EJA is a well-known series of instruments produced by Yokogawa, a leading Japanese company in the field of industrial automation. The EJA series includes various high-performance devices such as pressure transmitters, differential pressure transmitters, level transmitters, and flow meters. These instruments were first developed in 1994 by Yokogawa Electric Co., Ltd. and are based on advanced single-crystal silicon resonant sensor technology. Since their introduction, EJA transmitters have gained widespread recognition for their reliability and accuracy.
The development of the EJA series has earned prestigious awards, including the Daehan Memorial Award in Japan and the Quality Product Award from Hashimoto Taro. Additionally, EJA products have passed rigorous safety certifications in many advanced countries such as the United States, the United Kingdom, France, Germany, Russia, and China. The EJA transmitters manufactured by Chongqing Company maintain the same quality standards as those from Japan, and they have received positive feedback from users. They are also certified for use in power plants with capacities of 200MW, 300MW, and 600MW units.
This article provides an in-depth overview of how the EJA transmitter works and offers key considerations when selecting one. Let’s dive into the details.
**Key Features of the EJA Transmitter:**
1. The EJA transmitter is not easily affected by thermocouples or voltage drops along the wire, allowing the use of thinner and more cost-effective cables, which significantly reduces installation costs.
2. Capacitive interference can cause errors in the receiver resistance. For a 4-20mA two-wire loop, where the receiver resistance is typically 250Ω, the low resistance may lead to significant inaccuracies. Therefore, the EJA allows for longer wiring distances without compromising performance.
3. It is easy to add surge protection devices at the two-wire output port, enhancing safety against lightning strikes.
4. Each reading or recording device can switch between different channels without affecting measurement accuracy, regardless of wire length. This enables distributed data acquisition and centralized control, while the 4 mA zero level helps detect open circuits, short circuits, or sensor failures.
5. When the current source has sufficient output resistance, magnetic coupling-induced voltage in the wire loop will not significantly affect the signal. EJA transmitters can reduce interference using twisted pair cables. In contrast, three-wire and four-wire systems require shielded cables, and proper grounding of the shielding is essential.
**How Does the EJA Transmitter Work?**
The EJA transmitter converts differential pressure and pressure signals into frequency signals using two H-shaped vibrating beams located on a single-crystal silicon resonant sensor. These signals are sent to a pulse counter, and the difference in frequencies is processed by the CPU. The analog signal is then converted via an ADC to a 4–20mA output that corresponds to the input signal. A BRAIN/HART digital signal is superimposed on this analog signal for communication.
The built-in correction memory in the bellows assembly stores ambient temperature, static pressure, and input/output characteristic data. With CPU processing, the transmitter achieves excellent temperature, static pressure, and input/output characteristics. Digital communication is possible through the I/O port with external devices like handheld terminals (e.g., BT200, 275) or DCS cards with communication capabilities. High-frequency (2.4 kHz for BRAIN) or low-frequency (1.2 kHz for HART) digital signals are superimposed on the 4–20mA line without interfering with the analog signal during communication.
**Structure and Principle of the Resonant Beam:**
The core of the single-crystal silicon resonant sensor is made using micro-electro-mechanical systems (MEMS) technology. Two H-shaped beams are formed at the center and edge of the silicon chip, enclosed in a vacuum chamber to prevent air damping.
When the silicon resonant beam is placed in a magnetic field provided by a permanent magnet, it forms a positive feedback loop with a transformer and amplifier, causing the beam to oscillate. When pressure is applied to the upper and lower surfaces of the silicon wafer, a pressure difference causes deformation. The central beam experiences compression, while the edge beam experiences tension. This results in different strain effects on each beam, leading to frequency differences that correspond to the measured pressure.
**Performance Characteristics of EJA:**
- Excellent temperature influence characteristics
- Outstanding static pressure influence characteristics
- Strong one-way overvoltage resistance
**EJX Series Overview:**
The EJX series is a high-quality electronic differential pressure transmitter that uses a single-crystal silicon sensor for measuring flow, level, density, and pressure of liquids, gases, or vapors. It features a built-in display for static pressure readings and supports BRAIN or HART communication protocols. Additional features include fast response, remote protocol setting, self-diagnosis, and optional high/low pressure alarm outputs. It is available in FF fieldbus versions and is TUV certified, suitable for SIL2 applications except for the FF fieldbus type.
**How to Choose an EJA Transmitter:**
1. **Pressure Range:** Determine the maximum pressure in your system. It's recommended to select a transmitter with a range about 1.5 times higher than the maximum value to avoid damage from pressure fluctuations.
2. **Medium Type:** Consider the nature of the medium being measured. Corrosive or viscous substances may require special materials or chemical seals to protect the transmitter.
3. **Accuracy Requirements:** Accuracy depends on factors like nonlinearity, hysteresis, and temperature effects. Higher accuracy comes at a higher cost.
4. **Temperature Range:** Understand the operating and compensation temperature ranges to ensure stable performance under varying conditions.
5. **Output Signal:** Choose between mV, V, mA, or frequency outputs based on distance, noise levels, and system requirements.
6. **Excitation Voltage:** Select the appropriate excitation voltage based on the transmitter's design and system compatibility.
7. **Interchangeability:** Ensure the transmitter can be used across multiple systems to simplify integration and reduce calibration costs.
8. **Stability:** Check the long-term stability of the transmitter to minimize future maintenance and downtime.
9. **Enclosure and Installation:** Consider environmental factors like humidity, vibration, and installation space when choosing the housing.
10. **Connection Type:** Decide whether a short or long-distance connection is needed, and whether connectors or other components are required.
11. **Special Requirements:** Confirm the process connection interface and power supply requirements, and consider explosion-proof ratings if necessary.
By carefully considering these factors, you can select the most suitable EJA transmitter for your application, ensuring reliable performance and long-term value.
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