Hydrometry - Pressure Transducers

Quite a few days have passed since my last post about water level instrumentation, so I should probably mention my source again. Hyrdrometry, 3rd Edition, by Wubbo Boiten. (You can go read the archives if you want an official MLA style citation.) All of the pressure transducer material is based on pages 8-9.

Pressure Transducers:

Three other names commonly used for pressure transducers:
- Pressure Sensors
- Pressure Probes
- Pressure Transmitters

Function:
The basic function of a pressure transducer is to take pressure and turn it into an electric signal. The electric signal is processed, combined with known information about the water density, and converted to a height measurement. The height measurement is typically recorded on a data logger.

Here's a schematic:
Error:
There are a couple sources of error associated with pressure transducers:
1.) Air pressure above the water must be accounted for
2.) Since the pressure transducer is measuring pressure and converting it to water depth, the density of the water is a critical parameter. For instance, salt water has a different density than fresh water. Cold water has a different density than warm water.

Other Concerns:
Pressure transducers may be damaged if exposed to extreme pressures.

Characteristics:
Range: 0-40 feet
Accuracy: 0.1% of the full range

First Look at the Water Level Bench

Today I had the chance to work in the lab a little bit. I spent a while just sweeping up a lot of grit and dirt.

Since the bench has been completed I put some water in it and took a series of photos. Hopefully those will be useful for Ken to look at as he works on his software.

Ken also has been in contact with Ruby Mehrubeolgu, who has some interesting water/machine vision work going on down in Corpus Christi. A lot of grant solicitations emphasize collaboration between universities, so it is possible we could work together on something if interests, timing, etc, align.

BAE (Ag) Shop

I mentioned the BAE shop in my last post. Here's a link! It details all the machines and capabilities of the lab.

BAE Machine Shop

Keys to the Lab

Today I got my keys to the lab where I'll be working! It's an old lab that's been out of use for quite a while, so there is a lot of cleanup to do. I'm not sure how involved I will be in that, but it might contribute to a slower start to the project than I hoped.


I took a tour of the Ag Shop. It's excellent ~ all kinds of tools and machines in there to create stuff with. Hopefully after next week I'll have access to some of the tools in order to make a (very simple) camera mount. And for items I don't know how to create, it sounds like the shop personnel will be able to make just about anything.

In the meantime, I'm working on a preliminary set of deadlines. Dr. Birgand will look it over and help set it up on a realistic time table.

Gig

As of today I officially landed a part time gig to work as an undergraduate researcher with the NCSU BAE department this summer. (See archive posts for details on BAE.) I'll be working with Dr. Birgand and also with my friend Ken Chapman. We're working on some gadgetry to detect stream stage. More later!

Vacation Stream Gauge

In our travels through VA, WV, PA, OH, & IL, I have been seeing stream gauges all over the place!

Here's one on the Potomac River in Paw Paw, WV:

Hydrometry - Float Operated Gauges

Float gauges provide a mechanical means to measure water height.

Here are the basic components:

Float
Line (Tape, or Wire)
Pulley (Float wheel)
Counterweight

Recorder

The line is attached to the float on one end and a counterweight on the other. It hangs on a pulley. The recorder keeps record of the pulley shaft rotation, which can be translated into water height. Recorders have traditionally been paper on a drum, but more are being converted to digital recorders that keep track of the pulley shaft rotation.

Typically, float gauges are installed in stilling wells, which help to produce stable measuring conditions.

There are several sources of error that must be considered when using the float operated gauges.

Systematic Effects:
1.) Float lag: The float tends to lag behind the true water level as the float rises and falls. This can be minimized by selecting an appropriate diameter float and sufficiently large float wheel.

2.) Intake pipe effects: The intake pipe (horizontal pipe in figure to the right) diameter can also introduce measurement error. A diameter that is too large can reduce the effectiveness of the stilling effect of the well by allowing water to surge in and out of the well. However, a too small diameter can introduce measurement lag because of head loss in the intake pipe.

3.) Chimney Effect: How far the intake pipe extends into the river cross section also impacts measurement due to chimney effect. This can produce a systematic underestimation of the water height.

Other Concerns:
4.) Counterweight: The counterweight location must be configured so the counterweight does not hit the float as it drops. Ideally, the counterweight is never submerged, as the effect of bouyancy on the counterweight will affect measurement.

5.) Tape/Wire: Kinks or twists in the line can cause measurement error. In addition, surges of water can potentially unseat the wire from the pulley.

Float Gauges in Stilling Wells
(Source: USGS)