Piezometer – sensor, system or both?
Piezometers are probably the most used tool in geotechnical instrumentation but there is often confusion over terminology – is it a sensor, a system or a field instrument?
The photograph below shows a Bourdon Gauge and a Vibrating Wire Transducer connected on top of a vertical pipe. We have two measuring devices and one access pipe buried into the soil. But combine all the components together and you get a Piezometer or a field instrument.
The answer is therefore a Piezometer can be a sensor, a system or a field instrument ….. confused?
Typically our industry uses the term Piezometer as a collective term for all.
To understand why our industry calls them Piezometers and not Pressure Sensors or Pressure Transducers we need to go back in time to the origin of the word Piezometer.
Piezometerˌpʌɪɪˈzɒmɪtə
From Greek piezein “press or squeeze” and metron ”measure”.
Noun 19th Century
An instrument for measuring the pressure of a liquid or gas, or something related to pressure (such as the compressibility of liquid).
Device
Invented in 1822 by Hans Christian Oersted (1777-1851) to measure the compression coefficient of water.
A USA Army Engineer Waterways Experiment Station TECHNICAL REPORT NO. 6-654 Report I August 1964 it stated: “Piezometer tubes have been used for at least 38 years (so since 1926) to measure uplift pressure at the foundation of a dam”.
Piezometer tubes have been used for at least 38 years (so since 1926) to measure uplift pressure at the foundation of a dam
Beginning in 1925 at American Falls Dam in Idaho, systems of vertical wells have been placed across the contacts of the bases of concrete dams with the foundation rock.
In the 1940s, Arthur Casagrande developed a “piezometer” to measure pore water pressure used for the analysis of the compression of clays. It consisted of:
- Bottom rubber plug
- Porous point – 1.5” OD x 1.0” ID x 2 ft (later to be known as a piezometer tip)
- Rubber bush
- Saran tube standpipe 0.5” OD
He combined it in a borehole with
- Filter sand around the porous point
- Bentonite seal to measure a specific zone
This became known as a Casagrande Standpipe Piezometer and still today we refer to the porous tip as a Casagrande Piezometer and the riser pipe as the Standpipe. Both are individual products but once installed all become part of the Casagrande Piezometer system, pictured below.
The system became a product. The product became the name of the system!
Today Geotechnical pressure sensor models are typically defined by manufacturers as follows:
- Standpipe/Casagrande piezometers
- Vibrating Wire piezometers
- Strain Gauge piezometers
- Pneumatic piezometers
- Hydraulic piezometers
And yet all of the above need to be installed into a system to actually become a Piezometer.
Should we not therefore be calling the above sensors Pressure Sensors or Transducers? It would be braver men than us to try and change nearly 100 years of history although back in 1995 Geosense called them Pressure Transducers. We subsequently changed to Piezometer to match all other manufacturers but perhaps we should have tried harder to buck the trend!
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