Working principle of pressure transmitter

The two pressures of the measured medium in the pressure transmitter are introduced into the high and low pressure chambers, acting on the isolation diaphragms on both sides of the δ element (i.e. sensitive element), and transmitted to both sides of the measuring diaphragm through the isolation diaphragms and the filling liquid inside the element. Measure the electrodes on both sides of the diaphragm and insulating film to form a capacitor.

When the pressure on both sides is not consistent, it causes displacement of the measuring diaphragm, which is proportional to the pressure difference. Therefore, the capacitance on both sides is unequal. Through oscillation and demodulation, it is converted into a signal proportional to the pressure. The working principle and differential pressure transmitter are the same as absolute, but the difference is that the pressure in the low-pressure chamber is atmospheric pressure or vacuum.

The A/D converter converts the current of the demodulator into a digital signal, which is used by the microprocessor to determine the input pressure value. The microprocessor controls the operation of the transmitter. Additionally, it linearizes the sensor. Reset the measurement range. Engineering unit conversion, damping, square root, sensor fine-tuning and other calculations, as well as diagnosis and digital communication.

This microprocessor has 16 byte program RAM and three 16 bit counters, one of which performs A/D conversion.

The D/A converter fine tunes the digital signal from the microprocessor that has been calibrated, and these data can be modified using transmitter software. The data is stored in EEPROM and remains intact even when powered off.

The digital communication line provides a connection interface for the transmitter to external devices such as the 275 smart communicator or control systems using the HART protocol. This circuit detects the digital signal superimposed on the 4-20mA signal and transmits the required information through the circuit. The type of communication is frequency shift keying (FSK) technology and is based on the BeII202 standard.

feature

● High precision;
● Good stability;
● Two wire system (special four wire system is available);
Solid components, plug-in printed circuit boards;
Small, lightweight, sturdy and vibration resistant;
● Range and zero point can be continuously adjusted externally;
Positive migration can reach 500%; Negative transfer can reach 600%;
● Adjustable damping;
Good one-way overload protection characteristics;
No mechanical movable parts, low maintenance workload;
The entire series has a unified structure and strong interchangeability of components;
The membrane material in contact with the medium can be selected;
(316L, TAN, HAS-C, MONEL and other corrosion-resistant materials)
Explosion proof structure, all-weather use;
Intelligent HART fieldbus protocol.

Functional Parameters

● Target audience: liquids, gases, and vapors
Measurement range: 0-0.08kPa to 0-40MPa
Output signal: 4~20mA DC (special can be four wire system)
220V AC power supply, 0~10mA DC output
Power supply: 12-45V DC, usually 24V DC
(See Figure 2 for load characteristics)

● Load characteristics: Related to the power supply, the load capacity at a certain power supply voltage is shown in Figure 3. The relationship between load impedance RL and power supply voltage Vs is: RL ≤ 50 (Vs-12)
Indicator: pointer type linear indication with 0-100% scale and LCD display.
Explosion proof grade: a: Explosion proof type (Exd Ⅱ BT5 or Exd Ⅱ CT6)
b: Intrinsic safety type (Exia Ⅱ CT6 or Ex ib Ⅱ CT6)
Range and zero point: externally continuously adjustable
Positive and negative transfer: After the zero point undergoes positive or negative transfer, the absolute values of the upper and lower limits of the range and measurement range cannot exceed 100% of the upper limit of the measurement range.
The maximum positive migration is 500% of the minimum calibration range; The maximum negative transfer is 600% of the minimum calibration range
● Temperature range: Operating temperature range: -20~+88 ℃, (LT type: -25~+70 ℃)
Measuring element filled with silicone oil: -40~+104 ℃
When filling high-temperature silicone oil into flange type transmitters:+15~+315 ℃, ordinary silicone oil: -40~+149 ℃
Static pressure: 4, 10, 25, 32MPa
● Humidity: Relative humidity is 0~100% RH
● Volume change:<0.16cm3
Damping (step response): When filled with silicone oil, it is generally continuously adjustable between 0.2s and 1.67s

Technical data

(Without migration, under standard working conditions, filled with silicone oil, 316 stainless steel isolation diaphragm)
Accuracy:+(-) 0.075%
Dead zone: None (≤ 0.1%)
Stability: Absolute value of basic error not exceeding the maximum range within six months
Vibration influence: In any axial direction, when the vibration frequency is 200Hz, the error is ± 0.05%/g of the upper limit of the measurement range
Power supply impact: less than 0.0059%/V of the output range
● Load impact: If the power supply is stable, the load will not be affected

other

Isolation membrane: 316L stainless steel, Hastelloy C-276, Monel alloy, titanium or tantalum
Exhaust/drain valve: 316 stainless steel, Hastelloy C, Monel alloy
● Flanges and fittings: 316 stainless steel, Hastelloy C or Monel alloy
Contact medium "0" ring: nitrile rubber, fluororubber
● Filling fluid: silicone oil or inert oil
Bolt: 316L stainless steel
Electronic casing material: low copper aluminum alloy
● Pressure connection line: flange NPT1/4 center distance 54mm; joint NPT1/2 or M20 × 1.5 male threaded ball cone seal, with
When connecting, the center distance is 50.8, 54, and 57.2mm (NPT taper pipe thread complies with GB/T12716-91)
● Signal line connection hole: G1/2
Weight: 3.5kg (standard type, excluding options)

