Ⅰ. Four common faults of pressure transducers

The first fault is that when the pressure goes up, the pressure transmitter output cannot go up. In this case, check whether the pressure port is leaking or blocked first. If not, check the wiring method and the power supply. If the power supply is normal, simply pressurize it to see whether the output changes, or check whether the zero position of the sensor has output. If there is no change, the sensor has been damaged, possibly due to instrument damage or other problems in the whole system.

The second is in the first pressurization, the output of the pressure transmitter does not change, and the output of the transmitter changes suddenly after the pressure is released, and the zero position of the pressure transmitter cannot return after the pressure is relieved. It is very likely that the pressure sensor sealing ring has problem. Commonly, due to its specifications, the sealing ring is compressed into the pressure port of the sensor to block the sensor after it is tightened. When pressurized, the pressure medium can’t enter, but when the pressure is high, the sealing ring is suddenly burst open, and the pressure transducer is changed by the pressure. The best way to eliminate this fault is to remove the pressure transducer and directly check whether the zero position is normal. If the zero position is normal, you can replace the sealing ring and try again.

The third fault is the unstable output signal of the pressure transmitter. This fault may be a source of stress. The pressure source itself is an unstable pressure. It is very likely that the anti-interference ability of the instrument or pressure transducer is not strong, the vibrating of pressure transducer or the pressure transducer is faulty.

The fourth fault is the large deviation between the transmitter and the pointer pressure gauge. Deviation is normal, just confirm the normal deviation range.

The final type of fault is the effect of the position of the low differential pressure transmitter on the zero output. Due to its small measurement range, the sensing element will affect the output of low differential pressure transmitter. During installation, the pressure sensing part of the transmitter should be axial perpendicular to the direction of gravity. After installation and fixing, adjust the zero position of the pressure transmitter to the standard value.

Ⅱ. Anti-interference measures for pressure transducer

1. Maintain stability

Most pressure transducer will be unstable after overtime work, so it’s important to know how stable the pressure transducer is before you buy it. This advance work can reduce the trouble of future use.

2. Packaging of pressure transducer

The packaging of the pressure transducer, especially the frame, is often easy to ignore, but its shortcomings will gradually be exposed in the future use. When purchasing a pressure transmitter, you must consider the working environment of the pressure transducer in the future, how the humidity is, how to install, whether there will be strong impact or vibration, etc.

3. Select the output signal

What kind of output signal the pressure transducer needs: mV, V, mA, frequency output, and digital output, depends on many factors, including the distance between the sensor and the system controller or display, whether there is “noise” or other electronic interference signals, whether the amplifier is needed, and the location of the amplifier. For many devices with a short distance between the pressure transducer and the controller, the pressure transducer with mA output is the most economical and effective solution. If the output signal needs to be amplified, it is best to use a sensor with built-in amplification. For long-distance transmission or strong electronic interference signals, it is best to use mA output or frequency output.

The low differential pressure transmitter is a type of instrumentation equipment commonly used in modern industrial production, which can be called the “eye” of modern industrial production. In the production process, it can observe and control the pressure, flow, temperature, liquid level, and other measurement data of the entire production process. And then the data can be transferred to the logic algorithm in the central control system (DCS or PLC) through low DP data exchange. At the same time, adjustment is made by the regulator or control unit. Finally, the control of the production target values is completed. Thus, the low differential pressure transmitter is an important prerequisite for the establishment of a good control loop. Pressure transmitters and differential pressure transmitters are typical representatives of the “eye”, which are widely used due to their high precision and stability.

 

1. The classification of the low differential pressure transmitter 

The function of the low differential pressure transmitter, in simple terms, is to convert the displacement or force of the elastic load cell into a standard electrical signal. Based on the composition and working principle, it can be divided into force-balance transmitter and displacement transmitter while the latter is generally used in the glass production line. Based on the measurement accuracy and objective, it can be divided into the micro differential pressure transmitter, the differential pressure transmitter, and the pressure transmitter. And besides, according to the signal transmission and power supply mode, it can be divided into a two-wire transmitter and a four-wire transmitter.

