Wyatt Lo-Loss® PMT Flow Tubes



  • Lowest Pressure Loss
  • Short Laying Length
  • Economical Design
  • Best Documented Flow Tube on the Market


The Wyatt Lo-Loss® PMT is a differential-producing flow tube that maintains its accuracy over a wide range of flow rates. The hydraulic shape of the PMT incurs a lower permanent pressure loss than any other differential producing flow device. The PMT can be provided with either flanged or plain end connections. Wyatt offers full body pressure vessels and insert-type flow elements.


The PMT elements are often used in applications where the flow stream demands a specific material selection due to temperature and/or pressure, or the corrosive/erosive properties of the fluid being measured. PMT installations are found in power plants, refineries, chemical plants, cryogenic processes, water and sewage plants, custody transfer for steam and billing measurements.

Technical Specifications

Headloss Comparison


Within the specified pipe Reynolds number range and a normalized piping configuration, the Lo-Loss® PMT flow tube provides flow measurement uncertainties of:

± 1.00% for standard meters without flow calibration, and
± 0.25% for flow calibrated meters.

Pressure Loss:

The permanent pressure loss of the PMT flow tube, expressed as a percentage of the differential produced, is shown in Figure 1 and is the lowest of any differential-producing primary element.

Beta Ratio:

Wyatt Engineering offers Lo-Loss® PMT flow tubes with a wider range of diameter ratios (d/D) than any other manufacturer. By custom designing a PMT to your application’s flow conditions, Wyatt can provide an accurate and reliable primary element with the lowest permanent pressure loss.

Temperature Range:

Depending on the material of construction, the PMT can operate over a fluid temperature range of -425 °F to +1,200 °F (-250 °C and +650 °C).

Pressure Range/End Connections:

Flanged end connections per ANSI B16.1 for 125 PSIG and 250 PSIG service are available. A variety of other end connections is also available including: Mechanical joint flanges per AWWA C110 or C111, as well as plain-end designs.

Piping Requirements:

Designed for full-pipe flow, Lo-Loss® flow tubes can be mounted horizontally, vertically, or on an angle.  For recommended upstream piping, refer to Wyatt Engineering PMT Technical Manual.

Energy Considerations

Figure 1 compares the permanent pressure loss of the insert-type PMT design with that of other primary flow elements. Figure 2 illustrates the substantial savings that are realized when a PMT flow tube is used in a typical application. The pressure recovery of the PMT means reduced pumping costs. High beta ratio PMTs will recover up to 97.5% of the differential pressure produced. This is two to four times better than typical classical venturi devices, as well as most modified venturi meters.

Using venturi tubes instead of orifice plates can yield significant savings, and using the PMT flow tubes can provide further savings. For over 40 years, engineers have given their clients the benefits of efficiency and accuracy by doing just that.

Figure 2: Annual Pumping Cost Savings

Comparison of Differential Producers

Lo-Loss ® vs. Orifice Plate $1704
Lo-Loss ® vs. Flow Nozzle $1544
Lo-Loss ® vs. Short-Form Venturi $327
Lo-Loss ® vs. Long-Form Venturi $52

Available Options

The Lo-Loss® PMT design is furnished in three models, depending on the application.

PMT Available Options

Model PMT-UPMT is a fabricated full body venturi meter that is designed to meet ASMA Boiler and Pressure Vessel Code. The construction allows for a custom design to meet any set of process conditions. For flexibility, multiple pressure connections are available. To accommodate demanding applications with extreme temperature or pressure cycling, Wyatt can machine the element out of forged bar stock. Called the PMT-B, this design is most common for pipe sizes of 12 inches (300 mm) or less. Additionally, Wyatt can construct the BVT within a pipe shell. Called the PMT-F, this design is available with either flanged or plain end connections.

Model PMT-CI is a flanged cast iron venturi meter designed for either liquid or gas flow measurement. Typical throat materials are bronze and stainless steel. All valves, except for butterfly valves, may be direct coupled downstream for control purposes without any loss of accuracy. The PMT-CI can be supplied with flanges, mechanical joints, or plain ends.

Model PMT-IF is an insert-type venturi meter that is designed for insertion within the interior of the pipeline. The meter is secured in between two adjacent pipe flanges or welded directly in place. The insert-type venturi is best suited for liquids or gases that have low solids loading. The distinct advantage of the insert-type design is minimal weight, lower cost, and short laying length. The PMT-IF is constructed from all metallic components. An alternative style is the PMT-IL, which includes a metallic throat and mounting flange with a fiberglass or composite body. A third option is the PMT-IP, which is made entirely out of composite (or plastic) materials.