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What Is Differential Pressure Transmitter?

Dec 11, 2018

What is differential pressure transmitter?

What is differential pressure transmitter?The differential pressure transmitter is a typical self-balancing detection instrument that uses the working principle(differential pressure transmitter working principle) of negative feedback to overcome the influence of unfavorable factors such as component materials and processing techniques. 

The differential pressure transducer is used to prevent the medium in the pipe from directly entering the transmitter, and the pressure sensitive diaphragm and the transmitter are connected by a capillary filled with fluid. It is used to measure the level, flow and pressure of a liquid, gas or vapor and then convert it into a 4-20 mA DC signal output.

Know more about what is pressure transmitter. 

The working principle of the differential pressure transmitter is that the difference measured by the differential pressure transmitter is the pressure difference, that is, ΔP=ρgΔh. Since the oil tank is often cylindrical, the area S of the cross-section circle is constant, then the gravity G=ΔP·S=ρgΔh·S, S is constant, and G is inversely proportional to ΔP. That is, as long as the ΔP value is accurately detected, it is inversely proportional to the liquid level height h, and is proportional to the height difference Δh. When the temperature changes, although the oil volume expands or contracts, the actual liquid level rises or falls, and the detected 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 the problem.

Differential pressure level transmitter working principle:The differential pressure level transmitter is installed at the bottom of the liquid container and reflects the liquid level height by the gauge pressure signal. Such differential pressure type instruments include pneumatic and electric differential pressure transmitters and flange type liquid level transmitters, which are easy to install, easy to realize remote transmission and automatic adjustment, and are widely used in industry. In the liquid level measurement control process of the pharmaceutical, food and chemical industries, the container containing the liquid is often operated under pressure, and the conventional static pressure level transmitter cannot meet the measurement requirements.

Differential pressure level gauges have two pressure ports for gas phase and liquid phase. The pressure at the gas pressure point is the gas pressure in the equipment; the pressure at the liquid pressure point is affected by the gas pressure, and the static pressure of the liquid column. The difference between the liquid phase and the gas pressure is the static generated by the liquid column. pressure.

ⅰ.Overview of differential pressure transmitter.

Among the various types of instruments, the transmitter is the most widely used and widely used, and the transmitter is roughly divided into pressure.

Transmitter and differential pressure transmitter (DP sensor). Transmitters are commonly used to measure pressure, differential pressure, vacuum, liquid level,

Flow and density, etc. The transmitter has two-wire and four-wire systems, and two-wire transmitters are especially popular;

There are intelligent and non-intelligent points, and there are more and more intelligent transmitters; there are pneumatic and electric parts, and electric

In most cases, there are intrinsically safe (intrinsically safe) and explosion-proof types according to the application; according to the application conditions, the main types of transmitters are as follows: low (micro) pressure / low (micro) differential pressure Transmitter; medium voltage / medium differential pressure transmitter; high pressure easy to crystallize / pressure/high pressure transducer, differential pressure transmitter. Transmitter selection is usually based on installation conditions, environmental conditions, instrument performance, economy, and media usage. In practical applications, it is divided into direct measurement and indirect measurement;

Its uses include process measurement, process control and device interlocking. Common transmitters include ordinary pressure transmitters, differential pressure transmitters, single-lamp transmitters, dual-lamp transmitters, and plug-in flange transmitters.

What is differential pressure transmitter?

ⅱ.The pressure / differential pressure transmitter introduced pressure transmitter and differential pressure transmitter from the name of the measurement is the pressure and differential pressure (the difference between the two pressures), but the amount of indirect measurement can be a lot. For example, a pressure transmitter can measure the pressure in the device in addition to the pressure. When measuring the liquid level in an atmospheric pressure vessel, a pressure transmitter is required. When measuring the liquid level of a pressurized container, two pressure/differential pressure transmitters can be considered, that is, one lower limit of measurement, one upper limit of measurement, and their output signals.

The pressure rating of the transmitter must match the application. From an economic point of view, the outer membrane box and the insert part of the material are more important, to choose the right, but the connection of the flange can reduce the material requirements, such as the choice of carbon steel, chrome, etc., which will save a lot of money. 

