Instruction Manual
Model 4580
VW Pressure Transducer
and Barometer
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon®.
The information contained herein is believed to be accurate and reliable. However, Geokon
omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1995-2019 by Geokon
(Doc Rev Q, 04/30/19)
®
®
assumes no responsibility for errors,
Warranty Statement
Geokon warrants its products to be free of defects in materials and workmanship, under normal
use and service for a period of 13 months from date of purchase. If the unit should malfunction,
it must be returned to the factory for evaluation, freight prepaid. Upon examination by Geokon,
if the unit is found to be defective, it will be repaired or replaced at no charge. However, the
WARRANTY is VOID if the unit shows evidence of having been tampered with or shows
evidence of being damaged as a result of excessive corrosion or current, heat, moisture or
vibration, improper specification, misapplication, misuse or other operating conditions outside of
Geokon's control. Components which wear or which are damaged by misuse are not warranted.
This includes fuses and batteries.
Geokon manufactures scientific instruments whose misuse is potentially dangerous. The
instruments are intended to be installed and used only by qualified personnel. There are no
warranties except as stated herein. There are no other warranties, expressed or implied, including
but not limited to the implied warranties of merchantability and of fitness for a particular
purpose. Geokon is not responsible for any damages or losses caused to other equipment,
whether direct, indirect, incidental, special or consequential which the purchaser may experience
as a result of the installation or use of the product. The buyer's sole remedy for any breach of this
agreement by Geokon or any breach of any warranty by Geokon shall not exceed the purchase
price paid by the purchaser to Geokon for the unit or units, or equipment directly affected by
such breach. Under no circumstances will Geokon reimburse the claimant for loss incurred in
removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or software,
however, Geokon neither assumes responsibility for any omissions or errors that may appear nor
assumes liability for any damages or losses that result from the use of the products in accordance
with the information contained in the manual or software.
TABLE of CONTENTS
1. THEORY OF OPERATION .................................................................................................................................. 1
2. INSTALLATION .................................................................................................................................................... 2
2.1
PRELIMINARY TESTS............................................................................................................................................ 2
MODEL 4580 INSTALLATION ............................................................................................................................... 2
2.2
2.2.1 Barometer ................................................................................................................................................... 2
2.2.2 Piezometer................................................................................................................................................... 3
2.2.3 Pressure Transducer ................................................................................................................................... 4
SPLICING AND JUNCTION BOXES.......................................................................................................................... 4
2.3
ELECTRICAL NOISE .............................................................................................................................................. 4
2.4
LIGHTNING PROTECTION ..................................................................................................................................... 5
2.5
3. TAKING READINGS ............................................................................................................................................. 6
3.1
GK-404 READOUT BOX ....................................................................................................................................... 6
3.1.1 Operating the GK-404 ................................................................................................................................ 6
GK-405 READOUT BOX ....................................................................................................................................... 7
3.2
3.2.1 Connecting Sensors with a 10-pin Bulkhead ............................................................................................... 7
3.2.2 Connecting Sensors with Bare Leads .......................................................................................................... 7
3.2.3 Operating the GK-405 ................................................................................................................................ 7
GK-403 READOUT BOX (OBSOLE TE MODEL) ...................................................................................................... 8
3.3
3.3.1 Connecting Sensors with a 10-pin Bulkhead ............................................................................................... 8
3.3.2 Connecting Sensors with Bare Leads .......................................................................................................... 8
3.3.3 Operating the GK-403 ................................................................................................................................ 8
MEASURING TEMPERATURES ............................................................................................................................... 8
3.4
4. DATA REDUCTION .............................................................................................................................................. 9
4.1
PRESSURE CALCULATION .................................................................................................................................... 9
TEMPERATURE CORRECTION ............................................................................................................................. 11
4.2
BAROMETRIC CORRECTION ............................................................................................................................... 11
4.3
4.3.1 Vented Transducers................................................................................................................................... 13
