Pressure Transmitter vs. Differential Pressure Transmitter

Sino-Inst has summarized the following differences between them through years of experience:

Differential Structure

A pressure transmitter primarily consists of a load cell sensor (also called a pressure sensor), an amplifier circuit, and supporting components. It converts pressure changes in gases, liquids, or other media measured by the load cell sensor into an electrical signal. This electrical signal is provided to secondary instruments such as indicator alarms, recorders, and conditioners for display, indication, and adjustment.

A differential pressure transmitter primarily consists of a load cell sensor, a module circuit, a display head, a case, and process connections. Process connections include plates, nozzles, and venturi tubes. It converts received differential pressure signals from gases, liquids, or other media into a standardized electronic signal. This electronic signal is provided to secondary instruments such as indicator alarms, recorders, and conditioners for measurement, indication, and process adjustment.

Differential Working Principle

The primary difference between a differential pressure transmitter and a pressure transmitter lies in their measurement principle. A differential pressure transmitter measures the pressure difference generated by the flow of a substance. Pressure transmitters directly measure the pressure exerted on a substance. Furthermore, the differential pressure measured by a differential pressure transmitter can be converted into gas or liquid flow and level using appropriate formulas. Pressure transmitters are more suitable for measuring the pressure of liquids or gases.

The pressure-sensing electrical component of a pressure transmitter is generally a strain gauge, a sensitive device that converts the pressure on the measured object into an electrical signal. Strain gauges are primarily used in metal and semiconductor applications. Metal strain gauges are categorized into wire and foil. The strain gauge is typically bonded tightly to a mechanically strained substrate using a special adhesive.

When the substrate undergoes stress changes, the strain gauge also deforms, causing the resistance of the strain gauge to change, and thus the voltage applied to the resistor. Pressure transmitters can help companies monitor the operating status of fluid systems, address anomalies promptly, and improve production efficiency.

Differential pressure transmitters can also measure flow, liquid level, filter blockage, and heat exchanger efficiency. It offers high accuracy, stable measurement precision, and compatibility with a wide range of fluid media.

A differential pressure transmitter measures differential pressure, namely, ΔP = ρgΔh. As long as the ΔP value is accurately detected, it is inversely proportional to the liquid level height h and directly proportional to the height difference Δh. As a result, the measured pressure remains constant despite temperature fluctuations, even if the oil volume expands or contracts and the actual liquid level rises or falls.

A  prevents direct ingress of media from the pipeline into the transmitter. The pressure-sensing diaphragm and the transmitter are connected by a fluid-filled capillary tube.

Differential Applications:

First, both pressure transmitters and differential pressure transmitters can measure pressure. Second, pressure transmitters can measure liquid level. Differential pressure transmitters can also measure flow and liquid level.

Pressure Transmitters for Pressure Measurement

Pressure transmitters are widely used in various industrial automation environments, including water conservancy and hydropower, railway transportation, intelligent buildings, aerospace, military, petrochemical, oil wells, power generation, shipbuilding, machine tools, and pipelines. They measure the pressure of liquids, gases, or steam. A converter converts these pressure signals into a standard electrical output.

Pressure Transmitters for Liquid Level Measurement

The static pressure at a point in a liquid is proportional to the distance from that point to the liquid surface, i.e., P = ρgh.

Where: P = the pressure at the measured point, ρ = the density of the medium, g = the acceleration due to gravity, and h = the height from the measured point to the liquid surface.

For a given measured medium, ρ and g are constants, so changes in the measured point’s position relative to the liquid surface are solely related to the measured pressure, P.
To measure the liquid level in an open container, a single pressure transmitter is sufficient.

To measure the liquid level in a closed, pressurized container, two pressure transmitters or a differential pressure transmitter may be used. That is, one transmitter measures the lower limit and one transmitter measures the upper limit. Their output signals are subtracted to measure the liquid level. A differential pressure transmitter is generally used in this situation. If the liquid level and pressure in the container remain constant, it can also be used to measure the density of the medium.

