SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

SonTek/YSI
6.6. Temperature and Pressure Sensors
Temperature sensor
To measure temperature, the Argonaut uses a thermistor mounted on the inside of the transducer
head. The thermistor is coupled to a titanium pin exposed to the water to minimize the insulating
effects of the plastic housing. The temperature sensor has a specified accuracy of ±0.1°C.
Temperature data are sampled once per second during the averaging interval and the mean value
is recorded with each sample.
Pressure sensor
Cross Section of Argonaut Transducer Head
If the Argonaut includes a strain
gage pressure sensor, it is mounted
in a recessed hole on the top transducer
head as shown in Figure 17.
The sensor is recessed and isolated
from the water (for corrosion protection)
by a diaphragm and silicon
oil reservoir. The diaphragm
is held in place by a plastic retaining
plug that screws into the end
cap. The pressure sensor forms a
water tight seal using a bore seal o-ring. For added leak protection, the path into the Argonaut
housing is filled with epoxy. Under normal conditions, you should not need to access the sensor
directly. If you suspect problems, please contact SonTek before trying to access the sensor.
Data from the pressure sensor are sampled by the Argonaut electronics using a 12-bit A/D
converter. Data are converted to a 2-byte integer scaling the A/D counts by 16 (giving a total
count range of 0-65520, with a step size of 16 counts). Mean and standard deviation of pressure
are recorded with each sample. Pressure data are stored in counts that must be converted to
physical units (decibar) using three calibration constants.
The pressure calibration constants are measured at the factory and included with each system.
The constants are stored in each data file and can be accessed from the Argonaut direct command
interface. Pressure in counts is converted to decibar using the following formula.
dbar = PressOffset + (PressScale × Counts) + (PressScale_2 × (Counts 2 ))
where
68
Threaded
Retaining Plug
Diaphragm for
Silicon Oil Reservoir
Pressure Sensor
with Bore Seal O-Ring
Sensor Wires
Figure 17 – Argonaut Pressure Sensor Mounting
dbar = measured pressure (decibar)
PressOffset = offset calibration constant set at factory (decibar)
PressScale = linear (1 st order) calibration constant set a factory (decibar/count)
PressScale_2 = quadratic (2nd order) calibration constant set a factory (decibar /count 2 )
Counts = measured pressure (counts)
The values for PressOffset, PressScale, and PressScale_2 can be accessed three ways: through
the direct command interface (§3.12), from the data conversion software *.CTL file (see the
Argonaut Software Manual), or directly from the binary data file. In the first two methods,
PressOffset and PressScale are output with the units shown above and can be used directly in the
formula above. PressScale_2 is output with units of 10 -12 decibar/count 2 and must be multiplied
by (10 -12 ) before using in the above formula.
Argonaut Operation Manual Firmware Version 7.9 (May 1, 2001)