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.
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Heaps of instruments available, just google, the Fluke laser meter claims 1.5mm accuracy. If you're lucky a local rep might allow you to borrow one and try it out, that's the only way to find out if it'll work OK. Good luck...
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In a past life I was involved in wastewater. Back then the manual method of reading open channel meter heads was a hook gauge in a stilling well. It is very apparent when the tip of the hook reaches the surface in the stilling well and many have vernier scales,
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