Description
Product introduction:
● Viscous medium at high temperature
● Easily crystallized medium
● Precipitable medium with solid particles or suspended matter
● Strongly corrosive or highly toxic media
It can eliminate the phenomenon of leakage of the impulse tube to pollute the surrounding environment; it can avoid the tedious work of frequently supplementing the isolation liquid due to the unstable measurement signal when using the isolation liquid.
● Continuous and accurate measurement of interface and density
The remote transmission device can avoid the mixing of different instantaneous media, so that the measurement results truly reflect the actual situation of process changes.
● Occasions with high sanitary requirements
For example, in the production of food, beverage and pharmaceutical industries, not only is it required that the parts of the transmitter contacting the medium meet hygienic standards, and they should be easy to flush to prevent cross-contamination of different media.
Features:
Differential pressure transmitter is used to measure the level, density and pressure of liquid, gas and steam, and then convert it into a current signal output of 4-20mA DC. The 3151DP can also be simulated by the Communicator
● High accuracy
● The range and zero point are continuously adjustable externally
● Good stability
● Up to 500% of positive migration and 600% of negative migration
● Second-line system
● Adjustable damping and resistance to over-voltage
● Solid sensor design
● No mechanical moving parts and less maintenance
● Light weight (2.4kg)
● All series have unified structure and strong interchangeability
● Miniaturization (166mm total height)
● Optional diaphragm material
● Unilateral anti-overpressure
● Low-pressure cast aluminum alloy shell
smart
● Super measurement performance for pressure, differential pressure, liquid level and flow measurement
● Digital accuracy: + (-) 0.05%
● Simulation accuracy: + (-) 0.75% + (-) 0.1% F.S
● Full performance: + (-) 0.25F.S
● Stability: 0.25% for 60 months
● Range ratio: 100: 1
● Measurement rate: 0.2S
● Miniaturized (2.4kg) all stainless steel flange, easy to install
● The process connection is compatible with other products to achieve the best measurement
● The only sensor (patented technology) with H alloy sheath in the world, which achieves excellent cold and heat stability
● Smart transmitter with 16-bit computer
● Standard 4-20mA DC, with digital signal based on HART protocol, remote control
● Supports the mutual communication to the field bus and the upgrade of the technology based on the field control, through which they can be set and monitored.
Design principle:
As the name implies, the result measured by the differential pressure transmitter is the pressure difference, that is, △ P = ρg △ h. And because the oil tank is often cylindrical, the area S of the cross-section circle is constant, then, gravity G = △ P · S = ρg △ h · S, S is constant, and G is proportional to △ P. That is, as long as the △ P value is accurately detected, it is inversely proportional to the liquid level height h and proportional to the height difference Δh. When the temperature changes, although the oil volume expands or contracts, the actual liquid level increases or decreases. The pressure is always the same. If the user needs to display the actual liquid level, media temperature compensation can also be introduced to solve it.
Related parameters:
Use object: liquid, gas and steam
Measuring range: 0 ～ 0.1kPa to 0 ～ 40MPa
Output signal: 4 ～ 20mA DC (can be powered by four-wire 220V AC, 0 ～ 10mA DC output)
Power supply: 12 ～ 45V DC, generally 24V DC
Load characteristics: It is related to the power supply. The load capacity at a certain power supply voltage is shown in Figure 2. The relationship between the load impedance RL and the power supply voltage Vs is: RL≤50 (Vs-12)
Indication table: pointer type linear indication 0 to 100% scale or LCD display.
Explosion-proof: Flameproof ExdIICT6
Range and zero: continuously adjustable externally
Positive and negative migration: The absolute value of the upper and lower limits of the range and measurement range after the zero point undergoes positive or negative migration.
Do not exceed 100% of the upper limit of the measurement range. (Intelligent: range ratio 15: 1) The maximum positive migration amount is 500% of the minimum adjustment range; the maximum negative migration amount is 600% of the minimum adjustment range.
Temperature range: Amplifier operating temperature range: --29 to + 93 ° C (LT type: --25 to + 70 ° C).
Measuring element filled with silicone oil: -40 ～ + 104 ℃
Flange type transmitter when filling high temperature silicone oil: -20 ~ + 315 ℃, ordinary silicone oil: -40 ～ + 149 ℃
Static pressure: 4, 10, 25, 32MPa
Humidity: 5 to 95% relative humidity
Volume suction volume: <0.16cm3
Damping (step response): When filled with silicone oil, it is generally continuously adjustable between 0.2s and 1.67s.