ENVIRONMENTAL FACTOR S ............................................................................................................................... 13
4.4
5. MAINTENANCE .................................................................................................................................................. 14
6. TROUBLESHOOTING ........................................................................................................................................ 15
APPENDIX A. MODEL 4580 SPECIFICATIONS ................................................................................................ 16
A.1
MODEL 4580 PRESSURE TRANSDUCER ............................................................................................................. 16
THERMISTOR (SEE APPENDIX B ALSO) .............................................................................................................. 16
A.2
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION ..................................................................... 17
APPENDIX C. USE OF SECOND ORDER POLYNOMIAL TO IMPROVE ACCURACY ............................ 18
APPENDIX D. TYPICAL CALIBRATION REPORT .......................................................................................... 19
FIGURES
IGURE 1 - MODEL 4580-2 VIBRATING WIR E PRESSURE TRANSDUCER ......................................................................... 1
F
IGURE 2 - MODEL 4850-1 BAROMETER ........................................................................................................................ 1
F
IGURE 3 - MODEL 4580 WEIR MONITOR INSTALLATION .............................................................................................. 3
F
IGURE 4 - SUGGESTED LIGHTNING PROTECTION SCHEME ............................................................................................ 5
F
IGURE 5 - LEMO CONNECTOR TO GK-404 ................................................................................................................... 6
F
IGURE 6 - LIVE READINGS – RAW READINGS............................................................................................................... 7
F
IGURE 7 - SAMPLE MODEL 4580 CALIBRATION REPORT .............................................................................................19
F
TABLES
ABLE 1 - ENGINEERING UNITS MULTIPLICATION FACTORS ........................................................................................10
T
ABLE 2 - MODEL 4580 PRESSURE TRANSDUCER SPECIFICATIONS ..............................................................................16
T
ABLE 3 - THERMISTOR RESISTANCE VERSUS TEMPERATURE .....................................................................................17
T
EQUATIONS
QUATION 1 - DIGITS CALCULATION ............................................................................................................................. 9
E
QUATION 2 - CONVERT DIGITS TO PRESSURE .............................................................................................................. 9
E
QUATION 3 - TEMPERATURE CORRECTION .................................................................................................................11
E
QUATION 4 - BAROMETRIC CORRECTION ....................................................................................................................11
E
QUATION 5 - CORRECTED PRESSURE CALCULATION ..................................................................................................12
E
QUATION 6 - RESISTANCE TO TEMPERATURE .............................................................................................................17
E
1
Instrument Cable
Diaphragm
Wire
Electromagnetic
Tripolar Surge Arrestor
Thermistor
Seal
Screw
Coil
1. THEORY OF OPERATION
The Model 4580 Vibrating Wire Pressure Transducer is designed primarily for the measurement
of very small changes in pressure such as in weirs, streams, etc. A sealed version is also available
which can be used for accurate measurement of barometric pressure. However, the Model 45801 Barometer as shown in Figure 2 is specially designed for this purpose.
The transducer consists of a double diaphragm capsule secured in a pressure chamber and
connected to a vibrating wire strain gauge. Vented capsules are used where barometric effects
need to be eliminated, such as in the weir and stream level measurements. In the case where the
sensor is used as a barometer, the capsule is evacuated. The vibrating wire is attached to the
capsule on one end and to the rigid housing on the other.
External pressure applied to the capsule causes it to compress and exert a force on the vibrating
wire gauge and this produces a change in wire tension and frequency of vibration. The change in
tension is proportional to the pressure and, through calibration, the pressure can be very
accurately determined.
Figure 1 - Model 4580-2 Vibrating Wire Pressure Transducer
Figure 2 - Model 4850-1 Barometer
2
2. INSTALLATION
2.1 Preliminary Tests
Upon receipt of the pressure transducer the zero reading should be checked and noted (see
Sections 3 for readout instructions). A thermistor is included inside the body of the sensor (as
shown in Figure 1) for the measurement of temperature (see Section 3.4 for measurement
instructions).
Calibration data are supplied with each gauge and a zero reading at a specific temperature and
barometric pressure. Zero readings at the site should coincide with the factory readings within 20
digits after barometric and temperature corrections are made. The factory elevation is +580 ft.
Before March 21, 1995, factory barometric pressure readings were corrected to sea level;
readings after this date represent absolute pressure. (Barometric pressure changes with elevation
at a rate of ≈½ psi per 1,000 ft.) See Appendix D for a sample calibration report. Note that the
calibrations were performed with the sensor vertical, cable end up.
The sensor is very sensitive, and it will be observed that the readings change depending on the
position of the sensor. A 30 to 50 digit change between the sensor being vertically up and
vertically down is normal. This phenomenon will not be a problem if the sensor remains
stationary or is in the same orientation during use. Additionally, these sensors are sensitive to
shock, particularly the very low-pressure versions. A shift in the zero reading can occur during
shipping or rough handling. Normally this will not affect the operation of the sensor, but a shift
of more than 100 units should be checked out by monitoring the sensor for a few days at a
constant temperature, if possible.
