Differential Pressure Transmitters with 4-20mA for Air & Liquid DP Measurement

Differential pressure transmitters for measuring the DP of fluids and gases across particle filters and along a length of pipe to monitor flow.

Differential pressure transmitters provide precise and reliable measurement of pressure differences between two points, commonly utilizing a 4-20mA current loop output for robust signal transmission in industrial environments. These instruments are crucial for monitoring the health of particle filters by detecting increased pressure drops, enabling timely maintenance. They are also integral to flow measurement systems when paired with primary elements like orifice plates or venturi tubes, and for maintaining critical negative or positive pressure differentials in specialized environments such as cleanrooms and isolation rooms, ensuring operational efficiency and safety.

100 Pa bi-directional range differential air pressure transmitter - Bi-directional low range 4 to 20 milliamp differential pressure transmitter covering an air pressure range from -100 to +100 pascals, for cleanrooms, isolation rooms, and filter monitoring.
Bidirectional -50 to +50 Pa diff pressure transmitter with user calibration adjustment - Enhance your hvac or filter monitoring system with this accurate and durable low differential pressure transmitter featuring user-adjustable zero and span.

Find out more about Differential Pressure Transmitters to determine which product options and capabilities will best meet your application requirements.

Differential pressure (DP) transmitters featuring a robust 4-20mA current loop output signal are instrumental in a variety of industrial and research settings for precisely measuring the pressure difference between two points in a system. This type of analog signal is widely adopted in process control due to its high noise immunity, suitability for long-distance transmission without signal degradation, and its ability to power the device itself in 2-wire configurations, simplifying wiring and reducing installation costs. Furthermore, the live zero (4mA representing 0% of the calibrated range) allows for easy fault detection, as a current below 4mA typically indicates a wiring issue or transmitter malfunction.

One critical application for these transmitters is the monitoring of particle filter health. As particulate matter accumulates on a filter element, be it in an air handling unit, a hydraulic system, or a gas processing line, the resistance to flow increases. This increased resistance manifests as a higher differential pressure across the filter. By continuously measuring this DP, operators can determine the optimal time for filter cleaning or replacement, preventing excessive pressure drop, ensuring efficient system operation, and protecting downstream equipment. For instance, in industrial dust collection systems, DP transmitters monitor baghouse filter condition, while in pharmaceutical cleanrooms, they help maintain the efficacy of HEPA filtration systems.

Differential pressure transmitters are also fundamental in flow rate measurement when used in conjunction with primary flow elements such as orifice plates, venturi tubes, or averaging pitot tubes. These elements are installed in a pipe and create a restriction, causing a pressure drop that is proportional to the square of the flow velocity, as described by Bernoulli’s principle. The DP transmitter measures this pressure drop, and its 4-20mA output can then be fed into a flow computer, PLC, or DCS, which calculates the actual volumetric or mass flow rate. This method is common in chemical processing plants for monitoring reactant feed rates, in power plants for steam flow measurement, and in water treatment facilities for managing distribution lines.

The capability to measure slight pressure differences also makes these transmitters ideal for managing negative or positive pressure gradients between rooms or controlled environments. In cleanroom manufacturing facilities, maintaining a positive pressure hierarchy is crucial to prevent the ingress of contaminants from less clean adjacent areas. Conversely, in biomedical research laboratories or hospital isolation wards, negative room pressure is essential to contain airborne pathogens or hazardous substances, protecting the surrounding environment. DP transmitters provide the precise and reliable measurement needed to control HVAC systems and ensure these critical pressure differentials are consistently maintained. The selection of an appropriate DP range is paramount in these applications, often requiring transmitters sensitive to very low pressures, sometimes in the order of Pascals or inches of water column.