Outline dimension installation and connection diagram

On site wire connection diagram and circuit block diagram

Intelligent Circuit Block Diagram

Selection of pressure transmitter

1. What kind of pressure does the transmitter need to measure

First, determine the maximum value of the measured pressure in the system. Generally, it is necessary to choose a transmitter with a pressure range that is about 1.5 times larger than the maximum value. This is mainly due to peak values and continuous irregular fluctuations in many systems, especially in water pressure measurement and processing, which can damage pressure sensors. Continuous high pressure values or slightly exceeding the calibrated maximum value of the transmitter will shorten the lifespan of the sensor and also reduce accuracy. So a buffer can be used to reduce pressure spikes, but this will slow down the response speed of the sensor. So when choosing a transmitter, it is necessary to fully consider the pressure range, accuracy, and stability.

2. What kind of pressure medium

Viscous liquids and mud may block the pressure interface, and solvents or corrosive substances may damage the materials in direct contact with these media in the transmitter. The above factors will determine whether to choose direct isolation membranes and materials that come into direct contact with the medium.

3. What precision is required for the transmitter

The factors that determine accuracy include nonlinearity, hysteresis, non repeatability, temperature, zero offset scale, and the influence of temperature. But mainly due to nonlinearity, hysteresis, non repeatability, the higher the accuracy, the higher the price.

4. Temperature range of transmitter

Usually, a transmitter will calibrate two temperature ranges, one of which is the normal operating temperature and the other is the temperature compensation range. The normal operating temperature range refers to the temperature range when the transmitter is not damaged in its working state. If it exceeds the temperature compensation range, it may not meet its performance indicators for application.

The temperature compensation range is a typical range that is smaller than the operating temperature range. Working within this range, the transmitter will definitely achieve its expected performance indicators. Temperature changes affect its output from two aspects: zero drift and full-scale output. For example:+/- X%/℃ of full scale,+/- X%/℃ of reading,+/- X% of full scale when outside the temperature range, and+/- X% of reading within the temperature compensation range. Without these parameters, it will lead to uncertainty in use. Is the change in transmitter output caused by pressure changes or temperature changes. The temperature effect is the most complex part of understanding how to use a transmitter.

5. What kind of output signal is needed

The choice of mV, V, mA, and frequency output for digital output depends on various factors, including the distance between the transmitter and the system controller or display, the presence of "noise" or other electronic interference signals, the need for an amplifier, and the position of the amplifier. For many OEM devices with short distances between transmitters and controllers, using mA output transmitters is the most economical and effective solution.

If it is necessary to amplify the output signal, it is best to use a transmitter with built-in amplification. For long-distance transmission or strong electronic interference signals, it is best to use mA level output or frequency output.

If in an environment with high RFI or EMI indicators, in addition to choosing mA or frequency output, special protection or filters should also be considered.

6. What kind of excitation voltage to choose

The type of output signal determines the choice of excitation voltage. Many transmitters have built-in voltage regulation devices, so their power supply voltage range is large. Some transmitters are quantitatively configured and require a stable operating voltage. Therefore, the operating voltage determines whether to use a sensor with a regulator. When selecting a transmitter, the operating voltage and system cost should be considered comprehensively.

7. Do you need a transmitter with interchangeability
Determine whether the required transmitter can adapt to multiple usage systems. Generally speaking, this is very important, especially for OEM products. Once the product is delivered to the customer, the cost for calibration is quite high. If the product has good interchangeability, even changing the transmitter used will not affect the overall performance of the system.

8. The transmitter needs to maintain stability after timeout operation
Most transmitters will experience "drift" after excessive operation, so it is necessary to understand the stability of the transmitter before purchasing. This pre operation can reduce various troubles that may occur in future use.

9. Encapsulation of transmitter

The packaging of the transmitter is often overlooked as its rack, but this will gradually expose its shortcomings in future use. When choosing a transmitter, it is important to consider the future working environment, humidity, installation methods, and whether there will be strong impacts or vibrations.

10. What kind of connection should be used between the transmitter and other electronic devices

Do we need to use a short distance connection? If long-distance connection is used, is it necessary to use a connector?