【Daily Question 20090304】Selection of the principle for pressure transmitters

The structure of single-crystal silicon resonant transmitters is simpler than that of capacitive types; they offer stable performance with virtually no zero drift. However, the precision cannot be as high as that of capacitive types.

Single-crystal silicon resonant sensor: It is a single-crystal silicon chip on which microelectromechanical processing techniques are used to create two identical H-shaped resonant beams, which vibrate at a certain frequency. Its resonant frequency depends on the length and tension of the beam; the length of the beam is fixed, while the tension changes with pressure. Thus, the change in pressure is converted into a change in frequency; frequency difference technology is applied to the differential pressure, and the frequency difference signal is directly output to the CPU for processing and A/D conversion. It has the following features: 1. High precision: Since the sensor uses a single-crystal silicon resonant sensor, its excellent performance ensures high measurement accuracy; the precision of this transmitter can reach ±0.065%, which is unmatched by other transmitters. 2. Good stability: Thanks to the use of a characteristic correction memory in the circuit, the temperature and static pressure data collected are processed by the CPU to correct any measurement drift that may occur. At the same time, the intelligent converter uses large-scale integrated circuits and transforms the amplifier into an AISC type, reducing the number of components and enhancing the reliability of the amplifier itself, thereby ensuring that the instrument possesses very high stability and repeatability. Our factory has been using EJA smart transmitters since 1997, and to date there has been almost no need for maintenance; neither the zero point nor the range shifts, which makes them very popular among those responsible for transmitter maintenance. 3. Good hydrostatic characteristics: Due to the precise manufacturing process of the two resonant beams, their dimensions are exactly the same and they lie on the same surface; as a result, the frequency changes that occur under pressure are identical, with no variation in their difference. Therefore, the introduction of a static pressure gauge has almost no impact on the measurement, ensuring its accuracy. 4. It possesses excellent unidirectional compression properties: The transmitter has a very high capacity to withstand repeated pressure applied alternately on the high-pressure side and the low-pressure side, with values reaching up to 16 Mpa. Its compression time can reach 30 seconds. Its advantage is that it eliminates the need for a balance valve during installation, saving on costs. On the other hand, it can prevent the transmitter from being damaged due to unilateral pressure caused by misoperation or blockage of the pressure guiding tube, thus avoiding losses for the enterprise. 5. Wide measurement range: Within its typical operating range, the EJA smart transmitter has a span ratio of 100:1; it can also be 30:1 to 40:1. However, in addition to assessing whether a transmitter has a wide measurement range, it is also necessary to consider the materials used for various components of the transmitter as well as whether many additional specifications allow the device to be used for multiple purposes. The EJA transmitter excels in this regard: the materials used for its contact parts, the body, and the exhaust valve are all JIS SUS316, while the diaphragm box is made of JIS SUS316L (and the diaphragm itself is made of Hastelloy C-276). Such highly corrosion-resistant materials enhance the versatility of the transmitter. Therefore, a wide range ratio, versatile materials for the liquid-contacting components, and a reduction in additional specifications enhance the practicality and versatility of the transmitter, bringing convenience to designers and users. At the same time, it also reduces the number and variety of auxiliary tables, thereby cutting maintenance costs for enterprises. 6. Convenient configuration capabilities and self-diagnosis function: The EJA transmitters feature intelligent communication capabilities, allowing remote setting of parameters such as range, engineering units, calculation functions, damping time, and tag numbers via handheld terminals like BT200/HART375 or DCSs (with I/O cards that have field communication functions). It is also possible to perform fault diagnosis and zero-point adjustment on the transmitter ； Furthermore, the transmitter’s self-diagnosis function can display its operating status and fault information, thereby assisting maintenance personnel in analyzing and diagnosing faults. Capacitive pressure transmitter: It utilizes a variable capacitance mechanism that is simple in structure, durable, and highly stable. This variable capacitance consists of a diaphragm located in the pressure chamber along with insulating electrodes fixed to it; when pressure changes occur, the diaphragm bends slightly, which in turn changes the distance between the two electrodes. A specialized detection circuit is used to measure these minor changes in capacitance, followed by linear processing and temperature compensation. The sensor outputs a DC voltage or current signal that is proportional to the pressure being measured. The perfect combination of a sophisticated design, high-performance materials, and advanced detection circuits endows capacitive pressure transmitters with excellent performance. It has the following features: 1. High performance: The capacitive materials used in manufacturing the transmitter possess extremely stable physicochemical properties, enabling the product to achieve very high performance levels. Sensors with a precision of up to ±0.02% FS and stability better than ±0.05% FS offer such high performance, which is difficult to achieve with products based on other sensing principles. 2. Mechanical deformation: Even slight changes in the distance between the plates of a sensitive capacitor can result in measurable variations in the voltage signal. Minor mechanical deformations reduce the hysteresis and non-repeatability errors of the sensor, while also significantly increasing its speed. 3. Wide measurement range: These pressure transmitters feature a very wide measurement range; they can accurately measure pressures in the range of 25 Pa to 70 MPa, with extremely high stability. 4. Good long-term stability: Pressure transmitters exhibit higher stability compared to other similar products. Unlike other sensors such as metal strain gauges, capacitive pressure transmitters are minimally affected by creep, aging, and temperature changes. Almost all adverse factors have a smaller impact on the output stability of capacitive sensors compared to sensors of other types. The zero-point stability of the pressure transmitter can reach 0.05%FS/year. 5. High-output signal: The circuit of the pressure transmitter can directly convert minor changes in capacitance into a high-output signal, without the need for signal amplification. Piezoresistive sensors (thin-film type, type C) have the disadvantage of producing low-output signals and being susceptible to external signal interference; these are usually the main reasons for poor sensor stability, significant sensitivity to temperature, and susceptibility to electromagnetic interference. 6. Good corrosion resistance: The materials in contact with the medium in pressure transmitters are all made of high-quality stainless steel, allowing them to be compatible with many acid and alkali solutions, as well as corrosive gases or liquids. 7. Resistance to electromagnetic interference: Thanks to its high-output signal, interference-resistant design, and metal casing, the pressure transmitter has a high capacity to suppress external electromagnetic fields; it possesses interference resistance comparable to that of programmable controllers. 8. Operation in harsh environments: Pressure transmitters are highly durable; their industrial-grade versions can withstand at least 10^7 pressure cycles up to their full scale. If the operating pressure remains constant, the sensor’s service life can be virtually indefinite. Moreover, these industrial products can endure impacts of 100–200 kg as well as vibrations of at least 10–20.