Accuracy: ± 0.2%
Dead zone: None (≤0.1%)
Stability: the absolute value of the basic error within six months (one year for the intelligent type) that does not exceed the maximum range
Impact of vibration: When the vibration frequency is 200Hz in any axis, the error is ± 0.05% / g of the upper limit of the measurement range
Power effect: less than 0.005% / V of output range
Load effect: If the power supply is stable, the load has no effect.
Precautions:
1. Do not apply a voltage higher than 36V to the transmitter, resulting in damage to the transmitter;
2. Do not touch the diaphragm with a hard object, resulting in damage to the isolation diaphragm;
3. The measured medium is not allowed to freeze, otherwise the sensor element isolation diaphragm will be damaged, resulting in damage to the transmitter. If necessary, the transmitter must be temperature protected to prevent icing;
4. When measuring steam or other high-temperature media, its temperature should not exceed the limit temperature when the transmitter is used. Above the limit temperature used by the transmitter, a heat sink must be used;
5. When measuring steam or other high-temperature media, a heat pipe should be used to connect the transmitter and the pipe together, and use the pressure on the pipe to transmit to the transformer. When the measured medium is water vapor, an appropriate amount of water must be injected into the heat pipe to prevent the superheated steam from directly contacting the transmitter and damaging the sensor;
6. In the process of pressure transmission, the following points should be noted:
a. At the junction of the transmitter and the heat pipe, do not leak air;
b. Before starting to use, if the valve is closed, when using, the valve should be opened very carefully and slowly to prevent the measured medium from directly impacting the sensor diaphragm, thereby damaging the sensor diaphragm;
c. The pipeline must be kept clear, and the sediment in the pipeline will pop up and damage the sensor diaphragm.
Failure analysis:
1. Investigation method:
Review the fire, smoke, odor, power supply change, lightning strike, humidity, misoperation, and maintenance before the fault occurred.
2. Intuitive method:
Observe the external damage of the circuit, the leakage of the pressure tube, the overheating of the circuit, and the state of the power switch.
3. Detection method:
1) Open circuit detection: Separate the suspected part from other parts, check whether the fault disappears, and if it disappears, determine the fault, otherwise you can find the next step, such as: intelligent differential pressure transmitter can not work normally Hart remote communication You can disconnect the power supply from the body of the meter, and use the method of additional power on site to power the transmitter for communication to see if the cable is superimposed with an electromagnetic signal of about 2kHz and interferes with the communication.
2) Short circuit detection: Under the condition of ensuring safety, short the relevant part of the circuit directly. For example, if the output value of the differential transmitter is too small, the pressure guiding tube can be disconnected and the differential pressure signal can be directly sent from outside the pressure taking valve Directly to both sides of the differential pressure transmitter and observe the output of the transmitter to determine the blockage and leakage of the pressure-conducting pipeline.
3) Replacement test: replace the suspected faulty part and judge the faulty part. For example, if you suspect that the transmitter circuit board is malfunctioning, temporarily replace one to determine the cause.
4) Sub-division detection: Divide the measurement circuit into several parts, such as: power supply, signal output, signal transmission, signal detection, check by sub-section, from simple to complex, from the table and the inside, narrow down the scope and find the fault position.
Troubleshooting steps:
1. Check whether the power of the differential pressure transmitter is reversed, and whether the positive and negative poles of the power are connected correctly.
2. Measure whether the power supply of the transmitter has a 24V DC voltage; you must ensure that the power supply voltage to the transmitter is ≥ 12V (that is, the voltage at the input end of the transmitter is ≥ 12V). If there is no power supply, you should check whether the circuit is disconnected, whether the detection instrument is selected incorrectly (input impedance should be ≤250Ω), and so on.
3. If the pressure transmitter has a meter head, you need to check whether the meter head is damaged (you can short the two wires of the meter head first, if the short circuit is normal, it means that the meter head is damaged). If the meter head is damaged, Need to change the header.
4. If there is a problem with the differential pressure pressure transmitter, you can string the ammeter into the 24V power circuit and check whether the current is normal. If normal, the transmitter is normal. At this time, check whether other instruments in the loop are normal.
