Working Principle and Industrial Applications of Frequency-Type Pressure Transmitters

A frequency-type pressure transmitter is an industrial measuring instrument that converts changes in medium pressure into frequency signal output. Compared with conventional pressure transmitters that directly output voltage or current, it uses frequency as the core information carrier and features stable signals, strong interference resistance, and suitability for long-distance transmission. It is commonly used in chemical energy, machinery manufacturing, automated production lines, environmental monitoring, and supporting systems for intelligent equipment.

Its basic operating logic is not complicated: the measured pressure first acts on the sensing element, causing slight deformation of the elastic structure; the deformation is then converted into a frequency change through a vibrating beam, resonant element, oscillation circuit, or frequency modulation unit; after reading the frequency value, the control system calculates the actual pressure according to the preset calibration relationship. As pressure increases or decreases, the corresponding frequency changes linearly or nonlinearly after temperature compensation.

In industrial automation projects in 2026, pressure measurement requires not only accurate readings but also equipment capable of adapting to high temperature, high pressure, vibration, electromagnetic interference, and continuous operating conditions. As frequency signals are insensitive to voltage drops in transmission lines, frequency-type pressure transmitters have significant engineering application value in long-distance wiring, installations near high-power electrical equipment, and distributed control scenarios.

For purchasing personnel and technical evaluators, understanding the working principle of frequency-type pressure transmitters helps accurately assess range, accuracy, output type, process connection, temperature resistance range, and on-site compatibility, avoiding issues such as signal mismatch, installation difficulty, or increased maintenance costs caused by selecting a model solely based on the same “pressure range.”

I. Core Measurement Mechanism of Frequency-Type Pressure Transmitters

Conversion Process from Mechanical Deformation to Frequency Output

The front end of a frequency-type pressure transmitter typically consists of a pressure connection, isolation diaphragm, sensing core, and signal-processing components. When liquid, gas, or steam pressure enters the measuring chamber through the process connection, it pushes the metal diaphragm or ceramic sensing surface to produce elastic displacement. Although this displacement is usually only at the micron level, it is sufficient to alter the stress state of the sensing structure.

In products using resonant or vibrating-beam structures, pressure-induced stress changes alter the natural vibration frequency of the sensing element. An electronic oscillation circuit continuously excites the element and measures its actual oscillation frequency. For example, within a certain range, 10 kHz to 30 kHz can correspond to 0 MPa to 1.0 MPa, allowing the controller to calculate pressure based on this mapping relationship.

Another type of product first converts pressure changes into capacitance, resistance, or inductance changes, then converts them into pulse frequency through a frequency modulation circuit. Regardless of the internal design path, the ultimate purpose is to establish a stable, calibratable, and traceable relationship between output frequency and pressure, thereby meeting real-time measurement and closed-loop control requirements.

High-quality products are usually calibrated for zero point, full scale, linearity, repeatability, and temperature drift before leaving the factory. For a transmitter with an accuracy class of 0.25%FS and a full scale of 1.6 MPa, its permissible basic error generally needs to be controlled within approximately ±0.004 MPa. Actual specifications should also be confirmed in conjunction with the product manual and operating environment.

Why Frequency Signals Offer Strong Interference Resistance

During voltage signal transmission, line resistance, ground potential differences, and external electromagnetic fields may cause amplitude fluctuations. Frequency signals, however, mainly identify the number of pulses per unit time or the period length. As long as short-term amplitude disturbances do not reach the trigger threshold, they generally do not directly affect counting results. This gives frequency output certain advantages near variable frequency drives, motors, contactors, and high-power switching equipment.

However, interference resistance does not mean wiring practices can be ignored. Frequency-type pressure transmitters should still use shielded cables, proper grounding, and separate routing for power and signal circuits. Especially when cable lengths exceed 50 m, high-frequency equipment is present on site, or control cabinet grounding is complex, common-mode interference in the signal loop requires more careful control.

Another feature of frequency signals is their convenience for digital acquisition. PLC high-speed counters, industrial gateways, and embedded controllers can all read data through pulse counting or period measurement. When the acquisition period is set to 1 s and the sensor outputs 20,000 Hz, 20,000 pulse samples can theoretically be obtained, providing a sound data basis for filtering, averaging, and fault diagnosis.

In practical applications, the maximum input frequency, input level, duty cycle, and isolation method supported by the receiving end should be confirmed. If a PLC only supports ordinary counting inputs below 10 kHz while the transmitter's full-scale output is 25 kHz, a matching module must be configured, or an output solution such as 4-20mA or 0-10V that is more suitable for the existing system should be selected during model selection.