During use, we did not notice any difference between the two types of transmitters (perhaps due to lack of attention; please advise). The main capacitive pressure transmitters are Rosemount’s 1151 and 3051 series. Single-crystal silicon resonant piezoelectric transformers: Yokogawa’s 430 and 530 series in Japan. Principle of capacitive pressure transmitters: The two pressures of the medium being measured by the pressure transmitter are introduced into the high and low pressure chambers; these pressures act on the diaphragms on either side of the δ element (i.e., the sensitive element), and are transmitted to both sides of the measuring diaphragm through the diaphragms and the filling fluid inside the element. The measuring diaphragm and the electrodes on the insulating sheets on both sides each form a capacitor. When the pressures on both sides are not equal, it causes the measuring diaphragm to shift; the amount of this displacement is proportional to the pressure difference. As a result, the capacitances on the two sides become unequal. Through oscillation and demodulation processes, this is converted into a signal that is proportional to the pressure. The working principle of pressure transmitters and absolute pressure transmitters is the same as that of differential pressure transmitters; the difference is that the pressure in the low-pressure chamber is atmospheric pressure or vacuum. Working principle of the EJA series intelligent transmitters: The EJA series intelligent transmitters use single-crystal silicon resonant sensors. Microelectromechanical processing techniques are employed on the single-crystal silicon chip to create two H-shaped resonant beams of identical shape and size at the center and edges of its surface respectively. Since they are located within a micro vacuum chamber and do not come into contact with the filling fluid, they are free from the influence of air damping during vibration. The resonant beams convert pressure and differential pressure signals into frequency signals, which are sent to the pulse counter. The difference between the two frequencies is then transmitted directly to the CPU (microprocessor) for data processing. The result is an output signal of 4–20 mA DC corresponding to the input signal, and a BRAIN/HART digital signal is superimposed on this analog signal for communication purposes. ? The built-in characteristic correction memory in the diaphragm box stores the ambient temperature of the sensor, the static pressure, as well as the input/output characteristic correction data; through processing by the CPU, this enables the transmitter to achieve excellent temperature characteristics, static pressure characteristics, and input/output characteristics.

During use, we did not notice any difference between the two types of transmitters (perhaps due to lack of attention; please advise). The main capacitive pressure transmitters are Rosemount’s 1151 and 3051 series. Single-crystal silicon resonant piezoelectric transformers: Yokogawa’s 430 and 530 series in Japan. Principle of capacitive pressure transmitters: The two pressures of the medium being measured by the pressure transmitter are introduced into the high and low pressure chambers; these pressures act on the diaphragms on either side of the δ element (i.e., the sensitive element), and are transmitted to both sides of the measuring diaphragm through the diaphragms and the filling fluid inside the element. The measuring diaphragm and the electrodes on the insulating sheets on both sides each form a capacitor. When the pressures on both sides are not equal, it causes the measuring diaphragm to shift; the amount of this displacement is proportional to the pressure difference. As a result, the capacitances on the two sides become unequal. Through oscillation and demodulation processes, this is converted into a signal that is proportional to the pressure. The working principle of pressure transmitters and absolute pressure transmitters is the same as that of differential pressure transmitters; the difference is that the pressure in the low-pressure chamber is atmospheric pressure or vacuum. Working principle of the EJA series intelligent transmitters: The EJA series intelligent transmitters use single-crystal silicon resonant sensors. Microelectromechanical processing techniques are employed on the single-crystal silicon chip to create two H-shaped resonant beams of identical shape and size at the center and edges of its surface respectively. Since they are located within a micro vacuum chamber and do not come into contact with the filling fluid, they are free from the influence of air damping during vibration. The resonant beams convert pressure and differential pressure signals into frequency signals, which are sent to the pulse counter. The difference between the two frequencies is then transmitted directly to the CPU (microprocessor) for data processing. The result is an output signal of 4–20 mA DC corresponding to the input signal, and a BRAIN/HART digital signal is superimposed on this analog signal for communication purposes. ? The built-in characteristic correction memory in the diaphragm box stores the ambient temperature of the sensor, the static pressure, as well as the input/output characteristic correction data; through processing by the CPU, this enables the transmitter to achieve excellent temperature characteristics, static pressure characteristics, and input/output characteristics.