5. Whether the power supply is connected to the power input terminal of the transmitter, and connect the power cord to the power connection port.
Fault detection:
When detecting a differential pressure transmitter failure, we should understand that the working principle of the differential pressure transmitter allows us to find the cause more conveniently and quickly.
Working principle of differential pressure transmitter: The differential pressure from the double-sided impulse tube directly acts on the two-sided isolation diaphragm of the transmitter sensor, and is transmitted to the measuring element through the sealing liquid in the diaphragm. The measuring element will measure the measured The differential pressure signal is converted into a corresponding electric signal and passed to the converter, and then processed into a standard electric signal output through amplification and other processing.
Several common and practical measurement methods for differential pressure transmitters:
1. Combined with a throttling element, the liquid flow is measured using the differential pressure value generated before and after the throttling element.
2. Use the pressure difference generated by the liquid's own gravity to measure the height of the liquid.
3. Directly measure the pressure difference of liquid in different pipes and tanks.
During the measurement process of the transmitter, some faults often occur. The timely determination, analysis and processing of the faults are very important for the ongoing production. Based on the experience in daily maintenance, we summarized some judgment analysis methods and analysis processes.
Field calibration:
Calibration of conventional differential pressure transmitters:
First adjust the damping to zero, first adjust the zero point, and then add the full-scale pressure to adjust the full scale so that the output is 20mA. The on-site adjustment is fast. Here we introduce the quick adjustment method of zero and range. When adjusting the zero point, it has almost no effect on the full scale, but when adjusting the full scale, it has an effect on the zero point. Without migration, the effect is about 1/5 of the range adjustment, that is, the range is adjusted upward by 1 mA, and the zero point will move upward by about 0.2 mA ,vice versa. For example: input full-scale pressure is 100Kpa, the reading is 19.900mA, adjust the range potentiometer to make the output 19.900+ (20.000-19.900) × 1.25 = 20.025mA. If the range is increased by 0.125mA, the zero point will increase by 1/5 × 0.125 = 0.025. Adjust the zero-point potentiometer to make the output 20.000mA. After the zero and full-scale adjustments are normal, check the middle scales to see if they are out of tolerance. Fine-adjust if necessary. Then adjust the migration, linearity, and damping.
Calibration of Smart Differential Pressure Transmitters
It is not possible to calibrate the smart transmitter with the conventional method described above, because this is determined by the structure principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20mA current signal, in addition to the mechanical and electrical circuits, there is also a microprocessor to calculate the input data, so the calibration is different from the conventional method. In fact, the manufacturers have also explained the calibration of smart transmitters. For ABB's transmitters, for calibration, there are: "setting range", "resetting range", and "fine adjustment". Among them, the “setting range” operation is mainly to complete the configuration through the digital setting of LRV and URV, and the “resetting range” operation requires the transmitter to be connected to a standard pressure source, guided by a series of instructions, and changed by The transmitter directly senses the actual pressure and sets the value. The initial and final setting of the range directly depends on the actual pressure input value. But to see that although the transmitter's analog output is correctly related to the input value used, the digital reading of the process value will show a slightly different value, which can be calibrated by fine-tuning. Because each part needs to be adjusted separately and must be adjusted together, the actual calibration can be performed according to the following steps:
1. Do a 4-20mA trimming first to calibrate the D / A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required.
2. Do a full fine-tuning again to make the 4-20mA, digital reading coincide with the pressure signal actually applied, so a pressure signal source is needed.
3. Finally, do the re-ranging, and adjust the analog output 4-20mA to match the external pressure signal source. Its function is exactly the same as the zero (Z) and range (R) switches on the transmitter housing. .
How to choose:
The differential pressure transmitter measures the pressure difference of the medium in the process pipe or tank, and converts the measured differential pressure value into a current signal output through data conversion and square root. Selecting a differential pressure transmitter requires knowing the following parameters:
1.Differential pressure value
2.Medium
3.Working pressure of the medium
4. Working temperature of the medium
5.Is it smart or analog?
