Mounting Pressure Transducers & Water Quality Probes in Flumes & Weir Boxes

In-Situ Rugged Troll 100 pressure trandsucer Pressure transducers are a popular method of measuring the water level in flume or weir installations. They are used by water resource managers, engineers, and hydrologists to monitor flow rates in a range of applications, including: agriculture (runoff and water use), watersheds / catchments, storm water / CSO, dam seepage, and mine dewatering.

Likewise, water quality probes (typically pH, temperature, dissolved oxygen, ORP, and conductivity) are used to characterize the quality of flow, just like pressure transducers measure the rate of flow.

Both pressure transducers and water quality probes are mounted in similar ways when it comes to flumes and weirs – although water quality probes usually require the additional consideration of keeping the probe sensing element wetted at all times). Most transducers / probes are 1 to 1 ¾-inches [2.54 to 3.81 cm] in diameter, with the length varying from 5 1/2-inches [13.97 cm] and up.

Transducer / Probe Mounting

In general, there are four methods of mounting a pressure transducer / parameter probe (here after collectively referred to as probe) in a flume or weir box:

Securing the Probe to the Channel Floor

Here the probe is secured to the flume or weir box floor. The probe can be mounted on the floor itself or in a recessed cavity in the floor. The probe is usually held in place by spring clips or conduit straps. Of the two, spring clips allow the probe to be more easily removed / installed.

Like the probe, connection cabling is usually also surface mounted, either directly on the surface or in a recessed cavity. Conduit straps are usually used to keep the cable out of the flow stream. If the cable is left unsecured, there is risk in certain applications that the cable will get snagged, potentially allowing the probe to be pulled downstream.

Floor mounting is problematic for several reasons: the probe can be snagged and pulled out of position or lost, for deeper flumes it can be difficult to secure the probe (even with spring clips), and not only the probe, but also the cable, must be secured.

Removable Probe Holder

Removale probe holder for pressure transducers and parameter probes mounted in a parshall flume A removable probe holder is a stainless steel structure that is used to hold the probe rigidly in position on a flume or weir box’s sidewall, but which also allows for rapid removal. The probe is held in place on the holder by two spring clips – allowing quick mounting / dismounting. The holder is either mounted flush against the flume / weir box sidewall or it can be recessed in the sidewall with a molded-in cavity.

The probe holder is secured at the top of the flume / weir box by a bolt through the top stiffening flange. To remove the probe holder, simply remove the nut and lift the probe holder off of the mounting bolt.

Probe holders keep the probe firmly in place while allowing it to be exposed directly to the flow stream.

This direct exposure to the flow stream is a positive for accurate measurement, but also exposes the probe to ragging, debris, etc. The exposure can be somewhat minimized by recessing the probe in the sidewall (an option with fiberglass construction flumes and weir boxes).

Stilling Wells

galvanized parshall flume with stiling well and cover Stilling wells are chambers isolated from, but connected to, the main flow of water in a flume or weir. A small port allows the water in the stilling well to be refreshed so that the level follows that in main flow stream, but at the same time helps to dampen surges and waves that the flume or weir may be experiencing.

Stilling wells usually – but no always – extend below the floor of the flume or zero reference elevation of the weir box (normally 3-inches [7.62 cm]). This allows probes mounted in the well to remain wetted even under no-flow conditions. The wells themselves are usually 6 to 16-inches [15.24 to 40.64 cm] in diameter

Depending upon the probe’s length, it can be set horizontal on the floor of the stilling well or it can be mounted vertically (with some sort of restraint or mount).

Stilling wells do increase the width of flumes / weir boxes which may limit their use in space constrained applications. They also, usually, have open tops, which may allow the collection of leaves or debris.

Probe Wells

2-foot fiberglass H flume with probe well and approach section Probe wells are essentially small diameter stilling wells – usually 2 inches [5.08 cm] in diameter. Like stilling wells, probe wells have an inlet port that allows the flow in the well to refresh and their floors also extend below the floor of the flume – zero reference elevation of the weir, keeping the probe wetted.

However, the smaller diameter of the probe well means that well suited for space constrained installations and the probe is held nearly vertical without additional restraints or mounts. As a result, installing / removing a probe from a probe well is as easy as lifting it out / dropping it in. There is nothing to unclip (surface mounts / probe holders / potentially stilling wells).

Of the four approaches to probe mounting, the probe well has proven to be the most popular. It is economical, keeps the probe secure, keeps the probe wetted, and keeps the probe out of the flow stream.

Image Credit: IN-Situ