2.2 Model 4580 Installation
2.2.1 Barometer
The barometric sensors are calibrated at the factory and referenced to an absolute
barometric reading in millibars. Installation at the site is usually carried out by installing
the sensor in a vented enclosure with the sensor in a vertical position with the cable
exiting from the top of the sensor. (The sensor is position sensitive and is calibrated in the
vertical position.)
Transducers of this type have very high sensitivities and are usually somewhat susceptible
to shocks of the type that can occur during shipping. It is therefore advisable to obtain
concurrent readings with an accurate, local barometer following installation to be sure of
obtaining a proper onsite starting point for all measurements.
3
Transducer Cable
Vent Line
Moisture Trap
Cap
Slotted Pipe
Ground Surface
Weir
Transfer Pipe
Model 4580 Pressure Transducer
Water Level
2.2.2 Piezometer
When used as a piezometer or weir monitor, the vented version is usually chosen. In this
case, care must be taken to be sure the entire pressure cavity surrounding the capsule is
filled with water. A hole with a seal screw is provided on the body of the transducer to
allow water to flow through the pressure chamber. Flush water through the body of the
sensor with the water entering at the tip of the sensor and exiting through the seal screw.
Agitate the body of the sensor until no bubbles are seen exiting the seal screw port. The
screw should be replaced with the transducers held under water to prevent air from
reentering the sensor. (Air trapped in the chamber can cause inaccurate and/or unstable
pressure readings in changing temperature environments.)
The vented versions incorporate a moisture trap on the end of the vent line. Desiccant
inside the moisture trap prevents moisture from entering the inside of the sensor capsule.
The vent trap screw must be removed during the measurement period to equalize the
changes in barometric pressure both inside and outside the pressure capsule. (When the
desiccant turns from blue to pink it should be replaced.)
Figure 3 - Model 4580 Weir Monitor Installation
4
2.2.3 Pressure Transducer
The transducer has 1/4" NPT female thread on the front end allowing for connection to
various types of fittings. Both vented and unvented transducers are used in this version
and the choice depends upon the application and whether barometric changes must be
automatically eliminated from the readings.
The wrench flats on the transducer should be used when tightening fittings into the
transducer.
Prior to use, flush the transducer with water to remove all air from inside the sensor. This
can be done by removing the seal screw and pumping water through the sensor. Replace
the seal screw when bubbles no longer appear.
2.3 Splicing and Junction Boxes
Because the vibrating wire output signal is a frequency rather than a current or voltage,
variations in cable resistance have little effect on gauge readings. Therefore, splicing of cables
has no effect, and in some cases, can in fact be beneficial. For example, if multiple transducers
are installed in a borehole, and the distance from the borehole to the terminal box or datalogger is
great, a splice (or junction box) could be made to connect the individual cables to a single multiconductor cable. This multi-conductor cable would then be run to the readout station. For these
types of installations, it is recommended that the transducer be supplied with enough cable to
reach the installation depth, plus extra cable to pass through drilling equipment (rods, casing,
etc.).
Cable used for making splices should be a high-quality twisted pair type, with 100% shielding
and an integral shield drain wire. When splicing, it is very important that the shield drain
wires be spliced together. Splice kits recommended by Geokon incorporate casts that are placed
around the splice and then filled with epoxy to waterproof the connections. When properly made,
this type of splice is equal or superior to the cable in strength and electrical properties. Contact
Geokon for splicing materials and additional cable splicing instructions.
Junction boxes and terminal boxes are available from Geokon for all types of applications. In
addition, portable readouts and dataloggers are also available. Contact Geokon for specific
application information.
2.4 Electrical Noise
Care should be exercised when installing instrument cables to keep them as far away as possible
from sources of electrical interference such as power lines, generators, motors, transformers, arc
welders, etc. Cables should never be buried or run alongside AC power lines as they will pick up
the noise from the power cable, which will likely cause unstable readings. Contact the factory
concerning filtering options available for use with the Geokon dataloggers and readouts.
5
Transducer Cable
Model 4580 Pressure Transducer
LAB-3
(inside enclosure)
Earth Ground
2.5 Lightning Protection
In exposed locations, it is vital that the transducer be protected against lightning strikes. A
tripolar plasma surge arrestor, which protects against voltage spikes across the input leads, is
built into the body of the transducer. (See Figure 1.)