Selection basis:
⑴Measurement range, required accuracy and measurement function;
The environment that the measuring instrument faces, such as the industrial environment of petrochemical industry, the existence of hot (toxic) and explosive dangerous atmospheres, higher ambient temperature, etc .;
(3) Physical and chemical properties and conditions of the measured medium, such as working conditions such as strong acid, strong alkali, thick, easy to solidify crystallization and vaporization;
Changes in operating conditions, such as changes in medium temperature, pressure, and concentration. Sometimes it is necessary to take into account changes in gas and liquid concentrations and densities from the start-up to the normal production of the parameters;
The structure, shape and size of the container under test must be considered, such as towers, solution tanks, reactors, boiler drums, vertical tanks, spherical tanks, etc.
⑹Other requirements, such as environmental protection and hygiene requirements;
⑺ Engineering instrument selection must have unified considerations, requiring that specifications and varieties be reduced as much as possible, and spares and spare parts are reduced to facilitate management;
⑻The actual process:
① Depending on the physical and chemical properties of the medium and the degree of cleanliness, conventional differential pressure transmitters and float-type liquid level transmitters are preferred, and the material of the contacting medium must be selected;
② For some suspended matter, foam and other media, a single flange differential pressure transmitter can be used. Some plug-in double flange differential pressure transmitters that are easy to separate out and crystallize;
③ Consider what kind of equipment the test object belongs to. Such as tanks and tanks, the volume of the tank is small, the measurement range will not be too large, and the volume of the tank is large, the measurement range may be large;
④ For the liquid level of high-viscosity media and the liquid level of high-pressure equipment, because the equipment can not open the hole, you can choose to measure with the radio-frequency level gauge;
⑤ In addition to the measurement method and technical problems, there are also investment problems in the instrument.
Practical application:
In Wenzhou New Century Oil Depot Project, I applied this idea to the actual design.
Design conditions: 2000m3 oil tank, diameter d = 14.5m, the height of the actual oil inventory G can be obtained from the formula can also know its density ρh = 14m.
Primary meter: flange type flameproof differential pressure transmitter. The flange type is selected to prevent the sedimentation of the bottom of the tank and block the impulse tube. The range of the transmitter is 0 ～ 140kPa.
Secondary meter: use intelligent light column display alarm, universal signal input, can be used
3151DP differential pressure transmitter is used to prevent the medium in the pipeline from directly entering the transmitter. The pressure-sensitive diaphragm and the transmitter are connected by a capillary filled with fluid. It is used to measure the level, flow rate and pressure of liquid, gas or steam, and then convert it into 4-20mA DC signal output.
Differential pressure transmitter is suitable for the following measurement and control situations:● Viscous medium at high temperature
● Easily crystallized medium
● Precipitable medium with solid particles or suspended matter
● Strongly corrosive or highly toxic media
It can eliminate the phenomenon of leakage of the impulse tube to pollute the surrounding environment; it can avoid the tedious work of frequently supplementing the isolation liquid due to the unstable measurement signal when using the isolation liquid.
● Continuous and accurate measurement of interface and density
The remote transmission device can avoid the mixing of different instantaneous media, so that the measurement results truly reflect the actual situation of process changes.
● Occasions with high sanitary requirements
For example, in the production of food, beverage and pharmaceutical industries, not only is it required that the parts of the transmitter contacting the medium meet hygienic standards, and they should be easy to flush to prevent cross-contamination of different media.
Features:
Differential pressure transmitter is used to measure the level, density and pressure of liquid, gas and steam, and then convert it into a current signal output of 4-20mA DC. The 3151DP can also be simulated by the Communicator
● High accuracy
● The range and zero point are continuously adjustable externally
● Good stability
● Up to 500% of positive migration and 600% of negative migration
● Second-line system
● Adjustable damping and resistance to over-voltage
● Solid sensor design
● No mechanical moving parts and less maintenance
● Light weight (2.4kg)
● All series have unified structure and strong interchangeability
● Miniaturization (166mm total height)
● Optional diaphragm material
● Unilateral anti-overpressure
● Low-pressure cast aluminum alloy shell
smart
● Super measurement performance for pressure, differential pressure, liquid level and flow measurement
● Digital accuracy: + (-) 0.05%
● Simulation accuracy: + (-) 0.75% + (-) 0.1% F.S
● Full performance: + (-) 0.25F.S
● Stability: 0.25% for 60 months
● Range ratio: 100: 1
● Measurement rate: 0.2S
● Miniaturized (2.4kg) all stainless steel flange, easy to install
● The process connection is compatible with other products to achieve the best measurement
● The only sensor (patented technology) with H alloy sheath in the world, which achieves excellent cold and heat stability
● Smart transmitter with 16-bit computer
● Standard 4-20mA DC, with digital signal based on HART protocol, remote control
● Supports the mutual communication to the field bus and the upgrade of the technology based on the field control, through which they can be set and monitored.