Differential Pressure Transmitter Measuring Pressure

The operating principle of a differential pressure transmitter is based on Poisson’s law, which states that the pressure of a gas or liquid in a static state is proportional to its density. Therefore, when a pressure differential exists between two pressure points, parameters such as fluid flow, level, and density can be calculated by measuring the differential pressure.

A differential pressure transmitter typically consists of two pressure ports and a measuring chamber. When the medium enters the measuring chamber, it affects the pressure within the chamber, causing a change in the pressure within the chamber. The differential pressure transmitter calculates the differential pressure between the two pressure points by measuring the pressure difference within the chamber and converts it into a standard output signal.

There are two main measurement principles for differential pressure transmitters: one is static pressure measurement, which calculates the differential pressure by measuring the static pressure difference between the two pressure points. The other is dynamic pressure measurement, which calculates differential pressure by measuring the dynamic pressure difference generated by the fluid in the pipeline. Static pressure measurement is suitable for measuring low-speed fluids, while dynamic pressure measurement is suitable for measuring high-speed fluids.

Differential pressure transmitters measure liquid level:

The principle is that pressure inside a liquid varies at different heights. Pressure per unit area = liquid level height x liquid specific gravity.

For example, at a depth of one meter underwater, the pressure = 1000 mm x 1 g/mm³ = 1000 g/m² (= 1000 mmH²O = 0.1 kg/cm² = 0.001 MPa). By measuring the pressure at a specific height within the liquid, the height from that point to the liquid surface can be determined.

Because pressure above the container (liquid surface) can affect the measurement result, a differential pressure transmitter is needed to simultaneously measure the pressures above and below the liquid surface and offset them (measure the differential pressure) to determine the liquid surface height.

Differential pressure transmitters measure flow:

Differential pressure transmitters measure flow based on the Bernoulli equation and the throttling principle. The Bernoulli equation states that in closed systems, whether liquids or gases, energy conservation holds, whether at rest or in motion. When a fluid passes through a certain section of a pipe, if the cross-sectional area changes, the flow rate will change, resulting in a simultaneous change in pressure, known as differential pressure. This differential pressure is functionally related to the fluid’s flow rate.

Specifically, when a fluid flows through a restrictive device (such as an orifice plate, nozzle, or Venturi tube), the flow rate increases and the static pressure decreases, creating a static pressure difference before and after the restriction. This static pressure difference can be measured using a differential pressure gauge and converted to the fluid’s flow rate using a corresponding formula.

Differential Choosing Guide

The selection of a pressure transmitter is typically based on installation conditions, environmental conditions, instrument performance, cost-effectiveness, and the application medium. In practical applications, pressure transmitters are categorized as direct or indirect. Applications include process measurement, process control, and device interlocking. Common transmitters include standard pressure transmitters, differential pressure transmitters, single flange transmitters, double flange transmitters, and insert flange transmitters. The selection of differential pressure transmitters is based on the following factors:

(1) Measuring range, required accuracy and measuring function.

(2) The environment in which the measuring instrument is exposed, such as the industrial environment of the petrochemical industry, where there is a flammable (toxic) and explosive atmosphere, and a high ambient temperature.

(3) The physical and chemical properties and state of the measured medium, such as strong acid, strong alkali, viscosity, easy solidification, crystallization and gasification.

(4) Changes in operating conditions, such as changes in medium temperature, pressure and concentration. Sometimes, changes in gas and liquid concentration and density from start-up to when parameters reach normal production must also be considered.

(5) The structure, shape, size of the container of the measured object, the equipment accessories in the container and various inlet and outlet pipes must be considered, such as towers, solution tanks, reactors, boiler drums, vertical tanks, spherical tanks, etc.

Different Unit:

Differential pressure transmitters are designed and manufactured for gas and liquid pressure, and the unit is Pascal. While pressure transmitters are also designed for gas and liquid, but its unit is Newton or kilogram. Although they are used for the same media type, their units of measurement are different.