Diaphragm Seal Pressure Transmitter-FAQ 

1. What’s a pressure transmitter？

Pressure transmitters are among the most commonly used sensors in industrial practice. It involves many industries such as water conservancy and hydropower, rail transportation, intelligent construction, production automation, aerospace, military, petrochemical, oil well, electric power, ship, machine tool, pipeline, etc.

There are two main types of pressure transmitters: electric and pneumatic. The unified output signal of the electric kind is 0-10mA, 4-20mA, or 1-5V DC signal. The pneumatic type has a uniform output signal of 20 to 100 Pa of gas pressure.

The pressure transmitter is a device that converts pressure into a pneumatic signal or an electric signal for control and remote transmission.

It converts the physical pressure parameters such as gases and liquids sensed by the pressure measuring element sensor into standard electrical signals (e.g., 4~20mADC, etc.). It is then provided to secondary instruments such as display alarm meters, recorders, and regulators for measurement, indication, and process regulation.

2. What’s a direct mount pressure transmitter？

A direct mount pressure transmitter can be installed directly in the pipeline. There are also a variety of thread options. It allows precise measurement, online adjustment, and easy operation. The direct mount pressure transmitter has diffusion silicon and capacitive sensors with stable performance. The direct mount pressure transmitter has undergone tens of thousands of fatigue tests, strict temperature compensation, and a wide range of HART/PA/FF protocols available. It is also available with stainless steel housing for corrosive environments.

3. What’s the working principle of a direct mount diaphragm seal pressure transmitter？

The pressure electrical components for direct mount diaphragm seal pressure transmitters are generally resistance strain gauges. A strain gauge is a sensitive device that converts the pressure on the measured part into an electrical signal. The two most used strain gauges are metal and semiconductor strain gauges. There are two types of metal strain gauges: wire strain gauges and metal foil strain gauges. The strain gauges are usually bonded tightly to a mechanical strain-generating substrate using a unique adhesive. When the substrate is subjected to stress changes, the strain gauges are deformed together. As a result, it changes the resistance of the strain gauges, and thus a difference in the voltage applied to the opposition.

4. What are the technical specifications of a direct mount diaphragm seal pressure transmitter？

• Measuring medium: liquid, gas, or vapor
• Power supply: 24V DC (normal working voltage range: 10.5~40V DC)
• Output signal: 2-wire system, 4～20mA DC output, superimposed HART protocol digital communication.
• Measurement range.

Range code     Range range (KPa)        Measuring range

3              0.8~7.5KPa –            -7.5~7.5KPa

4              1.0~37.4KPa –          -37.4~37.4KPa

5            4.7~186.8KPa –         -100~186.8KPa

6              17.3~690Kpa           -100~690KPa

7              51.7~2068KPa          -100~2068KPa

8              172.3~6890KPa          0~6890KPa

9              689~20680KPa          0~20680KPa

0              1723~68900KPa         0~68900KPa

• Explosion-proof marking: Exd IICT6, intrinsically safe Exia IICT6
• Protection class: IP66
• Migration characteristics

1) The maximum positive migration rate is 500% of the minimum adjustment range

2)The maximum migration rate in the negative direction is 600% of the minimum adjustment range.

• Ambient temperature:-40～85℃（LCD meter head is -30～70℃）
• Explosion-proof type:-30～70℃
• Intrinsically safe type:-30～60℃
• The temperature of liquid connection:-40～105℃
• Storage temperature:-40～85℃
• Ambient humidity:0～100%RH
• Static pressure and overpressure limit: withstand pressure range: 100KPa~40MPa
• Both flanges can withstand pressure: 60Mpa
• Volume change: less than 0.16cm3
• Damping: 0.1~32.0s (adjustable)
• Starting time: 2s

 

5. What are the performance indicators of a diaphragm seal pressure transmitter？

• Range ratio: 100:1
• Accuracy class: 0.075 class, 0.1 class
• Range ratio effects

1) Class 0.075: ±0.075% of the adjustment range when the range ratio is 1:1 to 10:1, ±[0.025×(1+0.2×range ratio)] % when the range ratio is 10:1 to 100:1

2) Class 0.1: ±0.1% of the calibration range for a range ratio of 1:1 to 10:1, ±[0.05 x (1 + 0.1 x range ratio)] % for a range ratio of 10:1 to 100:1

• Stability

1) Class 0.075: 36 months error of ±0.2% of the maximum range

2) Class 0.1: ±0.25% relative to Class 0.075

• Temperature effect

1) Class 0.075: ±0.15%/28°C of maximum range for zero point error, including the total mistake of field and zero point, ±0.25%/28°C of the full range

2) Class 0.1: Less relative to 0.075, 0.2%/28°C and less relative to 0.075, ±0.3%/28°C

• Static pressure effects.