Design principle:
As the name implies, the result measured by the differential pressure transmitter is the pressure difference, that is, △ P = ρg △ h. And because the oil tank is often cylindrical, the area S of the cross-section circle is constant, then, gravity G = △ P · S = ρg △ h · S, S is constant, and G is proportional to △ P. That is, as long as the △ P value is accurately detected, it is inversely proportional to the liquid level height h and proportional to the height difference Δh. When the temperature changes, although the oil volume expands or contracts, the actual liquid level increases or decreases. The pressure is always the same. If the user needs to display the actual liquid level, media temperature compensation can also be introduced to solve it.
Related parameters:
Use object: liquid, gas and steam
Measuring range: 0 ～ 0.1kPa to 0 ～ 40MPa
Output signal: 4 ～ 20mA DC (can be powered by four-wire 220V AC, 0 ～ 10mA DC output)
Power supply: 12 ～ 45V DC, generally 24V DC
Load characteristics: It is related to the power supply. The load capacity at a certain power supply voltage is shown in Figure 2. The relationship between the load impedance RL and the power supply voltage Vs is: RL≤50 (Vs-12)
Indication table: pointer type linear indication 0 to 100% scale or LCD display.
Explosion-proof: Flameproof ExdIICT6
Range and zero: continuously adjustable externally
Positive and negative migration: The absolute value of the upper and lower limits of the range and measurement range after the zero point undergoes positive or negative migration.
Do not exceed 100% of the upper limit of the measurement range. (Intelligent: range ratio 15: 1) The maximum positive migration amount is 500% of the minimum adjustment range; the maximum negative migration amount is 600% of the minimum adjustment range.
Temperature range: Amplifier operating temperature range: --29 to + 93 ° C (LT type: --25 to + 70 ° C).
Measuring element filled with silicone oil: -40 ～ + 104 ℃
Flange type transmitter when filling high temperature silicone oil: -20 ~ + 315 ℃, ordinary silicone oil: -40 ～ + 149 ℃
Static pressure: 4, 10, 25, 32MPa
Humidity: 5 to 95% relative humidity
Volume suction volume: <0.16cm3
Damping (step response): When filled with silicone oil, it is generally continuously adjustable between 0.2s and 1.67s.
Accuracy: ± 0.2%
Dead zone: None (≤0.1%)
Stability: the absolute value of the basic error within six months (one year for the intelligent type) that does not exceed the maximum range
Impact of vibration: When the vibration frequency is 200Hz in any axis, the error is ± 0.05% / g of the upper limit of the measurement range
Power effect: less than 0.005% / V of output range
Load effect: If the power supply is stable, the load has no effect.
Precautions:
1. Do not apply a voltage higher than 36V to the transmitter, resulting in damage to the transmitter;
2. Do not touch the diaphragm with a hard object, resulting in damage to the isolation diaphragm;
3. The measured medium is not allowed to freeze, otherwise the sensor element isolation diaphragm will be damaged, resulting in damage to the transmitter. If necessary, the transmitter must be temperature protected to prevent icing;
4. When measuring steam or other high-temperature media, its temperature should not exceed the limit temperature when the transmitter is used. Above the limit temperature used by the transmitter, a heat sink must be used;
5. When measuring steam or other high-temperature media, a heat pipe should be used to connect the transmitter and the pipe together, and use the pressure on the pipe to transmit to the transformer. When the measured medium is water vapor, an appropriate amount of water must be injected into the heat pipe to prevent the superheated steam from directly contacting the transmitter and damaging the sensor;
6. In the process of pressure transmission, the following points should be noted:
a. At the junction of the transmitter and the heat pipe, do not leak air;
b. Before starting to use, if the valve is closed, when using, the valve should be opened very carefully and slowly to prevent the measured medium from directly impacting the sensor diaphragm, thereby damaging the sensor diaphragm;
c. The pipeline must be kept clear, and the sediment in the pipeline will pop up and damage the sensor diaphragm.