II. Comparison of Frequency-Type and Common Pressure Transmitter Output Methods

Differences Between Frequency Output, Analog Voltage, and Current Output

Common pressure transmitter outputs in industrial projects include frequency, 4-20mA, 0-10V, 0-5V, and 1-5V. No output method is absolutely superior; the key factors are site distance, control system interfaces, interference intensity, power supply conditions, and maintenance practices. Frequency output is suitable for counting-based acquisition and digital processing, while 4-20mA is a highly mature standard signal widely used in process industries.

When comparing 0-10V pressure transmitters with 4-20mA transmitters, voltage signals offer intuitive wiring for short-distance internal equipment connections, but are more susceptible to line voltage drops and interference as transmission distances increase. Using a current loop, 4-20mA is generally more suitable for on-site transmission over several hundred meters, and 4mA can serve as a “live zero” to help identify wire breaks or power supply abnormalities.

0-5V pressure transmitters are commonly used in small equipment, experimental devices, inside control cabinets, and low-cost control systems. A 1-5V pressure transmitter is often used with a 250Ω precision resistor to convert a 4-20mA signal into a 1-5V signal for reading by certain voltage input modules. Before wiring, the power supply polarity, signal common terminal, and load impedance must be confirmed.

The table below can be used for preliminary technical communication, but final model selection should still be based on the actual process medium, installation location, control system manual, and manufacturer technical specifications.

III. Evaluation of Sensing Element Technologies and Temperature Resistance Performance

How to Select Inductive, Strain Gauge, and Piezoresistive Pressure Transmitters

Whether a strain gauge pressure transmitter or a piezoresistive pressure transmitter is better cannot be determined solely by one accuracy indicator. Strain gauge products generally detect elastic body deformation through metal strain gauges and feature mature structures and broad applicability. Piezoresistive products use the piezoresistive effect of semiconductor materials to achieve high-sensitivity measurement and are commonly used in small-range and high-response applications.

The temperature resistance range of an inductive pressure transmitter should be evaluated comprehensively from four aspects: the sensor core, sealing materials, electronic module, and process isolation structure. The ambient temperature of standard industrial models is commonly -20℃ to 80℃, while the medium temperature may be -20℃ to 85℃ or higher. For high-temperature media above 150℃, heat dissipation structures, impulse pipes, isolators, or dedicated high-temperature models are often required.

Piezoresistive sensor cores are sensitive to temperature changes and therefore generally require temperature compensation. For outdoor equipment with significant temperature fluctuations, thermal oil pipelines, or steam auxiliary systems, zero temperature drift and span temperature drift should be closely reviewed, such as “±0.02%FS/℃” or “±0.05%FS/℃,” rather than relying only on room-temperature accuracy.

Medium compatibility should also be verified during model selection. Stainless steel wetted structures are generally suitable for air, water, and conventional hydraulic oil; mildly corrosive or high-viscosity media may require 316L, Hastelloy, ceramic diaphragms, or remote diaphragm seal solutions. Oxygen, hydrogen, food-grade, flammable, and explosive applications also require corresponding cleaning, certification, and explosion-proof configurations.

Reference Comparison of Main Technical Approaches

IV. Key Points for Installation Wiring and Thread Specification Confirmation

How to Confirm the M20×1.5 Pressure Transmitter Thread Specification

M20×1.5 is a common metric fine thread designation, where “M20” indicates a nominal thread diameter of approximately 20 mm and “1.5” indicates a pitch of 1.5 mm. It cannot be directly interchanged with specifications such as G1/2 or NPT1/2. Even if the external dimensions appear similar, differences in thread profile, sealing method, and pitch may result in installation failure or leakage.

To confirm the M20×1.5 pressure transmitter thread specification, check the equipment drawing, nameplate of the old component, process interface dimensions, and sealing type. M20×1.5 commonly relies on end-face sealing gaskets, bonded seals, or conical surface structures, whereas G threads generally use parallel pipe threads with sealing gaskets, and NPT threads rely on tapered pipe thread engagement for sealing. Their installation methods differ.

For high-pressure hydraulic systems, in addition to thread specification, installation hole depth, effective thread length, interface surface quality, and rated pressure resistance should be confirmed. If the system operating pressure is 25 MPa, it is recommended to allow an appropriate margin in the transmitter range and verify whether its overload capacity reaches 150% or 200% of the range to withstand start-stop impacts and instantaneous pressure peaks.

