PMTWyatt 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 125 or 250 PSIG flanges.  Wyatt offers both full body and insert-type primary elements


The cast iron PMT series of Lo-Loss® meters is designed to accurately and reliably measure the flow rates of water, wastewater, sludge, slurries, clean fluids, and gases in full pipe conditions. The PMT series of meters is ideally suited to applications where permanent pressure loss must be kept to a minimum, such as in gravity-fed systems, or where a savings due to lower pumping costs can be realized. The Model PMT-C is designed for rate-of-flow control applications, while the PMT-S incorporates a rugged design for cleaning the pressure taps for solid-bearing fluids.

Flow Measurement Accuracy

For pipe Reynolds numbers greater than 100 000 and in a normalized piping configuration, the Wyatt PMT Lo-Loss® provides a flow measurement accuracy of ± 0.25% with independent flow calibration and ± 1.00% without flow calibration. Lo-Loss® is a registered trademark of Wyatt Engineering.

Technical Specifications

Headloss Comparison


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

± 1.00% for standard meters and
± 0.25% for flow calibrated meters.

Pressure Loss:

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

Temperature Range:

Cast iron Lo-Loss® meters can handle process temperatures between -20 °F and +350 °F (-30 °C and +175 °C).

Pressure Range/End Connections:

Flanged end connections, per ANSI B16.1 for 125 PSIG and 250 PSIG service, are available. Various other end connections are also available, including: Mechanical joint flanges, per AWWA C110 and C111, and plain-end designs.

Piping Requirements:

Designed for full-pipe flow, Lo-Loss® flow meters can be mounted horizontally, vertically, or on an angle. Refer to Wyatt Engineering Technical Manual for the Lo-Loss® Meter for recommended upstream piping.

Energy Considerations

Figure 1 compares the permanent pressure loss of the insert Lo-Loss® design with that of other primary flow elements. Figure 2 illustrates the substantial savings that are realized when a Lo-

Loss® meter is used in a typical application. The pressure recovery of the Wyatt Lo-Loss® Meter means reduced pumping costs. High beta ratio Lo-Loss® meters 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 Lo-Loss® meter instead of venturi tubes can realize further savings. For over 40 years, engineers have given their clients the benefits of efficiency and accuracy by doing just that.

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

PMT Available OptionsThe Lo-Loss® design is furnished in four models, depending on the application.

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

Model PMT-C is a flanged cast iron primary element designed to utilize a direct-coupled butterfly valve in a rate-of-flow controller for liquid or gas service. The direct-coupling of the butterfly valve will not affect the discharge coefficient of the primary element. The meter outlet can be modified to accommodate a valve one or more line sizes smaller than the main piping run.

Model PMT-SModel PMT-S is a flanged primary element designed for wastewater, sludge, slurry, or other fluids with suspended solids. Manual vent cleaners are provided as standard; automatic vent cleaners or a sealed diaphragm system systems, either continuous or timed, are also available.

Model PMT-SC is a rate-of-flow controller designed for use in applications where the line fluid contains suspended solids. The primary element is provided with vent cleaners or a purge system and will accept a butterfly valve bolted directly to the downstream flange. This allows the PMT-SC to be used in many applications where other standard venturi or flow tubes cannot be used. Consult your local representative or Wyatt Engineering for information on other materials of construction.