Differential pressure is the most common level measurement technology, accounting for nearly 40% of all continuous level measurement worldwide. If you add in that a typical refinery runs more than 200 differential transmitters measuring flow, some sites have several hundred differential pressure transmitters. Differential pressure is the most field-proven measurement technology in process instrumentation, and it has been for decades.
This article covers how differential pressure flow measurement works, how differential pressure level measurement works, what each application requires, and how the SOR Measurement and Control 1800 Series fits both.
What Is a Differential Pressure Transmitter?
A differential pressure (DP) transmitter measures the difference between two pressure points in your process. Connect the high-pressure side port to one location and the low-pressure side port to another. The transmitter outputs that difference as a 4-20 mA signal, with optional HART or Modbus communication.
Where you tap the process determines what you measure. Across a flow restriction in a pipe, that pressure difference tells you the flow rate. At the bottom of a vessel, it tells you the liquid level.
Differential Pressure Flow Measurement
How It Works
You need a primary element in the pipeline, such as an orifice plate, venturi tube, or conditioning orifice plate. That restriction causes fluid to accelerate, dropping pressure on the downstream side.
The DP transmitter reads the pressure before and after the restriction. Furthermore, the flow rate in the pipe is proportional to the square root of the measured differential pressure. The basic volumetric flow formula is:
Volumetric Flow Rate = K x √(Differential Pressure / Density)
K represents the geometry of your primary element, provided by the manufacturer or calculated from pipe and bore dimensions. With water at 1.0 SG and a K of 4, a DP of 100 inH2O produces 40 GPM. At 50 inH2O, the flow drops to 28.3 GPM. The 1800 Series outputs the DP value in linear or square root form. Your PLC, DCS, or flow computer runs the calculation.
What You Need
- A primary element in the pipeline is to generate the pressure drop.
- Pipe penetrations upstream and downstream of that element to connect the transmitter.
- Know fluid density or specific gravity at operating conditions.
Where It Works Best
Differential pressure flow measurement is a good fit when other technologies hit their limits:
- Short pipe runs, especially with conditioning orifice plates that need only 2 diameters upstream and 2 downstream.
- Non-conductive liquids and gases where magnetic meters cannot be used.
- Large-diameter pipelines where Coriolis meters are not practical.
- High-pressure and high-temperature service, including steam.
- Flare gas, wellhead flow, feedwater, and transmission pipeline applications.
For comparison, Coriolis meters require 5 diameters upstream and 5 downstream. Magnetic meters need 5 upstream and 3 downstream. DP with a conditioning orifice needs 2 and 2. No moving parts means less maintenance and longer service life than turbine or vortex meters.
Differential Pressure Level Measurement
How It Works
Liquid creates pressure based on its height and density. A DP transmitter at the bottom of a vessel reads that hydrostatic pressure directly. For open tanks, the low-pressure side port is open to atmosphere. For closed or pressurized vessels, you connect the low-pressure side port to the vapor space at the top. That cancels the static pressure above the liquid and gives you the true level. The formula is:
Level = Differential Pressure / Specific Gravity
If your transmitter reads 52 inH2O on the high side, 0 on the low side, and your fluid has a specific gravity of 0.92, the level is 56.5 inches. The geometry of the tank does not change that calculation.
What You Need
- A process connection near the bottom of the vessel for the high-pressure side
- A known, consistent specific gravity for the liquid being measured.
- A second connection near the top for pressurized or closed systems.
Where It Works Best
Differential pressure level measurement holds up in installations where other technologies have trouble:
- Vessels with agitators, baffles, ladders, or internal platforms.
- Turbulent processes or applications with foam or vapor.
- Narrow or tall vessels where guided-wave radar or floats are impractical.
- Corrosive or high-temperature processes require separation between the transmitter and the media.
Common applications include pressure vessel monitoring, condensate tank tracking, chemical reactor level control, and wastewater treatment level monitoring. In filter and coalescer installations, multiple DP transmitters on one vessel track both level and differential pressure across the filter media.
The SOR Measurement and Control 1800 Series
SOR offers three 1800 Series differential pressure transmitters. All three include external push-buttons for setup and calibration and accept HART or Modbus programming. Each model fits a different installation scenario.
SOR 1800DP: Standard Differential Pressure Transmitter
The 1800DP connects to the process through standard impulse piping. It is the right choice for most flow and level applications where the fluid is compatible with direct connection.
An optional integral LCD lets technicians read the measurement at the device without a communicator. Three-valve and five-valve manifolds are available for installation and isolation.
1800DM: Direct Mount Diaphragm Seal
The 1800DM mounts diaphragm seals directly to the transmitter body. A fill fluid isolates the sensing element from the process, protecting it from fluids that would damage or plug standard impulse lines. Raised face, extended flange, and tri-clamp connections are available.
