The Thelmar Weir – Insertable Volumetric Weir for Flow Measurement in Pipes

Thelmar Insertable Volumetric Weirs are commonly used for temporary flow measurement in a pipe, or manhole; with relatively low flows.   The Thelmar weir is calibrated compound weir that is inserted into the invert of a pipe and incorporates a 90° v-notch for measuring lower end of the range; then opens into a rectangular weir at roughly 60 GPM.

Once in place, a thumbscrew adjustment holds the weir in the invert, and pushes the water through the weir opening.  An incremented weir plate allows you read instantaneous flow rate.   The weir is good to about 1/3 the pipe capacity.

We routinely get calls from customers who need a better way of estimating flows; without the expense of a more formal installation.

The Thelmar is perhaps most valuable when measuring low end flows, where there isn’t enough water depth required for flow metering resolution.

Individual volumetric weirs are available for 6, 8, 10, 12, 14 and 15 inch pipes.  Flow ranges by pipe diameter:

6”           57 – 46,000 Gallons Per Day (GPD)
8”           57 – 124,000 GPD
10”         57 – 234, 000 GPD
12”         57 – 361,000 GPD
15”         57 – 620,000 GPD

There are adapters for larger size pipes; however, those utilize a 15 inch weir and are limited to its flow range.

While Thelmar is most commonly used in short term installations, we’ve installed several for permanent operation. A bubbler flow meter can attach to an optional 1/8 inch stainless steel tube attached to the right side of the adjustable ring; making it easy to attach the tubing from above.  It runs down to the inside of the ring’s center bottom; so it doesn’t affect the function of the weir.