EQUIPMENT AND SUPPLIES--Need for In-place Calibration of Marsh-McBirney Model 250 VMFM (Velocity Modified Flow Meters)

In Reply Refer To:                                         February 25, 1981
EGS-Mail Stop 415



Subject:  EQUIPMENT AND SUPPLIES--Need for In-place Calibration of Marsh-McBirney
          Model 250 VMFM (Velocity Modified Flow Meters)

A number of Marsh-McBirney flow meters are being used by the U.S. Geological
Survey (USGS) to obtain flow data in storm sewers and in other places.
Recent studies made at the USGS Gulf Coast Hydroscience Center (GCHC) emphasize
the need for an in-place calibration of the meter.  Measured discharges can be
in error by as much as 60 percent or more without a calibration.  Copies of the
study reports may be obtained by request from the Surface Water Branch.

The studies show that several actions need to be taken when using the VMFM for
the monitoring of flow data.  These are:

1.  Disconnect the bubble gage from the VMFM velocity probe and move the
orifice 6 to 12 inches upstream or downstream from the probe.  The bubble
gage orifice must be located perpendicular to the flow to minimize drawdown
or pileup.

2.  Check the depth-measuring system of the VMFM using the procedures described
in the enclosure or using similar procedures.

3.  Calibrate the VMFM with the meter installed in the location and under
the conditions for which it is to be used.  Independent discharge measure-
ments need to be made for at least three discharges.  Discharge measurements
can be made using current meters, dilution methods, or other means which will
provide accurate measurements.

4.  Measuring the flow components of depth and velocity independently is
recommended.  The test showed that the determination of flow from the com-
ponents differed significantly from the flow which the VMFM computed.  The
independent measurement of depth and velocity will allow the investigator to
more easily determine if there are problems with the meter such as would occur
from trash catching on the velocity sensor.

Depending on the recording system used and/or the ability of the investigator,
separation of the components may require a factory modification of the meter.


If separated, the components will have to be recorded separately and the dis-
charge computed.  Questions concerning the separation of the components can
be directed to Jim Rorabaugh at the USGS Hydrologic Instrumentation Facility
Instrument Development Laboratory, GCHC (FTS 494-2161).

If the VMFM is modified to record depth and velocity separately, the meter
must be calibrated as modified.

                                         Marshall E. Moss
                                         Acting Chief, Surface Water Branch


WRD Distribtuion:  A, B, S, FO, PO

"This memorandum does not supersede any existing SWB memorandum."