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RSI Application Note
TN-019
Pressure Calibration Procedures for VMP-200 and XMP
Date: 2010-12-07, Rolf Lueck
Table of Contents
1.
Introduction .................................................................................................................................................. 1
2.
Pressure — Basic Concepts ........................................................................................................................... 1
2.2.
From Pressure transducer to AD Converter.......................................................................................... 2
2.3.
The Temperature Compensation Resistors .......................................................................................... 3
1. Introduction
This document explains how we “calibrate” the pressure channel on a VMP-200 instrument that uses the
µASTP-LP board (P049). It also applies to the pressure channel on an XMP (P043).
2. Pressure — Basic Concepts
The pressure channel uses the Keller Model PA-10L transducer with a full-scale range of 0 to 1000 dBar, but
some units may have other ranges. The manufacturer provides calibration information consisting of:



a sensitivity expressed in units of mV/Bar at 1 mA excitation,
a “zero” value representing the bridge voltage when the applied pressure is zero, in units of mV, and
the value of two resistors that must be added to the circuit for temperature compensation.
The pressure transducer cannot be calibrated when it is installed in the VMP-200. A history of over 50
pressure transducers from Keller indicates that the calibration performed by the manufacturer is accurate to
0.1% when it is compared against independent calibrations performed at RSI. Keller provides only two
coefficients, while our independent calibrations resolve a third coefficient representing the slight nonlinearity of the transducers. For most practical purposes, especially in shallow waters, the third coefficient
provides only a small correction and its inclusion changes the calculated pressure by less than 2 dBar over the
0 to 1000 dBar range compared to the results obtained by using only two (linear) coefficients.
Therefore, the Keller pressure transducers in the VMP-200 are not calibrated. The sensitivity and zero values,
provided by the manufacturer, can be used to determine the coefficients needed to convert pressure data
into physical units. These derived coefficients must be entered into the setup file used for data acquisition so
that they are available for the real-time display of depth and fall-rate, and can subsequently be used during
data processing to convert the raw, binary, data values into physical units.
It is important to note here that the values provided by the manufacturer cannot be entered directly into the
setup file. Here, I will show how the values provided by the manufacturer are converted into values that can
be placed into the setup file
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RSI Application Note
2.2.
TN-019
From Pressure transducer to AD Converter
Please, refer to the circuit schematics for P049R01. Page 5 shows the pressure circuit and page 8 shows the
AD Converter.
The pressure transducer is driven by a constant current of
IB 
1.024V
 0.8533 103 A
1200
(1)
which is accurate to better than 1000 PPM (parts per million). This is smaller than the excitation current used
for calibration by the manufacturer and must be taken into account. The gain of the transducer differential
amplifier is 7 and this too is accurate to better than 1000 PPM. Therefore, the voltage produced by the
pressure circuit is
VP  103 Z  G 
IB
P
103 S  G
3 
10
10
(2)
where:





Z is the “zero” value provided by Keller and is expressed in mV,
G = 7 is the gain of the transducer amplifier,
IB is the bridge excitation current (1),
S is the sensitivity provided by Keller in units of mV per Bar, and
P is the pressure in units of dBar, which is preferred by oceanographers over units of Bars.
The numeric values in (2) convert the numbers used by the manufacturer into standard values in units of
volts and amps. We can test (2) by inserting some typical numeric values for the sensitivity and zero of the
Keller transducer ( Z  0.1 and S  1.592 ). This gives
0.8533 103
P
103 1.592  7

3
10
10
3
4
VP  0.7 10  9.509 10 P
VP  103  (0.1)  7 
(3)
VP  0.9502 V at 1000 dBar
which is approximately the design voltage for a full-scale pressure.
The AD Converter has 16 bits and a full-scale range of 4.096 V. Therefore, the numeric value produced by the
pressure channel is
NP 
 216 
 216  P
IB
216
3
VP  103 Z  
G

10
S




G
4.096
103
 4.096 
 4.096  10
N P  112  Z  9.5570  S  P
To derive pressure from the VMP-200 data, we need the inverse of (4), which is
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(4)
RSI Application Note
TN-019
P
10 N P 102 Z

I B S G
IBS
11.719
0.10464
Z
NP
S
S
216

 16 103 .
4.096
P
(5)
With typical numeric values for sensitivity and zero, a pressure reading of N P  15000 counts gives a
pressure of P =986.7 dBar.
Therefore, the coefficients that must be entered into the setup file for pressure are:
11.719
Z,
S
0.10464
a1 
S
a0  
(6)
where S and Z are the sensitivity and zero values given in the calibration sheet by Keller and in the units given
by Keller.
The Matlab function XMP_P_coef.m will compute the coefficients for the setup file from the calibration
coefficients provided by Keller. It is called using the syntax
[a0, a1] = XMP_P_coef(Zero, Sensitivity)
The master copy of the function is located in \\Piggy\RSI\Products\odas_Library_V2. You should place a copy
into your Matlab path, if you do not already have it on your computer. The preferred directory is the one
containing the ODAS Library of Matlab functions.
2.3.
The Temperature Compensation Resistors
This manufacturer recommends the insertion of two resistors in to the bridge network to compensate the
transducer for its variation in output for changes in the temperature of the transducer. We always insert
these resistors. However, it is inevitable that the reading of the pressure transducer at zero (atmospheric)
pressure is not precisely zero when the above coefficients are applied to the data. Almost all of this “zero”
error is due to the temperature sensitivity of the transducer itself.
Users can correct this final zero error by using the “calibration” option of the data acquisition software and
taking a reading of several seconds when the instrument is on deck and close to the temperature expected in
the water. If data acquisition is started before the instrument is placed into water, then that segment of data
can also be used to get a reading for zero pressure. If a good zero pressure reading is available, then the
coefficient a0 can be replaced by the negative of this reading multiplied by the sensitivity coefficient a1. The
reading by the pressure transducer, when expressed in physical units, will then be zero when it is on deck at
atmospheric pressure.
End of Document
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