When selecting a differential pressure transmitter, engineers and technicians must consider factors such as the chemical compatibility of the wetted parts (diaphragms, seals) with the process fluid or gas, the required accuracy over the operating temperature and pressure range, and the static line pressure to which the transmitter will be exposed. For example, applications involving corrosive media might necessitate Hastelloy C or Monel diaphragms, while high static pressure applications, such as measuring DP across a valve in a high-pressure pipeline, require transmitters specifically designed to withstand these conditions without compromising measurement accuracy. The 4-20mA output allows for straightforward integration into existing control architectures, enabling automated monitoring and control based on real-time differential pressure readings.

Contents

Product Help

Liquid & Steam service installation

The transmitter should be installed below the process tapping points to a wall or other rigid mounting using the optional bracket assembly or similar rigid bracket. The transmitter should be piped up in compliance with the pressure transmitter installation guidelines with the process tappings taken to the side of the pipe work as illustrated in the adjacent diagram. The transmitter should be mounted within two degrees of the horizontal, small variations in mounting attitude will affect the transmitter zero point, however, this may be calibrated out during the initial commissioning procedure detailed later. When used on steam service the transmitter and impulse lines must be filled with water before system start up to prevent live steam damaging the transmitter.

Gas service installation

The transmitter should be installed above the process tapping points to a wall or other rigid mounting using the optional bracket assembly or similar rigid bracket. The transmitter should piped up in compliance with the pressure transmitter installation guidelines with the process tappings taken to the top of the pipe work as illustrated in the adjacent diagram. The transmitter is placed above the process measurement tapping to allow condensed liquid to drain back into the process lines. The transmitter should be mounted within two degrees of the horizontal, small variations in mounting attitude will affect the transmitter zero point, however, this may be calibrated out during the initial commissioning procedure detailed later.

Specifying differential pressure transmitter requirements

Some questions to ask when defining the product requirements for a differential pressure transmitter.

Differential pressure range

What is the highest differential pressure you need to measure?

Are the high and low pressure side always the same way round, or could they switch around? If the high and low pressure side is likely to reverse back and forth, then you will need to specify a bidirectional or compound range to cover the changing direction of pressure.

Do you need to change the dp range for a different process or location? Some differential pressure transmitters allow the range to be adjusted by the user. Rangeable dp transmitters are adjusted manually by internal screws or buttons which are accessed by removing a waterproof lid or cap. Intelligent dp transmitters are adjusted via a in-built user interface, digital interface and associated software, or a special user hand terminal.

Static line pressure

What could be the highest pressure measured at either of the two process connections?

This is the general system pressure and is often significantly higher than the differential pressure measurement range. The static line pressure rating of dp transmitters will vary enormously from one type to another, so it is important to pay special attention to this parameter, since it could lead to mechanical failure if an inadequate rating is selected.

Is there a risk that full static line pressure could be applied to one side of the differential pressure transmitter? The line pressure rating does not necessarily mean that this pressure can be applied to one-side only. Most manufacturers will make it clear on their product data sheets whether the maximum static line pressure can only be applied on both sides simultaneously, or is protected from it being applied on one side only.

Overpressure

Is there a chance that the normal operating differential pressure range could be exceeded by a pressure surge or a process problem?

Maximum overpressure is the amount of differential pressure that the transmitter can withstand beyond 100% of the dp range, without pushing the performance outside of specification limits.

The overpressure rating is typically expressed as multiples of the dp range, e.g. 2x, 4x.

Bidirectional or compound differential pressure ranges, which are a combination of a positive and negative pressure range component, should have two overpressure ratings.

If the dp transmitter will only allow full static line pressure to be applied to both sides at the same time, then take care to check the negative overpressure rating is adequate. Depending on the differential sensing technology utilised, the negative overpressure rating maybe significantly lower than the positive overpressure rating.

Where a differential pressure transmitter is protected against the full static line pressure being applied to one side only, then the positive and negative overpressure rating will equal the max static line pressure rating.

Checklist for differential pressure transmitter requirements

Define your differential pressure transmitter requirements using this checklist:

Differential pressure range?
Static line pressure?
Overpressure?
Media type?
Media temperature range?
Environmental conditions?
Signal output? 4-20mA
Power supply?
Measurement accuracy?
Electrical connection?
Process connection?
Certification?