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3.2
Water Depth Measurement – Ultrasonic
For Depth measurement the Starflow QSD uses Time-of-Flight (ToF) Ranging. This involves
transmitting a burst of ultrasonic signal upwards to the surface of the water and measuring the time
taken for the echo from the surface to be received by the instrument. The distance (water depth) is
proportional to the transit time and the speed of sound in water (corrected for temperature and
density). The maximum ultrasonic depth measurement is limited to 5m.
3.3
Water Depth Measurement - Pressure
Sites where the water contains large amounts of debris or air bubbles may be unsuited for ultrasonic
depth measurement. These sites are better suited to using pressure to determine the water depth.
Pressure based depth measurement may also be applicable to sites where the instrument cannot be
located on the floor of the flow channel or it cannot be mounted horizontally.
The Starflow QSD is fitted with a 2 bars absolute pressure sensor. The sensor is located on the
bottom face of the instrument and utilises a temperature compensated digital pressure sensing
element.
Where depth pressure sensors are used the atmospheric pressure variation will cause errors in the
indicated depth. This is corrected by subtracting the atmospheric pressure from the measured depth
pressure. A barometric pressure sensor is required to do this.
If the data logger used to read the Starflow QSD does not incorporate a barometric pressure sensor
then the One Wire Bus 6515A Reference Barometer option should be purchased. This mounts near to
the logger and is read by the Starflow QSD instrument which will then automatically compensate for
the atmospheric pressure variations ensuring an accurate depth measurement is achieved. This
enables Starflow QSD to report actual water depth (pressure) instead of barometric pressure plus
water head.
3.4
Temperature
A solid state temperature sensor is used to measure the water temperature. The speed of sound in
water and its conductivity is affected by temperature. The instrument uses the measured temperature
to automatically compensate for this variation.
3.5
Electrical Conductivity (EC) – 6537A Only
The 6537A Starflow QSD is equipped with the capacity to measure the conductivity of the water. A
linear four electrode configuration is used to make the measurement. A small current is passed
through the water and the voltage developed by this current is measured. The instrument uses these
values to calculate the raw uncorrected conductivity.
Conductivity is dependent on the temperature of the water. The instrument uses the measured
temperature to compensate the returned conductivity value. Both raw or temperature compensated
conductivity values are available.
Unidata Manual - 6527B and 6537A Starflow QSD User Manual 17 01 2020.docx
Manual – Starflow QSD Model 6527B and 6537A
Page 6
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