1800RM: Remote Mount Diaphragm Seals
The 1800RM ships with remote diaphragm seals already installed, connected to the transmitter body by flexible capillary tubing. You can locate the transmitter away from high temperatures, corrosive environments, or tight access points. Raised face, extended flange, and tri-clamp process connections are available.
A Houston filter/coalescer installation uses the SOR 1800RM with four DP transmitters (and 1 GP) on a single vessel, tracking both level and filter differential pressure simultaneously.
Why the SOR Measurement and Control 1800 Series
Plant engineers and maintenance technicians know differential pressure. It does not require a specialist to commission, troubleshoot, or replace. The 1800 Series delivers the specifications to compete with the major producers at a better price, with shorter lead times from the Lenexa, Kansas manufacturing facility.
View the full SOR 800 Series and 1800 Series application list(PDF) for a breakdown of uses across oil and gas, power, chemical, and water treatment.
SOR Controls Group works with manufacturers, municipalities, OEMs, and skid builders who need reliable instrumentation, competitive pricing, and short lead times. If you are replacing existing DP transmitters or specifying new flow and level loops, contact SOR to discuss your application.
Frequently Asked Questions: SOR 1800 Series Differential Pressure Transmitters
Q: What is differential pressure flow measurement?
A: Differential pressure flow measurement calculates flow rate by measuring the pressure drop across a primary element, such as an orifice plate or venturi tube, installed in the pipeline. As fluid passes through the restriction, downstream pressure drops. The DP transmitter measures that difference, and your flow computer or control system converts it to a volumetric flow rate using the formula: Flow Rate = K x √(DP / Density).
Q: Why use differential pressure flow measurement instead of a Coriolis or magnetic meter?
A: DP flow measurement has no moving parts, works on liquids and gases, handles non-conductive fluids, and fits large-diameter pipes where Coriolis meters are impractical. Conditioning orifice plates reduce straight-run requirements to 2 diameters upstream and downstream, compared to 5 or more for most competing technologies. The upfront cost is also significantly lower for equivalent accuracy in standard process conditions.
Q: What is differential pressure level measurement?
A: Differential pressure level measurement determines liquid level by reading the hydrostatic pressure at the bottom of a vessel. For open tanks, the low-pressure side port is open to atmosphere. For pressurized or closed vessels, the low-pressure side port connects to the vapor space at the top, canceling out any static head pressure above the liquid. The formula is: Level = Differential Pressure / Specific Gravity.
Q: Can the SOR 1800 Series handle both differential pressure flow measurement and level measurement?
A: Yes. The same transmitter family handles both applications. For flow, you connect across a primary element such as an orifice plate or venturi tube. For level, you connect to the bottom and top of a vessel using impulse lines or remote seals. The transmitter outputs the differential pressure value; your control system performs the flow or level calculation.
Q: What is the difference between the 1800DP, 1800DM, and 1800RM?
A: The 1800DP connects to the process via standard impulse piping and suits most flow and level applications. The 1800DM uses diaphragm seals mounted directly to the transmitter body, protecting the sensing element from corrosive or viscous fluids. The 1800RM ships with remote diaphragm seals on flexible capillary tubing, separating the transmitter from high-temperature or hard-to-access process locations.
Q: What output options does the SOR 1800 Series offer?
A: The 1800 Series outputs 4-20 mA as standard. Optional outputs include HART digital communication for DCS and SCADA integration, 1-5 VDC low-power for battery-powered or solar applications, and Modbus RTU for RS-485 networks. The optional integral LCD provides local indication at the device without a communicator.
Q: What is the accuracy of the SOR 1800 Series?
A: The 1800 Series delivers ±0.075% full-scale accuracy as standard. That specification matches or exceeds products from Emerson, Yokogawa, and Endress+Hauser at a more competitive price point.
Q: What certifications does the SOR 1800 Series carry?
A: The 1800 Series holds ATEX and IECEx certifications for intrinsically safe and flameproof installations, plus CSA certification for explosion-proof applications in North America. This covers most hazardous area classifications in oil and gas, chemical, and power generation environments.
Q: How do I select the right differential pressure range for flow or level?
A: For differential pressure flow measurement, work with your primary element manufacturer to determine the expected DP at maximum flow. For differential pressure level measurement, calculate the maximum hydrostatic pressure at the bottom tap using vessel height and fluid specific gravity. The SOR 1800 Series covers a range from ±0.87 psid to ±1,450 psid with a 10:1 turndown ratio for flexibility across changing process conditions.
Q: What is the lead time for the SOR 1800 Series?
A: Contact your local SOR rep directly for current stock, pricing, and lead time on your specific configuration and quantities.
