Siemens pressure transmitter is a kind of equipment which converts pressure into pneumatic signal or electric signal for control and remote transmission. It can convert the physical pressure parameters such as gas and liquid sensed by the load cell sensor into standard electrical signals (such as 4 ~ 20mADC), which can be supplied to the secondary instruments such as indicating alarm instrument, recorder and regulator for measurement, indication and process adjustment.

Siemens Transmitter 7ME3050-2BA10-1BA1

Figure 1: Siemens Transmitter 7ME3050-2BA10-1BA1

1.Key Features

* It can detect a wide range of media, including oil, water and paste compatible with 316 stainless steel and 304 stainless steel, and has a certain anti-corrosion ability.
* High accuracy, high stability, imported sensor, good linearity, high temperature stability.
* Small volume, light weight, easy to install, debug and use.
* Stainless steel fully enclosed shell, good waterproof.
* The pressure sensor can directly sense the measured liquid level pressure, which is not affected by medium foaming and deposition.

2.Working Principle

The electrical component of pressure transmitter to sense pressure is generally resistance strain gauge, which is a sensitive device to convert the pressure on the measured part into an electrical signal.
The most widely used resistance strain gauge are metal resistance strain gauge and semiconductor strain gauge. There are two kinds of metal resistance strain gauges: wire strain gauge and foil strain gauge. Usually, the strain gauge is tightly bonded to the matrix which produces mechanical strain by a special adhesive. When the stress of the matrix changes, the resistance strain gauge also deforms together, which changes the resistance of the strain gauge and the voltage applied to the resistance.


3.1 Check the Size of the Mounting Hole

If the size of the mounting hole is not suitable, the threaded part of the sensor will be easily worn during the installation process. This will not only affect the sealing performance of the equipment, but also make the pressure sensor can not give full play to its role, and may even cause safety hazards.
Only suitable mounting hole can avoid thread wear (thread industry standard 1 / 2-20 UNF 2b). Generally, the mounting hole can be detected by mounting hole measuring instrument to make appropriate adjustment.

3.2 Keep the Mounting Hole Clean

It is very important to keep the installation hole clean and prevent the molten material from blocking to ensure the normal operation of the equipment.
Before the extruder is cleaned, all pressure sensors should be removed from the barrel to avoid damage.
When the sensor is removed, the molten material may flow into the mounting hole and harden. If the residual molten material is not removed, the top of the sensor may be damaged when the sensor is installed again.
The cleaning tool pack can remove the molten residue. However, repeated cleaning process may deepen the damage to the sensor caused by the mounting hole. If this happens, measures should be taken to raise the position of the sensor in the mounting hole.

3.3 Choose the Right Location

When the pressure sensor is installed too close to the upstream of the production line, the unmelted material may wear the top of the sensor.
If the sensor is installed too far back, there may be a stagnant zone of molten material between the sensor and the screw stroke, where the molten material may degrade and the pressure signal may be distorted.
If the sensor is too deep into the barrel, the screw may touch the top of the sensor during rotation and cause damage.
Generally speaking, the sensor can be located on the barrel in front of the filter screen, in front of and behind the melt pump or in the mold.

3.4 Clean Carefully

All sensors should be removed before cleaning the extruder barrel with wire brush or special compound. Because these two cleaning methods may cause damage to the vibration film of the sensor.
When the barrel is heated, the sensor should also be removed and the top should be wiped with a soft cloth that will not produce wear. At the same time, the holes of the sensor should also be cleaned with a clean drilling machine and guide sleeve.

pressure transmitter set in factory

Figure 2: Pressure transmitter set in factory.

4. Correct Usage

The following conditions should be considered when using the pressure sensor.

* Prevent the transmitter from contacting with corrosive or overheated medium;
* Prevent the sediment in the pipe;
* When measuring the liquid pressure, the pressure tap should be opened on the side of the process pipe to avoid the deposition of slag;
* When measuring the gas pressure, the pressure tap should be opened on the top of the process pipe, and the transmitter should also be installed on the top of the process pipe, so that the accumulated liquid can be easily injected into the process pipe;
* The pressure pipe should be installed in the place where the temperature fluctuation is small;
* When measuring steam or other high temperature medium, it is necessary to add condenser such as buffer tube (coil), and the working temperature of transmitter shall not exceed the limit;
* When freezing occurs in winter, anti freezing measures must be taken for the transmitter installed outdoors to prevent the liquid in the pressure port from expanding due to freezing volume and leading to sensor damage;
* When measuring the liquid pressure, the installation position of the transmitter should avoid the impact of liquid (water hammer phenomenon), so as to avoid the sensor over-voltage damage;
* When wiring, put the cable through the waterproof joint (accessory) or flexible pipe and tighten the sealing nut to prevent rainwater from leaking into the transmitter shell through the cable.