The isolated pressure transmitter is good for threaded connection, which saves money and is easy to install. For the selection of common pressure and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the medium temperature used can be ignored, because the common type is pressurized into the meter and the long-term working temperature is normal temperature. 

However, the normal type of maintenance is larger than the isolation type. The first is the insulation problem. When the temperature is below zero, the pressure guiding tube will freeze. The transmitter will not work or even be damaged. This will increase the heat tracing and incubator. From an economic point of view, when the transmitter is selected, only the crystallization medium can be a common type of transmitter, and for the low pressure and easy crystallization medium, the purge medium can also be added for indirect measurement (as long as the process allows the use of purge liquid or gas) The application of ordinary transmitters requires maintenance personnel to carry out regular inspections, including whether the various pressure guiding tubes are leaking, whether the cleaning medium is normal, whether the insulation is good, etc. 

As long as the maintenance is good, a large amount of ordinary transmitters are used for one-time investment. It will save a lot. Pay attention to the combination of hardware maintenance and soft maintenance during maintenance. From the selection of the measuring range of the transmitter, the general transmitter has a range adjustable range, and the range of the range that Zui will use is set in the range of 1/4~3/4 of its range, so the accuracy will be Guaranteed, it is even more important for differential pressure transmitters. 

In practice, some applications (level measurement) need to migrate the measurement range of the transmitter, calculate the measurement range and migration according to the current installation location, and migrate with positive and negative migration. At present, the smart transmitter has been quite similar. It is characterized by high precision, large adjustable range, and very convenient adjustment and good stability. It should be considered when selecting the type. According to the design specifications, in the selection of engineering design, whether pneumatic or electric transmitters are used, because of their special features, should be comprehensively considered and analyzed according to the specific conditions of the device. 

What is differential pressure transmitter?

The following points can be used for selection: 

※ centralized operation and response speed; 

※ whether it is compatible with DCS computer operation; 

※ economy, reliability and maintenance; 

※ safety (explosion-proof, power failure, gas source failure Etc); 

※ environmental conditions and transmission distance. Generally speaking, the following conditions are suitable for the selection of pneumatic transmitter: 

※ The distance from the transmitter to the display adjustment unit is short, usually not more than 150 meters; 

※The process material is flammable and explosive medium and relative When the humidity is very large; ※There is a requirement for less investment in the instrument and the response speed is not fast; 

※ Generally, small and medium-sized enterprises require easy maintenance and economical reliability; in large devices mainly based on electric instruments, some on-site regulating circuits are not required to be introduced into the center. The control room is operated centrally.

The following conditions are preferred for the selection of electric actuators:

The distance between the transmitter and the display adjustment unit is more than 150 meters; the large enterprise requires central control with high centralized management; the object with DCS computer for control and management; the requirement for fast response, complex information processing and computation. In fact, in modern production facilities, they are mixed and selected for their respective characteristics.

ⅲ. The selection of pressure transmitter From a physics point of view, the pressure on any object includes the atmospheric pressure and the pressure of the measured medium (generally referred to as gauge pressure). The sum of the pressures of the two parts acting on the object to be measured is called the absolute pressure. P absolutely = P table + Instrument for measuring absolute pressure at atmospheric pressure is called absolute pressure gauge. For ordinary industrial pressure gauges, the gauge pressure value is measured, that is, the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure value, the measured gauge pressure value is called positive gauge pressure; when the absolute pressure is less than the atmospheric pressure value, the measured gauge pressure value is negative value, which is called negative gauge pressure, that is, vacuum degree. The meter that measures the degree of vacuum is called a vacuum gauge.

(1) In order to ensure the accuracy of pressure measurement, the minimum pressure measurement value should be higher than 1/3 of the measurement range of the pressure gauge;

(2) For applications requiring high distance measurement or high measurement accuracy, pressure sensors or pressure transmitters should be used;

(3) When the measurement accuracy requirement is not high, a resistive or inductive, Hall effect remote transmission pressure transmitter can be selected;

(4) The pneumatic base type pressure indicating regulator is suitable for local pressure indication adjustment;

(5) Pressure transmitter and pressure switch should be reasonably selected according to the explosion-proof requirements of the installation site.

