurement of cumulative flow, automatic data recording, computer management, etc. For the mass flow controller

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