Failure analysis:
1. Investigation method:
Review the fire, smoke, odor, power supply change, lightning strike, humidity, misoperation, and maintenance before the fault occurred.
2. Intuitive method:
Observe the external damage of the circuit, the leakage of the pressure tube, the overheating of the circuit, and the state of the power switch.
3. Detection method:
1) Open circuit detection: Separate the suspected part from other parts, check whether the fault disappears, and if it disappears, determine the fault, otherwise you can find the next step, such as: intelligent differential pressure transmitter can not work normally Hart remote communication You can disconnect the power supply from the body of the meter, and use the method of additional power on site to power the transmitter for communication to see if the cable is superimposed with an electromagnetic signal of about 2kHz and interferes with the communication.
2) Short circuit detection: Under the condition of ensuring safety, short the relevant part of the circuit directly. For example, if the output value of the differential transmitter is too small, the pressure guiding tube can be disconnected and the differential pressure signal can be directly sent from outside the pressure taking valve Directly to both sides of the differential pressure transmitter and observe the output of the transmitter to determine the blockage and leakage of the pressure-conducting pipeline.
3) Replacement test: replace the suspected faulty part and judge the faulty part. For example, if you suspect that the transmitter circuit board is malfunctioning, temporarily replace one to determine the cause.
4) Sub-division detection: Divide the measurement circuit into several parts, such as: power supply, signal output, signal transmission, signal detection, check by sub-section, from simple to complex, from the table and the inside, narrow down the scope and find the fault position.
Troubleshooting steps:
1. Check whether the power of the differential pressure transmitter is reversed, and whether the positive and negative poles of the power are connected correctly.
2. Measure whether the power supply of the transmitter has a 24V DC voltage; you must ensure that the power supply voltage to the transmitter is ≥ 12V (that is, the voltage at the input end of the transmitter is ≥ 12V). If there is no power supply, you should check whether the circuit is disconnected, whether the detection instrument is selected incorrectly (input impedance should be ≤250Ω), and so on.
3. If the pressure transmitter has a meter head, you need to check whether the meter head is damaged (you can short the two wires of the meter head first, if the short circuit is normal, it means that the meter head is damaged). If the meter head is damaged, Need to change the header.
4. If there is a problem with the differential pressure pressure transmitter, you can string the ammeter into the 24V power circuit and check whether the current is normal. If normal, the transmitter is normal. At this time, check whether other instruments in the loop are normal.
5. Whether the power supply is connected to the power input terminal of the transmitter, and connect the power cord to the power connection port.
Fault detection:
When detecting a differential pressure transmitter failure, we should understand that the working principle of the differential pressure transmitter allows us to find the cause more conveniently and quickly.
Working principle of differential pressure transmitter: The differential pressure from the double-sided impulse tube directly acts on the two-sided isolation diaphragm of the transmitter sensor, and is transmitted to the measuring element through the sealing liquid in the diaphragm. The measuring element will measure the measured The differential pressure signal is converted into a corresponding electric signal and passed to the converter, and then processed into a standard electric signal output through amplification and other processing.
Several common and practical measurement methods for differential pressure transmitters:
1. Combined with a throttling element, the liquid flow is measured using the differential pressure value generated before and after the throttling element.
2. Use the pressure difference generated by the liquid's own gravity to measure the height of the liquid.
3. Directly measure the pressure difference of liquid in different pipes and tanks.
During the measurement process of the transmitter, some faults often occur. The timely determination, analysis and processing of the faults are very important for the ongoing production. Based on the experience in daily maintenance, we summarized some judgment analysis methods and analysis processes.
Field calibration:
Calibration of conventional differential pressure transmitters:
First adjust the damping to zero, first adjust the zero point, and then add the full-scale pressure to adjust the full scale so that the output is 20mA. The on-site adjustment is fast. Here we introduce the quick adjustment method of zero and range. When adjusting the zero point, it has almost no effect on the full scale, but when adjusting the full scale, it has an effect on the zero point. Without migration, the effect is about 1/5 of the range adjustment, that is, the range is adjusted upward by 1 mA, and the zero point will move upward by about 0.2 mA ,vice versa. For example: input full-scale pressure is 100Kpa, the reading is 19.900mA, adjust the range potentiometer to make the output 19.900+ (20.000-19.900) × 1.25 = 20.025mA. If the range is increased by 0.125mA, the zero point will increase by 1/5 × 0.125 = 0.025. Adjust the zero-point potentiometer to make the output 20.000mA. After the zero and full-scale adjustments are normal, check the middle scales to see if they are out of tolerance. Fine-adjust if necessary. Then adjust the migration, linearity, and damping.