1) Zero point error: zero-point error is ±0.5% of the maximum range

2) Range error: The range error is -1.5 ± 0.2 5% of the maximum range

The zero point error and range error are systematic errors. You can eliminate this pressure difference by adjusting the pressure transmitter to the actual static pressure before installation.

• Power influence: less than 0.05% of the output range
• Vibration impact: in any axis, caused by the error of the maximum range of ± 0.05% / g
• Load effect: no-load effect in the load working area as long as the voltage input to the transmitter is above 10.5V
• Installation influence: It can produce an error in the zero position of up to 0.25KPa. However, you can correct it. It has no effect on the measuring range and no effect on the rotation of the measuring body relative to the flange.
• Electromagnetic radiation: by IEC801 standard

6. What are the structural instructions of a  diaphragm seal pressure transmitter？

• Isolation diaphragm: 316 stainless steel, Hastelloy C, Monel alloy, tantalum, gold plated
• Sensor housing: 316 stainless steel, Hastelloy C
• Electrical housing: low copper aluminum alloy / 316 stainless steel
• Coating: Conductive oxide inner layer, the polyester epoxy outer layer
• Lead process connection: flange, coupling 1/2-1/4 NPT (tapered pipe thread)
• Electrical connection: M20X1.5 male process connection, M20X1.5 female process connection, G1/2 male process connection, 1/2-1/4 NPT process connection, other connections as specified by the user
• Weight: 1.2 kg (excluding options)

7. What are the main advantages of a diaphragm seal pressure transmitter？

1) Reliable operation and stable performance

2)Special V/I integrated circuit, few peripheral devices, high reliability.

The direct mount pressure transmitter is straightforward to maintain. It’s also compact and light in weight. And it is extremely convenient to install and commission.

3) Aluminum alloy die-casting shell

three-terminal isolation, electrostatic spraying protection layer, robust and durable.

4) 4-20mA DC 2-wire signal transmission

strong anti-interference ability, long transmission distance.

5) LED, LCD, pointer three kinds of indication meter head, the field reading is very convenient.

You can apply it to the measurement of viscous, crystalline, and corrosive fluids.

6) High accuracy, high stability.

In addition to the original imported sensor has been corrected with laser, the comprehensive temperature drift and non-linearity of the whole machine in the use of temperature range for adequate compensation.

8. What are the selection rules of the diaphragm seal pressure transmitter?

1) Based upon the type of the pressure to the measured:

The main pressure transmitters are gauge pressure, absolute pressure, differential pressure, etc.

Gauge pressure is defined as less than or greater than atmospheric pressure. Absolute pressure is defined as higher than the absolute pressure zero point. Differential pressure is defined as the difference between two pressures.

2) Based on the measured pressure range:

In general, according to the actual measurement pressure for the measurement range of 80% selected.

To consider the maximum pressure of the system. In general, the pressure transmitter pressure range maximum should reach the maximum pressure value of the system 1.5 times.

Some water pressure and process controls have pressure spikes or continuous pulses. These spikes can reach five or even ten times the “maximum” pressure and can cause damage to the transmitter. Continuous high-pressure pulses, close to or exceeding the maximum pressure rating of the transmitter, can shorten the useful life of the transmitter. However, increasing the transmitter pressure rating will sacrifice the resolution of the transmitter. Therefore, you can use a buffer in the system to attenuate spikes, which will reduce the transducer’s response.

Pressure transmitters are generally designed to withstand a maximum pressure for 200 million cycles without degrading performance. Therefore, when selecting a transmitter, you can find a compromise between system performance and transmitter life.

3) Based on the medium to be measured:

According to the different measurement media, pressure transmitters can be divided into dry gas, gas-liquid, strong corrosive liquid, viscous liquid, high-temperature gas-liquid, etc. According to the different media, choose the correct model, which will help extend the transmitter’s life.

4) Based on the maximum overload of the system:

The system’s maximum excess load should be less than the transmitter’s overload protection limit. Otherwise, the life of the transmitter will be affected, and it may even damage the transmitter. Typically, the safety overload voltage of a pressure transmitter is two times the full scale.