What is differential pressure transmitter?

ⅳ. The selection of differential pressure transmitter

  The differential pressure transmitter is selected according to the following points:

(1) Measurement range, required accuracy and measurement function;

(2) The environment faced by the measuring instrument, such as the petrochemical industrial environment, has the presence of a hot (toxic) and explosive atmosphere, and has a high ambient temperature;

(3) Physicochemical properties and state of the measured medium, such as strong acid, strong alkali, viscous, easy to solidify crystallization and vaporization;

(4) Changes in operating conditions, such as changes in medium temperature, pressure, and concentration. Sometimes it is necessary to take into account changes in the concentration and density of the gas and liquid phases from the start of the drive to the normal production of the parameters;

(5) The structure, shape and size of the container to be tested, equipment accessories in the container and various inlet and outlet pipe ports must be considered, such as towers, solution tanks, reactors, boiler steam drums, vertical tanks, spherical tanks, etc.;

(6) Other requirements, such as environmental protection and sanitation;

(7) There must be a unified consideration in the selection of engineering instruments, and it is required to reduce the number of specifications as much as possible and reduce spare parts for profit management;

Actual process situation:

1 Consider which type of device the measured object belongs to. For tanks and tanks, the volume of the tank is small, the measurement range is not too large, the tank volume is large, and the measurement range may be large;

2 To see the physical and chemical properties and cleanliness of the medium, the conventional differential pressure transmitter and the float type liquid level transmitter are preferred, and the material of the contact medium part is also selected;

3 For some suspensions, foams and other media, single-lange differential pressure transmitters are available. Some insertable double-blue differential pressure transmitters that are easy to precipitate and easy to crystallize;

4 The level of the high-viscosity medium and the liquid level of the high-pressure equipment, because the equipment can not be opened, can be measured by the radiation level gauge;

5 In addition to measurement methods and technical problems, there are also investment problems with instruments. In summary, the selection of the transmitter is technically feasible, economically reasonable, and convenient to manage.

ⅴ.Differential pressure transmitter installation guide

1. Differential pressure transmitter wiring:

(1) Remove the electronic housing.

(2) Connect the positive lead to the “PWR/COMN” terminal and the negative lead to the “-” terminal. Be careful not to connect the live signal cable to the test terminal (test). Powering up will damage the test diode in the test circuit. Shielded twisted pairs should be used for optimum measurement. To ensure proper communication, a 24AWG or higher cable should be used.

(3) Seal the unused conduit interface on the transmitter housing with a conduit plug.

(4) Re-tighten the cover.

2. Installation requirements for differential pressure transmitter:

(1) Introduce the outer tube (positive pressure) of the elbow sensor into the three-valve group and connect it to the positive pressure chamber of the differential pressure transmitter, and introduce the inner (negative pressure side) pressure tube of the elbow sensor into the three-valve assembly. To the negative pressure chamber of the differential pressure transmitter;

(2) The installation position of the differential pressure transmitter on the process pipeline is related to the measured medium. In order to obtain a good installation effect, the following conditions should be considered:

a. Prevent the transmitter from coming into direct contact with corrosive or overheated measured media;

b. Prevent the dross from depositing and clogging in the pressure guiding tube;

c. The length of the pressure guiding pipe on both sides of the positive and negative pressure should be the same as possible;

d. The liquid column indenter in the pressure pipe on both sides of the positive and negative pressure should be balanced;

e. The impulse piping is installed where the temperature gradient and temperature fluctuations are minimal.

(3) When measuring the liquid flow rate, the differential pressure transmitter should be installed beside or below the pipe to be tested so that the air bubbles are discharged into the pipe;

(4) When measuring the gas flow rate, the differential pressure transmitter should be installed beside or above the pipe to be tested so that the accumulated liquid can easily flow into the pipe;

(5) When measuring the steam flow, the differential pressure transmitter should be installed under the pipe to be tested so that the condensate can be filled in the pressure pipe.