Siemens Transmitter 7ME6140-1RG10-1AA3

Figure 3: Siemens Transmitter 7ME6140-1RG10-1AA3

5. Advantages

The pressure transmitter has the characteristics of reliable operation and stable performance.

V/I integrated circuit, less peripheral devices, high reliability, easy maintenance, small size, light weight, easy installation and debugging.

Aluminum alloy die casting shell, three end isolation, electrostatic spray plastic protective layer, durable;

4-20mA DC two-wire signal transmission has strong anti-interference ability and long transmission distance;

LED, LCD, pointer three kinds of indicator, field reading is very convenient. It can be used to measure viscous, crystalline and corrosive media;

High accuracy, high stability. In addition to the imported original sensor has been corrected by laser, the comprehensive temperature drift and nonlinearity of the whole machine in the use temperature range are finely compensated.

6. Common Problems

* when installing, the pressure sensitive parts of the transmitter should be perpendicular to the gravity direction.
If the installation conditions are limited, the zero position of the transmitter should be fixed and adjusted to the standard value.

* The residual pressure can't be released, so the zero position of the sensor can't come down again.
The best way to solve this problem is to remove the sensor and directly check whether the zero position is normal. If it is normal, replace the sealing ring and try again.

* If the output of the pressure transmitter does not change, then the output of the pressure transmitter suddenly changes, and the zero position of the pressure relief transmitter cannot return.
This phenomenon is most likely caused by the sealing ring of pressure sensor.

* Whether it meets the power supply requirements; whether there are wiring errors between the power supply and the transmitter and load equipment.
If there is no voltage on the terminal of the transmitter or the polarity is reversed, the transmitter will have no voltage signal output.

* Generally, the shell of pressure sensor and transmitter shall be grounded. Signal cable shall not be mixed with power cable. Strong electromagnetic interference shall be avoided around the sensor and transmitter. Sensors and transmitters should be periodically verified according to industry regulations.

* When selecting the pressure sensor and transmitter, the user should fully understand the working conditions of the pressure measurement system, and make reasonable selection according to the needs, so as to make the system work in the best state and reduce the project cost.

* Through the spacer and the filling liquid in the element, it is transmitted to both sides of the measuring diaphragm. The measuring diaphragm and the electrodes on both sides of the insulating sheet form a capacitor.

* The pressure transmitter is required to be inspected once a week and once a month, mainly to remove the dust in the instrument, carefully check the electrical components, and often check the output current value. The pressure transmitter has weak current inside and must be separated from the strong current outside.

Siemens Transmitter 7ME3400-3LC00-6EA2

Figure 4: Siemens Transmitter 7ME3400-3LC00-6EA2

7. Influencing Factors

There are many factors affecting the accuracy of the transmitter in the circuit, including the following.

7.1 Effects of Nonlinear Elements

Siemens transmitters in Germany are mostly AC converters (small transformers). The secondary power frequency AC signal is rectified, filtered and stabilized to obtain the final DC signal. Because rectifier diodes are nonlinear devices, their voltage and current curves are nonlinear.

7.2 Influence of Transmitter Core

Iron core material is used as magnetic medium in conventional transmitter. On the one hand, the nonlinear characteristics of ferromagnetic materials (the initial region and saturation region of magnetization meow line) are not an ideal linear transmission relationship, so it will inevitably affect the accuracy of the transmitter.

On the other hand, due to the hysteresis of ferromagnetic materials, the iron core will also affect the accuracy of the transmitter. Generally, in the power frequency range, the conventional silicon steel lag angle changes from 0 ° to 15 ° and the existence of this lag angle is equivalent to increasing the composition of reactive power. Because the conventional power transmitter calculates the voltage and current signals through the multiplier, the lag angle will also affect the accuracy of the power transmitter.

7.3 Influence of Operational Amplifier

The conventional power transmitter is mostly composed of operational amplifiers. The temperature has a great influence on the operation of the operational amplifier. When the temperature changes, the "zero" point drift makes the working point unstable, which directly affects the accuracy and reliability of the transmitter.

7.4 Influence of Value Selection Setting

Although the setting value of the transmitter is close to the full value as far as possible, the transmitter often can not work in the linear region, resulting in errors.

7.5 Error Caused by Impedance Mismatch

7.6 Impact of System Imbalance

Generally speaking, the calculated power of conventional transmitter is approximately considered to be balanced, but it is actually unbalanced, which often affects the accuracy of transmitter.

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