5) Based on the required accuracy level:

The measurement error of the transmitter is divided according to the accuracy level, with different accuracy levels corresponding to other fundamental error limits (expressed as a percentage of the full range output). In practice, according to the control requirements of the measurement error and in line with the principle of economic use to choose.

6) Based on the operating temperature range of the system:

The temperature of the measurement medium should be in the working temperature range of the transmitter. If over-temperature is used, it will produce a large measurement error and affect the life of the transmitter. During the production of the pressure transmitter, the influence of temperature will be measured and compensated for. It ensures that the measurement errors arising from their temperature effects are within the limits required by the accuracy class. On higher temperature occasions, you can consider the choice of a high-temperature type pressure transmitter or take the installation of condenser tubes, radiators, and other auxiliary cooling measures.

7) Based on the compatibility of the measurement medium and the contact material:

In some measuring situations, the measuring medium is corrosive. In this case, it is necessary to select materials or unique processes compatible with the measuring medium. In addition, it ensures that the pressure transmitter is not damaged.

8) Based on the pressure interface form:

Usually, the threaded connection (M20 x 1.5) is the standard interface form.

9) Based on the power supply and output signal:

Typically pressure transmitters are DC-powered and offer a wide choice of output signals. It includes 4 to 20mA, 0 to 5VDC, etc. It can also have a digital output of 232 or 485.

10) Based on the working environment of the site and other:

Whether there is vibration and electromagnetic interference, etc. Relevant information should be provided when selecting the type to take the corresponding treatment.

9. What are the operating instructions of a  diaphragm seal pressure transmitter?

• Daily maintenance

Check the size of the mounting holes: If the mounting holes are not the right size, they will quickly wear the threaded part of the sensor during the installation process. It affects the sealing performance of the equipment and prevents the pressure sensor from functioning fully. It may even create a safety hazard. Only suitable mounting holes can prevent wear of the threads (industry standard for threads is 1/2-20 UNF 2B). You can usually check it with a mounting hole gauge to make the appropriate adjustments.

2) Keeping the mounting holes clean: It is essential to keep the mounting holes clean and prevent melt blockage to ensure proper operation of the machine. Before cleaning the extruder, you should remove all pressure sensors from the barrel to avoid damage. The melt flows into the mounting holes and hardens when the sensor is removed. If this residue is not removed, damage can be caused to the top of the sensor when it is reinstalled. The purging kit will eliminate this fusion remains. However, repeated cleaning processes may deepen the damage caused to the sensor by the mounting holes. If so, you should take action to lift the sensor into the installation orifice.

3) Choose the proper position: If the pressure sensor is installed too close to the upstream end of the line, unmelted material may wear away at the top of the sensor. If the sensor is mounted too far back, it may create a melted-material stagnation zone between the sensor and the screw stroke. It is where melt degradation may occur, and the pressure signal may be distorted. If the sensor is too far into the barrel, the screw may hit the sensor as it rotates. If the sensor is too far into the barrel, the screw may touch the top of the sensor during rotation and damage it. Generally, the sensor can be located on the barrel in front of the screen, behind the melt pump, or in the mold.

4) Clean carefully： Before cleaning the extruder barrel with a wire brush or special compound, you should remove all sensors. It is because both cleaning methods may cause damage to the vibration film of the sensor. When the barrel is heated, you should also remove the sensor. Wipe its top with a soft cloth that will not cause abrasion. The holes in the sensor also need to be cleaned with a clean drill and guide bush.

• Correct use

You should take the following conditions into account during the use of the pressure transducer.

1) Preventing the transmitter from coming into contact with corrosive or overheated media.

2) Preventing dross from being deposited in the conduit.

3) When measuring liquid pressure, the pressure take-off port should be open on the side of the process pipe to avoid sedimentation and slag accumulation.

4) When measuring gas pressure, you should open the pressure take-off port at the top of the process pipe, and the transmitter should also be installed at the top of the process pipe so that accumulated liquid can be quickly injected into the process pipe

5) You should install the pressure guide pipe in a place where temperature fluctuations are negligible.

6) When measuring steam or other high-temperature media, it should be connected to a buffer tube (coil) and another condenser. It should not cause the working temperature of the transmitter to exceed the limit.

7) In winter, when freezing occurs, the transmitter installed outdoors must take anti-freezing measures to avoid the liquid in the pilot port due to icing volume expansion, leading to sensor damage.

8) When measuring liquid pressure, the transmitter should be installed in a position that avoids the impact of the liquid (water hammer phenomenon). It is to prevent overpressure damage to the sensor.