YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
76 ADVField Operation Manual (September 1, 2001)
4.9. Optional Sensors (External): Resonance/Frequency Pressure Sensors
This section describes the use of
fre-
quency pressure sensors
with ADVField
systems. SonTek supports two types of
frequency pressure sensors: (1) the Druck
RPT (
DRUCK
), a silicon resonant trans-
ducer made by Druck, Inc and (2) the
Paroscientific digiquartz transducer
(
PAROSFREQ
) made by Paroscientific, Inc.
4.9.1. Overview
Typically, SonTek systems include
analog
pressure sensors
(strain gage) that provide
sufficient accuracy and stability (0.1 to
0.25 %) at a moderate cost. These sensors
are usually installed within the system head (Figure 24). They are internal to the system and are
usually referred to as
PRESSURE
or
PRESSURE SENSOR
in our documents and command interface.
Some applications, however, require better long-term stability (e.g., tide studies, wave interaction
with topography, long-term water-level monitoring). To accommodate these needs, SonTek has
integrated
frequency pressure sensors
, which offer an order of magnitude better accuracy and
stability (0.01%). The first supported sensor was the Paroscientific digiquartz with a serial inter-
face (
PAROS
). These sensors did not fit inside our systems and were mounted externally (Figure
24). The next supported sensor, the Druck RPT, did fit inside an ADVOcean head. However, for
historical purposes, all
non-strain gage sensors
are usually referred to as
EXTERNAL PRESSURE
SENSOR
in our documents and direct command interface (§3.9.4).
4.9.2. Serial External Pressure Sensors (PAROS)
As mentioned, the first external pressure sensor we supported was the
PAROS
. The
PAROS
is con-
nected to an ADVField by a short cable. The differences, when compared with a strain-gage are:
•
The
PAROS
is an absolute sensor. It reads 1 dbar in air; a strain gage reports 0 dbars in air.
•
Sampling rate is limited to 4 Hz (ADVs and Hydras) due to serial interface limitations.
4.9.3. Druck RPT External Frequency Pressure Sensor (DRUCK)
Although the
PAROS
provides the required accuracy, its size and external location cause addi-
tional complications when mounting and deploying a system. SonTek, in collaboration with
Druck, Inc., now offers the Druck RPT (resonant pressure transducer). This sensor is very com-
pact and fits inside the same mounting socket as the regular strain gage (Figure 24). The
DRUCK
sensors are available for Hydra systems. The differences, when compared with a strain-gage are:
•
The
DRUCK
is an absolute sensor. It reads 1 dbar in air; a strain gage reports 0 dbars in air.
•
Requires a special interface to sample the frequency signal (installed on Recorder board).
•
Sampling rate is limited to 10 Hz (ADVs and Hydras) because of the time required to sample
the frequency signal with sufficient precision.
•
Requires calibration matrix (specific to each sensor) to be loaded into the processor (system
CPU or a user PC) to obtain valid pressure readings.
•
Available in 50-psia pressure rating only (equivalent to
≈
22-m sealed gage).
External Paroscientific
Pressure Sensor
Power and
communication cable
Internal pressure sensor
(strain gage or Druck RPT)
SonTek ADVO
Figure 24 - ADVOcean with
external pressure sensors