This is a short post about a simple device I tried making for measuring the velocity of water.

A pitot tube is a device for converting a fluid velocity to a pressure (which is easier to measure). Recall from this post that velocity is one of the components of the energy of a fluid. A tube with a bend in it that stagnates the velocity converts that energy to height, with the relationship h = v2/(2g).

Using some copper pipe I had left over from a previous project, along with a wooden dowel and a length of vinyl tubing, I tried making a pitot tube. The following image shows the assembled apparatus as well as using it to take a measurement.

Pitot tube: full view (left) and in use (right)

The water level in the vinyl tubing has risen around 2 cm. That equates to a velocity of 0.63 metres per second (2 ft/sec). Looking at what it would take to reach higher levels, 5 cm equates to a velocity of 1 m/s, 10 cm is 1.4 m/s, 15 cm is 1.7 m/s, and 20 cm is 2 m/s.

I didn't have a way to cross-check the velocity measurement, but I felt like the pitot tube reading was on the low side. Placing the pitot tube into the flow created turbulence in its immediate vicinity, altering the local velocities from what they were before it was inserted into the stream*. One way to mitigate this might be to use a longer length of tube facing into the stream (before the elbow).

*Taken more generally, the fact that attempting to measure a process inevitably affects it is a fundamental issue in physics and other fields.

To test my pitot tube, I biked to a suitable spot along the Nashwaak River in Marysville (actually, I went as far as an old train bridge to enjoy the ride and the view). This is one of my recommended activities for people to do in Fredericton, NB. There is a nice view along this trail of the old cotton mill in Marysville and the remains of the dam that powered it.

Old dam and mill in Marysville

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