I didn’t notice the difference between the two when using them again; I’m still learning*. . . . . . .

There is not much difference in the use of these two types of transmitters; it mainly depends on the system to which they are connected. It’s important to choose the right digital protocol. Additionally, people usually consider the price, and there is fierce competition in terms of pricing between the two mainstream models, 3051 and EJA

The two pressures of the medium being measured by the pressure transmitter are introduced into the high and low pressure chambers; these pressures act on the diaphragms on either side of the δ element (i.e., the sensitive element), and are transmitted to both sides of the measuring diaphragm through the diaphragms and the filling fluid inside the element. The measuring diaphragm and the electrodes on the insulating sheets on both sides each form a capacitor. When the pressures on both sides are not equal, it causes the measuring diaphragm to shift; the amount of this displacement is proportional to the pressure difference. As a result, the capacitances on the two sides become unequal. Through oscillation and demodulation processes, this is converted into a signal that is proportional to the pressure. The working principle of pressure transmitters and absolute pressure transmitters is the same as that of differential pressure transmitters; the difference is that the pressure in the low-pressure chamber is atmospheric pressure or vacuum. The A/D converter converts the current from the demodulator into a digital signal, whose value is used by the microprocessor to determine the input pressure value. A microprocessor controls the operation of the transmitter. Additionally, it performs sensor linearization. Reset the measurement range. Unit conversions for engineering purposes, damping, square root calculations, sensor fine-tuning, as well as diagnosis and digital communication. This microprocessor has 16 bytes of RAM for programs, and three 16-bit counters, one of which performs A/D conversion. The D/A converter fine-tunes the digital signal data coming from the microprocessor and having been corrected; these data can be modified using transmitter software. The data is stored in EEPROM, remaining intact even when power is lost. The digital communication line provides a connection interface for the transmitter to external devices, such as a Type 275 intelligent communicator or control systems that use the HART protocol. This circuit detects the digital signal superimposed on the 4-20mA signal and transmits the required information through the loop. The communication type is Frequency Shift Keying FSK technology, in accordance with the BELL202 standard.

There is not much difference in the use of the two types of transmitters; single-crystal silicon resonant transmitters basically have no zero drift, while capacitive transmitters exhibit significant zero drift during operation

Monocrystalline silicon resonant transmitters have virtually no zero drift, while capacitive transmitters exhibit noticeable zero drift during use, but they are more accurate

We equip the devices with the 1151 model: there are both pressure transmitters and differential pressure transmitters.

Single-crystal silicon resonant pressure transmitter: 1. High precision 2. Good stability 3. Excellent static pressure performance 4. Good unidirectional compression characteristics 5. Wide measurement range 6. Convenient configuration capabilities and self-diagnosis functions

For single-crystal silicon resonant types, EJA is mainly used, while for capacitive types, ROSEMENT is primarily utilized

Everyone has discussed the details thoroughly, so I’ll just briefly talk about the situation here. We have both types of situations in our case; everyone should decide which one to use by considering the discussions above along with the actual conditions of their own factories

There is not much difference in the use of these two types of transmitters; it mainly depends on the system to which they are connected. It’s important to choose the right digital protocol. Additionally, people usually consider the price, and there is fierce competition in terms of pricing between the two mainstream models, 3051 and EJA

There is not much difference in the use of the two types of transmitters

The precision can reach 0.075%; it mainly depends on the price and the service provided by the manufacturer. For applications involving low differential pressures, such as furnace negative pressure, capacitors are a better choice, while silicon resonators are more suitable for applications with absolute pressure. Specifically, silicon resonators from Japanese companies like Yokogawa should be used; for capacitors, Emerson or Rosemount options work well； Using SMAR for capacitors works very well too

It seems to me that there isn’t much difference in the use of the two transmitters; we use both here.

We didn’t notice any difference either; it’s probably just due to a lack of attention. We mostly use EJA, with a small portion using Rosemount.

Last time, I had a technical discussion with the field sales representatives from Honeywell; in terms of temperature and pressure compensation, resonant types are superior to capacitive types, especially when it comes to pressure – capacitive types cannot measure pressure directly, only pressure differences. While the resonant type can combine differential pressure, static pressure, and temperature. I’m not sure if what I said is correct; please everyone review it.