Choosing Instruments

Critical Parameters

Each project presents a unique set of critical parameters. The designer must identify those parameters and then select instruments to measure them. What information is required for the initial design? What information is required for evaluating performance during and after construction? When the parameters are identified, the specification for instruments should include the required range, resolution, and precision of measurements. See instrument performance specifications below.

Complementary Parameters

The behavior of a soil or rock mass typically involves not one, but many parameters. In some cases, it may be sufficient to monitor only one parameter, but when the problem is more complex, it is useful to measure a number of parameters and to look for correlation between the measurements. Thus it is common practice to choose instruments that provide complementary measurements.

For example, inclinometer data indicating increased rate of movement may be correlated with piezometer data that shows increased pore pressures. The load on a strut, calculated from strain gauge data, should correlate with convergence data provided by inclinometer behind a retaining structure.

Another benefit of selecting instruments to monitor complementary parameters is that at least some data will always be available, even if one instrument fails.

Ground Conditions

Ground conditions often determine the choice of instrument. For example, a standpipe piezometer is a reliable indicator of pore-water pressure in soil with high permeability, but is much less reliable in soil with low permeability. A large volume of water must flow into the standpipe to indicate even a small change in pore-water pressure. In soils with low permeability, the flow of water into and out of the standpipe is too slow to provide a timely indication of pore-water pressure. A better choice in this case would be a diaphragm-type piezometer, which offers faster response since it is sensitive to much smaller changes in water volume.

Environmental Conditions

Temperature and humidity also affect instrument choice. Instruments such as hydraulic piezometers and liquid settlement gauges have limited use in freezing weather. In tropical heat and humidity, simple mechanical devices may be more reliable than electrical instruments.

Personnel and Resources at the Site

Consider the personnel and resources at the site when choosing instruments. Do technicians have the skills required to install and read a particular type of instrument? Are adequate support facilities available for maintenance and calibration of the instrument?

Data Acquisition

An automatic data acquisition system may be required when:

1. There is a need for real-time monitoring and automatic alarms;
2. Sensors are located at a remote site or in a location that prevents easy access;
3. There are too many sensors for timely manual readings;
4. Qualified technicians are not available.

If a data acquisition system is required, the choice of instruments should be narrowed to those that can be connected to the system easily and inexpensively.

Instrument Life

Are readings needed only during construction or will they be needed for years afterwards? Instruments, signal cables, and protective measures should be selected accordingly. Some instruments are excellent for short-term applications, but may exhibit excessive drift over the long term.

Instrument Quality

The difference in cost between a high-quality instrument and a lesser-quality instrument is generally insignificant when compared to the total cost of installing and monitoring an instrument. For example, the cost of drilling and backfilling a borehole is typically 10 to 20 times greater than the cost of the piezometer that goes in it. It is false economy to install a cheaper, less reliable instrument. It is expensive and sometimes impossible to replace a failed instrument. Even when it is possible to replace the instrument, the original baseline data is no longer useful.

Instrument Performance

Instrument performance is specified by range, resolution, accuracy, and precision. The economical designer will specify minimum performance requirements, since the cost of an instrument increases with resolution, accuracy, and precision.

Range is defined by the highest and lowest readings the instrument is expected to produce. The designer typically specifies the highest values required.

Resolution is the smallest change that can be displayed on a readout device. Resolution typically decreases as range increases. Sometimes the term "accuracy" is mistakenly substituted for resolution. Resolution is usually many times better than accuracy and is never expressed as a plus/minus value.

Accuracy is the degree to which readings match an absolute value. Accuracy is expressed as a ± value, such as ± 0.5mm, ±1% of reading, or ±1% of full scale.

Precision or repeatability is often more important than accuracy, since what is usually of interest is a change rather than an absolute value. Every time a reading is repeated, the value returned by the instrument is slightly different. Precision is expressed as a ± value representing how close repeated readings approach a mean reading.