HMA 1200 Technical Manual

Revision 2016-1-1

Copyright ©, 2016 by HMA Group.

The information contained in this document is subject to change without prior notice.

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Contents

1. Introduction:................................................................................................................................................................ 3

2. Specifications: ............................................................................................................................................................. 4

3. Vibrating Wire Piezometer Operation .............................................................................................................. 5

4. Calibration and Interpreting Readings ............................................................................................................. 6

4.1 Electrical Connection ............................................................................................................................................. 6

4.2 Calibration Sheet Interpretation ...................................................................................................................... 7

4.3 Data Reduction ......................................................................................................................................................... 8

4.4 Pressure Head/Water Level Calculation ....................................................................................................... 9

4.5 Barometric Compensation .................................................................................................................................. 9

5. Installation Procedure .......................................................................................................................................... 10

5.1 Installation in Standpipes ................................................................................................................................. 10

5.2 Installation in Boreholes (traditional sand layer method) ................................................................. 10

5.3 Installation in Boreholes (direct grout method) ..................................................................................... 11

5.4 Installation in Fills and Embankments ....................................................................................................... 12

6. Instrument Troubleshooting ............................................................................................................................. 13

7. Field Installation Records Sheet ....................................................................................................................... 14

8. Standard Temperature vs Resistance Values ............................................................................................. 15

9. Definitions ................................................................................................................................................................. 16

10. HMA Group Conditions of Supply ............................................................................................................... 17

Important Note

Always ensure that the zero reading is taken at installation.

For further information, see the data reduction and installation sections of this manual.

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1. Introduction:

The HMA Geotechnical Vibrating Wire Piezometer has been designed to easily measure
remote fluid pressures in earthen masses.

The Vibrating Wire Piezometer functions on the principle of tensile wire vibration. As the
native reading taken is a frequency measurement, water penetration, temperature
variations and contact resistance do not affect the output unlike several other types of
electrical instrumentation.
The piezometers can be read using a handheld vibrating wire readout unit , or a standalone
datalogger.
Providing the HMA Geotechnical Vibrating Wire Piezometers are installed in accordance
with the prescribed techniques, the instrument has the capacity to operate indefinitely.

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2. Specifications:

Piezometer

Pressure Ranges (kPa)

350, 700,1000 2000, 3000, 5000

Over Range

1.5 x Rated Pressure

Resolution

0.025% full scale

Accuracy

< 0.5% full scale

Operating Temperature

-20 to 60⁰C

Filters

0.5 and 20 micron

Dimensions

19mm dia, 136mm long

Weight

0.11kg

Signal Output

Continuous Gauge Frequency

Frequency Range

2000 - 3500 Hz

Cable

Description

2 Pair Twisted 22AWG

Conductor Gauge

22AWG

Insulation Material

PP Compound

Nominal Thickness (mm)

0.2

Nominal OD (mm)

1.2

Drain Wire Gauge

24AWG

Applicable Standards

AS/NZS 1125, AS/NZS 3808

Electrical Properties

Max. Conductor DC Resistance @
20⁰C

56.95 Ohms per km

Voltage Test: Core to Core

1kV AC for 1 Minute

Voltage Test: Core to Screen

1kV AC for 1 Minute

Mechanical Properties

Operating Temperature

-15 to 90⁰C

Min. Bend Radius

63mm

Approximate Mass

5.83kg/100m

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3. Vibrating Wire Piezometer Operation

The HMA Geotechnical Vibrating Wire Piezometer provides a reliable static pressure output to be
utilised in a number of applications, such as:

• Water level monitoring

• Soil deposit pressure monitoring

• Compacted fills for dams

• Mining applications (Dewatering, Pumping, Backfill)

• Slope stability

• Seepage

The piezometers are based upon the principle of vibrating wire resonance. The instrument consists
of a vibrating wire element connected to a sensitive, perpendicular diaphragm. Exerting pressure,
such as pore water pressure, on the diaphragm will cause it to deflect therefore altering the tension
and corresponding resonant frequency of the vibrating wire.
Each time a measurement is taken, electromagnetic coils adjacent to the vibrating wire pluck the
instrument wire and then measure the resonant frequency of the wire. The frequency of the wire is
read by a handheld readout or a datalogger.

Figure 1. Vibrating Wire Piezometer construction

Hand held readouts and dataloggers usually return a Digits (B) reading, being the frequency
squared divided by 1000 (Hz2 x 10-3) as this value is directly proportional to the pressure applied to
the diaphragm. By using vibration frequency as a measurement method, water penetration, lead
wire resistance and contact resistances are deemed negligible. Long runs of instrumentation cable
can be installed also, unlike other forms of piezometers.
The stainless steel housing of the piezometer ensures resistance to corrosive environments.

Standard filters are 20-micron pore diameter sintered stainless steel. However, a variety of filter
permeabilities are available to meet different application requirements.

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4. Calibration and Interpreting Readings

Prior to shipment each piezometer is individually calibrated with respect to applied pressure.
Although comparatively minimal, further corrective calibrations regarding changes in barometric
pressure and temperature may also be necessary. It should also be noted that the manufacturers
factory elevation is 110 metres above sea level, meaning that further corrections for site elevation
may be required.

The site test reading of each piezometer should be checked and noted upon delivery. Theoretically
once calibrated the site test reading should closely match the factory reading.

All Geotechnical Systems Vibrating Wire Piezometers have been calibrated to international
standards using either pneumatic or deadweight pressure testers. Both testers are traceable to the
national standards at the National Bureau of Standards (U.S.A).

Each Vibrating Wire Piezometer ships with a calibration sheet provided by HMA Geotechnical.

4.1 Electrical Connection

Figure 2. Typical HMA Geotechnical Vibrating Wire Piezometer cable

Conductor Definitions

Red

Vibrating Wire

Black

Vibrating Wire

Green

Thermistor

White

Thermistor