Special care should be taken to prevent the temperature of the differential pressure transmitter from contacting the medium above the limit temperature used by the transmitter when measuring steam or other high temperature media.

ⅵ.Differential pressure transmitter calibration procedure

Calibration of conventional differential pressure transmitters:

First adjust the damping to zero state, first adjust the zero point, then adjust the full scale pressure to full scale, so that the output is 20mA, the on-site adjustment is fast, here introduces the zero adjustment of the zero point and range. Zero adjustment has almost no effect on fullness, but the fullness has an effect on zero. When there is no 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, and the range potentiometer makes the output 19.900+(20.000-19.900)×1.25=20.025mA. The range increases by 0.125mA, then the zero point increases by 1/5×0.125. =0.025. The zero point potentiometer makes the output 20.000 mA. After the zero and full scale adjustments are normal, check the middle scales to see if they are out of tolerance. Fine-tune if necessary. Then carry out the migration, linear, and damping adjustment work.

Calibration of SMART differential pressure transmitter :

It is not acceptable to calibrate the smart transmitter using the conventional method described above, as this is determined by the structural principle of the HART transmitter. 

Smart Differential Pressure Transmitter Working Principle

Because the smart transmitter is between the input pressure source and the generated 4-20 mA current signal, in addition to the mechanical and electrical circuits, there is also the operation of the micro-processing chip on the input data, so the calibration is different from the conventional method. In fact, the manufacturer also has a description of the calibration of the smart transmitter. For example, ABB's transmitter has the following functions: “set range”, “heavy quantitation” and “fine adjustment”. The “set range” operation is mainly through the digital setting of LRV and URV to complete the configuration work, while the “re-quantity” operation requires that the transmitter be connected to the standard pressure source and guided by a series of instructions. The transmitter directly senses the actual pressure and sets the value. The initial and final settings of the range are directly dependent on the actual pressure input value. However, it should be noted that although the analog output of the transmitter is correctly related to the input values used, the digital readings of the process values will display slightly different values, which can be calibrated by fine-tuning the item. Since each part has to be adjusted separately and must be adjusted, the actual calibration can be carried out as follows:

1. Perform a 4-20 mA trimming to correct the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure signal source is required.

2. Make another full-scale fine-tuning so that the 4-20 mA, digital reading matches the actual applied pressure signal, so a pressure signal source is required.

3. Finally, the heavy-quantity process is performed, and the analog output 4-20 mA is matched with the applied pressure signal source by the adjustment, and its function is exactly the same as that of the zero-adjusting (Z) and measuring range (R) switches on the transmitter casing. 

Selecting a Differential Pressure Transmitter on China-transmitters.com . Huaheng Instrument offers over10 china differential pressure transmitter products.About 65% of these are differential  pressure transmitters,20% are differential pressure sensors,and 15% are sensors. A wide variety of china differential pressure transmitter options are available to you, such as resistance sensor, hall sensor, and optical sensor. You can also choose from differential pressure sensor, industrial. As well as from analog sensor, digital sensor, and switching transducer. And whether china differential pressure unit is round, oil pressure sensor, or digital. The pressure range measured varies among differential pressure transmitters, but some can be adjusted to measure pressure in a wider or narrower range than the given range for finer control of the equipment.A differential pressure unit measures and transmits the difference between two pressure points in a system. You can also customize Differential Pressure Transmitter orders from us,we are OEM/ODM manufacturers. Our differential pressure transmitter price, in line with the budget of most projects.We are experienced China exporter for your online sourcing. Update your electrical products and buy from these credible suppliers with the latest China production technology. We hope to keep every buyer up to date with this fastest moving electronic industry and the latest products trends.  You can also contact our buyer service and get some buying guides. China differential pressure transmitter products are most popular in North America, Mid East, and Eastern Europe. You can ensure product safety by selecting from certified suppliers with ISO9000