Technical information

© CLEAPSS 2005 1635 ICT in the Science Department
Sensors with a linear output will usually have switched ranges, any one of which
is likely to be inadequate for monitoring daylight over time. The change in light
level when the Sun goes behind a dense cloud can easily involve a factor > 10. For
advanced work, the spectral quality of the light can be of significance and the
spectral response of sensors does vary. See section 17.3.5 (Light) for a discussion.
Magnetic field
Semiconductor Hall-effect probes produce a voltage that is linearly related to the
magnetic field strength perpendicular to the wafer and make the basis of useful
sensors. Many such sensors will give positive or negative outputs, depending upon
the direction of the field. The sensor may have to be reversed to obtain a response
of the desired polarity.
Oxygen
The problems of maintenance, calibration, temperature sensitivity and use are all
discussed at length in section 17.3.8 (Oxygen).
pH
A pH meter with combination electrode is one of the more familiar electronic
meters in school use and most of those now sold have outputs fitted so that they
could be used as sensors with computers. The attraction of using a pH sensor is
largely that it is considerably cheaper than an instrument with its own meter. See
section 17.3.9 (pH) for information on storage and revival of probes.
Position/
Movement
Position sensors may be described as ‘movement’ sensors but actually respond to
the position of the attached arm, not to its movement. Sophisticated software
could perform a differentiation and obtain a velocity value but normally these
sensors are used to record position as it varies with time. (Velocity/Motion sensors
are available, see below.)
There may be a trade-off between robustness and sensitivity with these devices.
For biological investigations with the sensor attached to a growing plant or simil-
ar, it is necessary to use a sensor with a low-friction bearing with a variable
capacitor to sense displacement and this could be somewhat delicate. For studying
the simple harmonic motion of a weighted ruler in physics, a more robust design
is adequate and, therefore, preferable.
Pressure
Three types of pressure sensor are mentioned in section 17.3.10 (Pressure); mano-
meter sensors are no longer in fashion.
Pressure sensors that respond to pressure changes of the order of +1 atmosphere
use either semiconductor sensors (which have strain gauges as part of a small
silicon diaphragm) or an aneroid barometer bellows. Such sensors can be used to
demonstrate gas laws and effectively replace mercury-filled manometers.
A barometer sensor has a pressure sensor with more sensitive circuitry and no
connection for tubing. Its main application is in automatic weather stations.
Sound
Sound level can be a parameter of interest in environmental studies and a sensor
covering a wide range of sound levels in one range would be useful for recording
sound levels with any datalogging device or computer. See section 17.3.11
(Sound).
Temperature
Temperature sensors may use a variety of sensitive probes. Thermocouple types
are very linear and accurate in measuring temperature changes or differences.
Semiconductor thermistor probes are very linear and stable in calibration but
tend to be more sluggish in response than thermocouples if the sensitive part is
enclosed in a sheath. Unenclosed semiconductor probes are now rarely employed;
if used in solutions, problems with electrical leakage currents tend to occur.
Thermometer probes are unlikely to approach the response speed of a mercury-in-
glass thermometer when immersed in a liquid. Speed of response in air is usually
similar (but not as good) as mercury-in-glass. See also section 17.3.12 (Temper-
ature) and section 10.7 (Thermometers).
Velocity
Useful motion sensors are available, eg, from Data Harvest. Some distance sens-
ors (eg, Harris SensorMeter) use appropriate software to generate velocity and
acceleration data.