During installation, avoid gripping the electrical housing directly and forcibly tightening it. Use the wrench flats corresponding to the process connection instead. For pump outlets, compressor pipelines, or hydraulic stations with pulsating pressure, buffer tubes, dampers, or pressure buffer fittings can be installed to reduce the impact of continuous shocks on sensing elements and connection points.

Wiring Method for 1-5V Pressure Transmitter Output Signals

Before wiring a 1-5V pressure transmitter, first distinguish whether it is an independent three-wire voltage output product or a two-wire 4-20mA transmitter with an external 250Ω resistor for conversion. The former generally includes power positive, power negative, and signal output terminals; the latter requires a precision resistor to be correctly connected in series within the current loop, with the 1-5V voltage signal obtained from both ends of the resistor.

Using a 4-20mA signal with a 250Ω resistor as an example, according to U=I×R, 4mA corresponds to 1V and 20mA corresponds to 5V. The controller analog input terminals should be connected across the resistor, and the signal common terminal must be correctly connected to the system reference ground according to the electrical drawings. If the resistor is connected in the wrong position or the input terminal is connected to the wrong loop, readings of 0, full-scale drift, or power supply abnormalities may occur.

For frequency-type pressure transmitters, wiring considerations shift to supply voltage, pulse output type, and PLC input method. Confirm whether the output is NPN, PNP, push-pull, open collector, or differential signal, and verify the common terminal wiring of the PLC high-speed counter. In 24VDC power systems, reversed polarity may prevent normal operation. Although some models have reverse polarity protection, this should not replace standardized wiring practices.

After initial power-on, it is recommended to verify the zero output under no-pressure or known standard pressure conditions, then gradually load to 25%, 50%, 75%, and 100% of the range to check response. If frequency jumps, unstable voltage, or significant deviation in PLC display values are found, check power supply ripple, shield grounding, loose terminals, parameter scaling, and sensor installation orientation in sequence.

V. Selection and Maintenance Recommendations for Complex Operating Conditions

Determining Pressure Transmitter Configuration Based on Process Conditions

Pressure transmitter selection should specify at least 6 basic items: measured medium, normal operating pressure, maximum pressure, medium temperature, output signal, and process connection. For measurement and control systems involving liquid level, differential pressure, weighing, flow, and similar applications, pressure parameters often also need to be analyzed together with installation height, pipeline static pressure, density changes, and control valve operating conditions.

When high-temperature, high-pressure, or highly corrosive media are present on site, wetted materials and isolation solutions should be confirmed first. When strong vibration and frequent pulsation are present, structural strength, damping capability, and vibration resistance rating should be considered. When the system needs to connect to a DCS, PLC, or remote monitoring platform, the interface compatibility of 4-20mA, RS485, frequency, or voltage signals should be confirmed in advance.

The advantage of pneumatic pressure transmitters is their suitability for certain non-electric or special explosion-proof environments, but their pipeline maintenance, response speed, and system integration convenience are generally inferior to modern electronic products. Whether the maintenance cost of an electric pressure transmitter is high mainly depends on whether it uses a modular design, has on-site diagnostic functions, offers available spare parts, and whether the installation environment remains within rated conditions over the long term.

Zero-point inspections, seal checks, and signal verification are recommended according to the equipment maintenance cycle. For critical stations in continuous operation, output comparison can be performed once every 6 months. For conditions with significant high temperature, high pressure, corrosion, or vibration, the inspection interval can be shortened according to the equipment risk level. If measurement deviation continues to increase, first rule out blocked pressure ports, liquid accumulation in impulse lines, wiring oxidation, and power supply fluctuations before deciding whether factory recalibration or replacement is necessary.

Frequency-type pressure transmitters achieve highly reliable measurement through the path of “pressure change—sensing element deformation—frequency change—control system conversion.” Their value lies not only in accurate readings, but also in signal stability and system adaptability in complex industrial environments. Combined with proper selection of range, material, thread, temperature resistance rating, and output interface, they can provide continuous and reliable pressure data for automation control systems.

Xi'an Shenghongchuang Instrument Co., Ltd. and its subsidiary Shaanxi Qinkong Sensor Technology Co., Ltd. can provide industrial measurement and control products including pressure transmitters, level transmitters, differential pressure transmitters, and intelligent digital display control instruments for applications such as machinery manufacturing, chemical energy, metallurgy and building materials, logistics and warehousing, environmental monitoring, and automated production lines. Non-standard customization is also supported based on actual operating conditions.

To determine the range, output frequency, M20×1.5 connection, temperature resistance rating, or PLC wiring solution for a frequency-type pressure transmitter, please compile the medium name, pressure range, temperature, installation connection, and control system model, and submit them to professional technical personnel for targeted selection confirmation.