Hach Sigma 950 Instrument Manual page 39

7. Enter the ambient air temperature at the transducer location. For optimum results,
allow enough time (100 minutes) to ensure that the sensor is at equilibrium with the
surrounding ambient temperature.
The speed of sound in air varies with the temperature of the air. The ultrasonic sensor is
equipped with temperature compensation to help eliminate the effect of temperature
variation under normal site conditions.
8. Press to continue.
ACCEPT
Liquid Depth
Liquid depth requires knowing the level or depth of the liquid in the channel that is
contributing to flow. Liquid depth calibration is the recommended calibration method for
the in-pipe ultrasonic sensor.
Continue from Step 8, above:
1. Select the Liquid Depth method.
2. Take a physical measurement of the liquid depth (head) and enter the value.
3. Press when finished.
ACCEPT
Sensor Height
Sensor height calibration is generally used when access to the primary device is difficult
(such as confined space entry in a manhole) or when there is no liquid flowing during
installation of the flow meter. This calibration method requires compensation for the
internal deadband in the sensor housing. Measurement uncertainty increases to 1.07 cm
(0.035 ft) for a ±30 cm (±1 ft) change in level from the calibration point.
Use this method only if the Liquid Depth method is not an option.
Continue from Step 8, above:
1. Measure the distance from the bottom of the sensor to the zero flow point. Add 18 cm
(7.09 in.) to the measured distance to obtain the total zero flow distance for the
in-pipe sensor. Refer to
2. Select the Sensor Height calibration method and enter the total zero flow distance
from Step 1.
3. Press when finished.
ACCEPT
Setting the Invisible Range
Note: When programming the invisible range, 18 cm (7.09 in.) must be added to the desired range
to compensate for the internal deadband distance between the sensor, the reflector, and the bottom
of the sensor housing.
The 950 Flow Meter is equipped with an invisible range feature to prevent false echoes
from tops of channel walls, ladder rungs, shelves, etc. A
user-selected range is defined that is invisible to the flow meter. Do not extend the
invisible range to where it meets or overlaps the highest expected level in the channel.
Have a gap of at least 5 cm (2 in.) between the invisible range and the highest expected
level. Only objects beyond the invisible range can
be detected.
Figure 6 on page 38.
Sensor Installation
37