Additional lightning protection measures available include:
• Placing a Lightning Arrestor Board (LAB-3), in line with the cable, as close as possible to
the installed transducer. (See Figure 4.) These units utilize surge arrestors and transzorbs to
further protect the transducer. This is the recommended method of lightning protection.
• Terminal boxes available from Geokon can be ordered with lightning protection built in. The
terminal board used to make the gauge connections has provision for the installation of
plasma surge arrestors. Lightning Arrestor Boards (LAB-3) can also be incorporated into the
terminal box. The terminal box must be connected to an earth ground for these levels of
protection to be effective.
• If the instruments will be read manually with a portable readout (no terminal box), a simple
way to help protect against lightning damage is to connect the cable leads to a good earth
ground when not in use. This will help shunt transients induced in the cable to ground, away
from the instrument.
Figure 4 - Suggested Lightning Protection Scheme
6
3. TAKING READINGS
3.1 GK-404 Readout Box
The Model GK-404 Vibrating Wire Readout is a portable, low-power, handheld unit that can run
continuously for more than 20 hours on two AA batteries. It is designed for the readout of all
Geokon vibrating wire gauges and transducers; and is capable of displaying the reading in either
digits, frequency (Hz), period (µs), or microstrain (µε). The GK-404 also displays the
temperature of the transducer (embedded thermistor) with a resolution of 0.1 °C.
3.1.1 Operating the GK-404
Before use, attach the flying leads to the GK-404 by aligning the red circle on the silver
Lemo connector of the flying leads with the red line on the top of the GK-404 (Figure 5).
Insert the Lemo connector into the GK-404 until it locks into place.
Figure 5 - Lemo Connector to GK-404
Connect each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
To turn the GK-404 on, press the “ON/OFF” button on the front panel of the unit. The
initial startup screen will be displayed. After approximately one second, the GK-404 will
start taking readings and display them based on the settings of the POS and MODE
buttons.
The unit display (from left to right) is as follows:
• The current Position: Set by the POS button. Displayed as a letter A through F.
• The current Reading: Set by the MODE button. Displayed as a numeric value
followed by the unit of measure.
• Temperature reading of the attached gauge in degrees Celsius.
Use the POS button to select position B and the MODE button to select Dg (digits).
(Other functions can be selected as described in the GK-404 Manual.)
The GK-404 will continue to take measurements and display readings until the unit is
turned off, either manually, or if enabled, by the Auto-Off timer.
If no reading displays or the reading is unstable, see Section 6 for troubleshooting. For
further information, consult the GK-404 manual.
7
3.2 GK-405 Readout Box
The GK-405 Vibrating Wire Readout is made up of two components: The Readout Unit,
consisting of a Windows Mobile handheld PC running the GK-405 Vibrating Wire Readout
Application; and the GK-405 Remote Module, which is housed in a weatherproof enclosure and
connects to the vibrating wire gauge to be measured. The two components communicate
wirelessly. The Readout Unit can operate from the cradle of the Remote Module, or, if more
convenient, can be removed and operated up to 20 meters from the Remote Module.
3.2.1 Connecting Sensors with a 10-pin Bulkhead
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.2.2 Connecting Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.2.3 Operating the GK-405
Press the button labeled “POWER ON”. A blue light will begin blinking, signifying that
the Remote Module is waiting to connect to the handheld unit.
Launch the GK-405 VWRA program on the handheld PC by tapping on “Start”, then
“Programs”, then the GK-405 VWRA icon. After a few seconds, the blue light on the
Remote Module should stop flashing and remain lit, indicating that the remote module
has successfully paired with the handheld PC. The Live Readings Window will be
displayed on the handheld PC. Figure 6 shows a typical vibrating wire output in digits
and thermistor output in degrees Celsius.
If the no reading displays or the reading is unstable, see Section 6 for troubleshooting
suggestions. For further information, consult the GK-405 Instruction Manual.
Figure 6 - Live Readings – Raw Readings
8
3.3 GK-403 Readout Box (Obsolete Model)
The GK-403 can store gauge readings as well as apply calibration factors to convert readings to
engineering units. The following instructions explain taking gauge measurements using Modes
“B” and “F”.
3.3.1 Connecting Sensors with a 10-pin Bulkhead
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
3.3.2 Connecting Sensors with Bare Leads
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.3.3 Operating the GK-403
1) Turn the display selector to position “B” (or “F”).
2) Turn the unit on.
3) The readout will display the vibrating wire output in digits (refer to Equation 1 in
Section 4). The last digit may change one or two digits while reading.
