Small Hand-held Distance Measurement Instrument Needed

Dear Sirs,
            I am an Instrumentation Technician with job duties including the
periodic verification / calibration of Ultrasonic Open Channel (Parshall
Flume) flow monitoring instruments. Currently, the procedure that we have
been employing for verifying flow readings would be "sticking" the flume
with a Steel Rule placed with the blade in-line with the flow (resolution of
the Rule is 1/16 inch), and "dipped" to the surface of the liquid in the
flume (we look for the "wake" created by the Rule to let us know we have
"dipped" far enough, but unfortunately, we sometimes have a problem of
consistency of "sticking depth" (how far the Rule is "dipped" into the flow)
as there are a number of us that must do these inspections. Consistency from
one person to the next is "problematic" at best.
            Now, I have given you folks the problem above, and am asking the
following: Do any of you good folks here know of an instrument that we could
use (ultrasonic / laser) that would negate the need for "sticking" these
flumes as we have been doing? The distance to be measured is less than 36
inches. Any input, (pro, or con) would be appreciated.