Calibration of Smart Differential Pressure Transmitters
It is not possible to calibrate the smart transmitter with the conventional method described above, because this is determined by the structure principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20mA current signal, in addition to the mechanical and electrical circuits, there is also a microprocessor to calculate the input data, so the calibration is different from the conventional method. In fact, the manufacturers have also explained the calibration of smart transmitters. For ABB's transmitters, for calibration, there are: "setting range", "resetting range", and "fine adjustment". Among them, the “setting range” operation is mainly to complete the configuration through the digital setting of LRV and URV, and the “resetting range” operation requires the transmitter to be connected to a standard pressure source, guided by a series of instructions, and changed by The transmitter directly senses the actual pressure and sets the value. The initial and final setting of the range directly depends on the actual pressure input value. But to see that although the transmitter's analog output is correctly related to the input value used, the digital reading of the process value will show a slightly different value, which can be calibrated by fine-tuning. Because each part needs to be adjusted separately and must be adjusted together, the actual calibration can be performed according to the following steps:
1. Do a 4-20mA trimming first to calibrate the D / A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required.
2. Do a full fine-tuning again to make the 4-20mA, digital reading coincide with the pressure signal actually applied, so a pressure signal source is needed.
3. Finally, do the re-ranging, and adjust the analog output 4-20mA to match the external pressure signal source. Its function is exactly the same as the zero (Z) and range (R) switches on the transmitter housing. .
How to choose:
The differential pressure transmitter measures the pressure difference of the medium in the process pipe or tank, and converts the measured differential pressure value into a current signal output through data conversion and square root. Selecting a differential pressure transmitter requires knowing the following parameters:
1.Differential pressure value
2.Medium
3.Working pressure of the medium
4. Working temperature of the medium
5.Is it smart or analog?
Selection basis:
⑴Measurement range, required accuracy and measurement function;
The environment that the measuring instrument faces, such as the industrial environment of petrochemical industry, the existence of hot (toxic) and explosive dangerous atmospheres, higher ambient temperature, etc .;
(3) Physical and chemical properties and conditions of the measured medium, such as working conditions such as strong acid, strong alkali, thick, easy to solidify crystallization and vaporization;
Changes in operating conditions, such as changes in medium temperature, pressure, and concentration. Sometimes it is necessary to take into account changes in gas and liquid concentrations and densities from the start-up to the normal production of the parameters;
The structure, shape and size of the container under test must be considered, such as towers, solution tanks, reactors, boiler drums, vertical tanks, spherical tanks, etc.
⑹Other requirements, such as environmental protection and hygiene requirements;
⑺ Engineering instrument selection must have unified considerations, requiring that specifications and varieties be reduced as much as possible, and spares and spare parts are reduced to facilitate management;
⑻The actual process:
① Depending on the physical and chemical properties of the medium and the degree of cleanliness, conventional differential pressure transmitters and float-type liquid level transmitters are preferred, and the material of the contacting medium must be selected;
② For some suspended matter, foam and other media, a single flange differential pressure transmitter can be used. Some plug-in double flange differential pressure transmitters that are easy to separate out and crystallize;
③ Consider what kind of equipment the test object belongs to. Such as tanks and tanks, the volume of the tank is small, the measurement range will not be too large, and the volume of the tank is large, the measurement range may be large;
④ For the liquid level of high-viscosity media and the liquid level of high-pressure equipment, because the equipment can not open the hole, you can choose to measure with the radio-frequency level gauge;
⑤ In addition to the measurement method and technical problems, there are also investment problems in the instrument.
Practical application:
In Wenzhou New Century Oil Depot Project, I applied this idea to the actual design.
Design conditions: 2000m3 oil tank, diameter d = 14.5m, the height of the actual oil inventory G can be obtained from the formula can also know its density ρh = 14m.
Primary meter: flange type flameproof differential pressure transmitter. The flange type is selected to prevent the sedimentation of the bottom of the tank and block the impulse tube. The range of the transmitter is 0 ～ 140kPa.
Secondary meter: use intelligent light column display alarm, universal signal input, can be used