4) The thermistor reading will be displayed above the gauge reading in degrees
centigrade.
5) Press the “Store” button to record the value displayed.
If the no reading displays or the reading is unstable, see Section 6 for troubleshooting
suggestions. The unit will automatically turn off after approximately two minutes to
conserve power. Consult the GK-403 Instruction Manual for additional information.
3.4 Measuring Temperatures
All vibrating wire transducers are equipped with a thermistor, which gives a varying resistance
output as the temperature changes. The white and green leads of the instrument cable are
normally connected to the internal thermistor. Geokon readout boxes will read the thermistor and
display the temperature in degrees C. (High temperature versions use a different thermistor,
which must be read using an ohmmeter.)
To read temperatures using an ohmmeter:
1) Connect an ohmmeter to the green and white thermistor leads coming from the instrument.
Since the resistance changes with temperature are large, the effect of cable resistance is
usually insignificant. For long cables a correction can be applied, equal to approximately
14.7 Ω per one thousand feet (48.5Ω per km). Multiply this factor by two to account for both
directions.
2) Look up the temperature for the measured resistance in Appendix B, Table 3.
9
4. DATA REDUCTION
4.1 Pressure Calculation
The basic units utilized by Geokon for measurement and reduction of data from vibrating wire
pressure transducers are the “digits” displayed on the GK-404 or GK-405 reading box.
Calculation of digits is based on the following equation:
2
Digits =
1
Period
Equation 1 - Digits Calculation
× 10
-3
or Digits=
For example, a transducer reading 9000 digits on channel B of the GK-404 or GK-405 readout
corresponds to a period of 333.3 µs and a frequency of 3000 Hz. Digits are directly proportional
to wire tension and applied pressure.
To convert digits to pressure using a linear calibration coefficient the following equation applies:
(See Appendix C for use of second order polynomial)
P
uncorrected
Equation 2 - Convert Digits to Pressure
= (R1 – R0) × G × F1
Where:
R0 represents the initial reading taken at installation (usually the zero reading).
R1 represents the current reading.
G is the calibration factor required to convert from “digits” to the engineering units given on the
calibration report. (See Appendix D for a sample calibration report.)
F1 is an optional multiplier to convert to any other required engineering units (see Table 1).
For example:
If;
The initial reading on channel B (R0) = 2509,
The current reading (R1) = 7023
The calibration factor (G) = -0.0001954 psi/digit.
Then;
Pressure = (7023 - 2509) × -0.0001954 = 0.882 psi
Hz
1000
2
10
From
→
↓
HG
psi
1
.036127
.43275
.0014223
1.4223
.49116
.019337
14.696
.014503
14.5039
.14503
145.03 "H2
O
27.730
1
12
.039372
39.372
13.596
.53525
406.78
.40147
401.47
4.0147
4016.1
'H2O
2.3108
.08333
1
.003281
3.281
1.133
.044604
33.8983
.033456
33.4558
.3346
334.6
mm
m H20
.70432
.025399
.304788
.001
1
.34532
.013595
10.332
.010197
10.197
.10197
101.97
"HG
2.036
.073552
.882624
.0028959
2.8959
1
.03937
29.920
.029529
29.529
.2953
295.3
mm HG
51.706
1.8683
22.4196
.073558
73.558
25.4 1 760
.75008
750.08
7.5008
7500.8
atm
.06805
.002458
.029499
.0000968
.0968
.03342
.001315
1
.000986
.98692
.009869
9.869
mbar
68.947
2.4908
29.8896
.098068
98.068
33.863
1.3332
1013.2
1
1000
10
10000
bar
.068947
.002490
.029889
.0000981
.098068
.033863
.001333
1.0132
.001 1 .01
10
kPa
6.8947
.24908
2.98896
.0098068
9.8068
3.3863
.13332
101.320
.1
100
1
1000
MPa
.006895
.000249
.002988
.0000098
.009807
.003386
.000133
.101320
.0001
.1
.001
1
The calibration report gives the pressure in certain engineering units. These can be converted to
other engineering units using the multiplication factors shown in Table 1.
m H20
"HG
mm
atm
mbar
bar
kPa
MPa
To
H20
psi
704.32 25.399 304.788 1 1000 345.32 13.595 10332 10.197 10197 101.97 101970
"H2O
'H2O
Table 1 - Engineering Units Multiplication Factors
mm H20
(Note: Due to changes in specific gravity with temperature, the factors for mercury and water in
the above table are approximate.)