 

2.  The application of the low differential pressure transmitter

Based on what we have learned, it is realized that the displacement transmitter is more approved by the market than the force-balance transmitter in today’s measurement field. The same is true in the design process of the glass production line. However, in terms of differences in measurement objectives and accuracy, there are still differences in the selection of the low differential pressure transmitter.

(1) The high-precision micro-differential pressure transmitter is used in the furnace to measure the pressure of the kiln, which is one of the fundamental safety and measurement objects. At this moment, the pressure pipe of P1 needs to go deep inside the furnace wall while the pressure of P2 is the atmospheric pressure at the temperature close to that of the furnace. The pressure pipe that is finally connected to the low differential pressure transmitter should be the average value of the pressure on both sides of the furnace. Next, based on the principle of the transmitter, the gauge pressure inside the furnace can be obtained through P1-P2.

(2) The low differential pressure transmitter and the orifice plate flowmeters throttling device can be used for the measurement of natural gas, hydrogen, oxygen, and combustion-supporting airflow. A disk with a central hole is inserted into the pipeline. When the fluid passes through the orifice plate, the cross-section of the flow beam is reduced, causing a local difference in flow velocity and a relatively significant pressure difference. The flow rate of the fluid is proportional to the square root of the pressure difference before and after the throttle element under certain conditions. In the formula, Q is the measured flow, F0 is the opening area of the orifice plate, α is the flow coefficient, ρ is the liquid density, P1 is the inlet pressure of the orifice plate and P2 is the outlet pressure of the orifice plate.

In addition to its important role in the key measurement areas in the glass production line, the low differential pressure transmitter has many derivative applications, such as the measurement of the height change of the glass liquid level and the liquid level height, etc, which will not be described in detail here.

 

3. The development of the low differential pressure transmitter

Compared with liquid, it is more complicated to measure gas flow due to its compressibility. The volumetric flow rate Q of the gas is a function of the differential pressure (P1-P2) and the density ρ while the density is a function of the current temperature and pressure of the gas. In the practical application, there will be a large measurement error due to the difference between the current density of the medium and the density during design. Therefore, temperature and pressure compensation should be performed on it.

In the past, designers install both thermal resistance and a pressure transmitter near the location where the gas pipeline is set up the flowmeter. All three groups of signals are entered into the DCS system, and the compensation calculation is completed with the formula in the computer system. However, this model is gradually being replaced nowadays. With the emergence of a new generation of multi-parameter intelligent transmitters, this type of transmitter comprehensively improves the accuracy, reliability, and long-term stability of the low differential pressure transmitter by using a microprocessor as a basis.

The low differential pressure sensor is powered by 5V with an output of 0.25-4V. Although it is the analog output, the internal linearization and temperature compensation are all realized digitally, ensuring high accuracy and resolution ratios (up to 0.05 Pa). It is thus unsusceptible to temperature.

The low differential pressure sensor has two specific output modes, including linear and square output which are completely interchangeable. Relying on simple peripheral circuits, such as 0-10V output or other interfaces, high-quality transmitters can be produced.

Its leading performance is based on the integration of sensing elements, signal amplification, and A/D conversion into the same silicon chip. The thermal sensor element measures differential pressure. Compared with other thermal differential pressure sensors, it requires less air and can work under the extremely harsh environment in a safe and reliable way. Compared with the diaphragm sensor, the low differential pressure sensor has a wider measuring range, more stable performance and more excellent renaturation in the section of low differential pressure. In addition, the low differential pressure sensor can withstand large instantaneous pressure and is not sensitive to the installation direction.

The low differential pressure sensor has the following characteristics:

1. Accuracy:

(1) Digital accuracy: ±0.05%

(2) Full performance: ±0.25F.S

2. Strong performance for pressure measurement, differential pressure measurement and flow measurement

3. Range ratio: 100:1

4. Stability: 0.25%, 60 months

5. Measurement rate: 0.2S

6. Stainless steel cover that is easy to install and ensures excellent cold and thermal stability

7. Process connections are compatible with other products for optimal measurement

8. Adoption of the intelligent transmitter with a 16-bit computer

9. Standard current output of 4-20mA, protection mechanism for mis-wiring, environmental design, and compliance with RoHS standard

1. Calibration is needed by pressure transmitter

After being used for a while, the measurement performance of the pressure transmitter may be changed, and the phenomena of zero drift and increasing measurement error will appear. Therefore, it is necessary to carry out regular calibration of the pressure transmitter. The calibrator should be 3 times more accurate than the required calibration accuracy. Standard pressure values of 0%, 25%, 50%, 75%, and 100% are input successively into the transmitter via the pressure calibrator in a forward and reverse manner, to detect whether the output signal of the electric current is within the allowed accuracy range. If out of range, it needs to be calibrated.

 

2. Calibration methods of pressure transmitter

The calibration methods are as follows (4~20mA output):

(1) Input the standard pressure of 4mA with pressure calibrator and adjust the ZERO potentiometer to make sure that the electric current output falls within the accuracy range;

(2) Input the standard pressure of 20mA with pressure calibrator and adjust the SPAN potentiometer to make sure that the electric current output falls within the accuracy range;

(3) ZERO and SPAN potentiometers are mutually restrictive. Repeat step (1) and step (2) until both the current outputs of 4mA and 20mA meet the accuracy requirements, and then check whether the current outputs of 25%, 50%, and 75% meet the accuracy requirements. If so, it indicates the calibration of pressure transmitter produced by pressure transmitter manufacturer is qualified.

Alpha Instrument Inc. is specialized in the development and manufacturing of low differential pressure sensors, which have been used in heating ventilation air conditioning, critical environment, medical industry, semiconductor equipment, and other fields. Our innovatively designed products provide long-term stability, higher sensitivity, and higher overvoltage capacity. Welcome to contact us to buy!

Ⅰ. Preparation work before installation of pressure transmitter

1. Check the equipment: Since the equipment is designed by different pressure transmitter suppliers and features various models, it is necessary to determine the transmitter corresponding to each tag number according to the range, design and installation mode, and the material required by the process medium.

2. Determine the installation position: A variety of pressure transmitters can be installed anywhere if they are waterproof and dustproof. However, considering the convenience of daily operation and maintenance, and to extend service life and ensure reliability, the installation position should meet the following requirements:

(1) There is enough working space around, and the distance from adjacent objects (in any direction) is more than 0.5m;

(2) There is no serious corrosive gas around;

(3) It is not exposed to the surrounding thermal radiation and direct sunlight;

(4) To prevent the interference caused by the vibration of transmitter and pressure conducting tube (capillary), the pressure transmitter should be installed in the place without vibration.

 

Ⅱ. Installation requirements of pressure transmitter

1. Installation requirements of pneumatic pressure transmitter

(1) The pressure transmitter should be installed on the vertical or horizontal pipeline without strong vibration.

(2) The surrounding environment should be free of impurities corrosive to carbon steel, copper and alloy.

(3) For the convenience of maintenance and zero adjustment, adequate space should be reserved on the top and sides of the equipment.

(4) The pressure transmitter should be installed near (generally no more than 5m) the site requiring pressure measurement as much as possible.

(5) When the measured pressure is zero after installation, the output pressure of pressure transmitter should be 20KPa, otherwise it should be adjusted.

(6) The air source used for pressure transmitter should be filtered and purified compressed air.

 

2. Installation requirements of electric pressure transmitter

(1) Installation requirements for measuring gas pressure

A. When measuring the gas pressure, the pressure-relief point of the pressure measuring pipeline must be selected at the upper half of the pipeline to prevent liquid accumulation in the pipeline.

B. Flush valve should be installed at the front of pressure transmitter to prevent liquid or dirt from entering the transmitter.

C. When installing the pressure transmitter on the pipe throttling device, the pressure measuring point must be located within the area where the vertical line and the horizontal line meet at a 45-degree angle.

(2) Installation requirements for measuring steam pressure

When measuring the steam pressure, the pressure measuring point must be located within the area where the vertical line and the horizontal line meet at a 45-degree angle. The gas collector should be installed at the highest point of the pressure measuring point and the gas should be discharged regularly to ensure the measurement accuracy of the transmitter.

(3) Installation requirements for measuring the pressure of corrosive medium

When measuring the pressure of corrosive medium, an isolation device should be installed in front of the pressure transmitter and space liquid should be injected into it.