In the special case where the Model 4580 is being used as a Barometer it is necessary to add the
atmospheric pressure at the time of the initial reading. This pressure would best be obtained
locally as an absolute value not corrected for height above sea level. If a local measurement
cannot be obtained then it is permissible to use the barometric pressure recorded on the
calibration report, see Appendix D.
For example:
Suppose the Linear gauge factor is 0.004586 kPa/digit, and the initial reading is 4908 at a
barometric pressure of 990.3 mbar which, converted to kPa equals 99.03 kPa. A subsequent
reading of the barometer equals 5024.
The new barometric pressure = 99.03 + (5024 - 4908) x (0.004586) = 99.56 kPa or 995.6 mbar.
However, where the barometer is being used as a means of correcting for barometric pressure
changes on nearby unvented vibrating wire piezometers then only the change of barometric
pressure is required which in the above case is + 0.53 kPa. This value must be subtracted to
correct for the barometric pressure change
Correction for Barometric pressure changes on unvented piezometers and pressure transducers is
explained in Section 4.3.
11
4.2 Temperature Correction
Careful selection of materials is made in constructing the vibrating wire pressure transducer to
minimize thermal effects, however, most units still have a slight temperature coefficient. Consult
the supplied calibration report (see Appendix D for a sample calibration report) to obtain the
coefficient for a given sensor.
Since piezometers are normally installed in a tranquil temperature environment, corrections are
not normally required. If that is not the case for a selected installation, corrections can be made
using the internal thermistor for temperature measurement. See Section 3.4 for instructions
regarding obtaining the sensor temperature.
The temperature correction equation is as follows:
P
temperature
Equation 3 - Temperature Correction
= (T1-T0) × K × F1
Where;
T1 represents the current temperature.
T0 represents the initial temperature.
K is the thermal coefficient from the supplied calibration report.
F1 is an optional multiplier to convert to engineering units (see Table 1).
The calculated correction would then be added to the Pressure calculated using Equation 2. If
the engineering units were converted remember to apply the same conversion to the calculated
temperature correction!
4.3 Barometric Correction
Since the unvented pressure transducer is hermetically sealed, it responds not only to water
pressure changes but also to changes in atmospheric pressure. That being the case, corrections
may be necessary. For example, ignoring a barometric pressure change from 29 to 31 inches of
mercury would result in ≈1 psi of error (or ≈2.3 feet if monitoring water level in a well).
The barometric correction equation is as follows:
P
barometric
Equation 4 - Barometric Correction
= (S1-S0) × F2
Where;
S1 represents the current barometric pressure.
S0 represents the initial barometric pressure.
F2 is a multiplier to convert the barometric pressure units (see Table 1) to the required units.
12
The calculated correction is subtracted from the pressure calculated using Equation 2. If the
engineering units were converted remember to apply the same conversion to the calculated
barometric correction!
The user should be aware that this correction scheme assumes ideal conditions. In reality,
conditions are rarely ideal. For example, if a well is sealed and not connected hydraulically to the
atmosphere, barometric effects on the water pressure can be attenuated. This will result in an
error due to applying an unnecessary or excessive correction. Geokon recommends, in these
cases, to independently record barometric pressure changes and correlate these with observed
pressure changes to arrive at a correction factor.
An alternative to measuring the barometric pressure and making barometric correction is to use
pressure transducers that are vented to the atmosphere. (as described in Section 4.3.1.) With
vented transducers, barometric pressure finds its way to both the inside and the outside of the
pressure capsule and is thus automatically canceled.
Equation 5 describes the pressure calculation with both temperature and barometric corrections
applied.
P
= (R1 - R0) × G + (T1-T0) × K - (S1-S0) × F2
corrected
Equation 5 - Corrected Pressure Calculation
For example, using the typical calibration report shown in Appendix D, showing a 7 kPa
Pressure Transducer (vented) used to measure changes of water level, ΔL mm:
R0 = 8168 digits
R1 =9660 digits
T0 = 15.1° C
T1 = 23.8° C
S0 = 29.3" Hg.
S1 = 30.8" Hg.
G = 0.001875 kPa/digit
K = -0.0007823 kPa/°C
F1 = 101.95 (to convert kPa to mm H2 O)
F2 = 345.44 (convert "Hg. to mm H20)
P
uncorrected
P
temperature
P
barometric
P
corrected
P
corrected
= (9660 -8168) × 0.001875 =2.798 kPa
= (23.8 - 15.1) × -0.0007823 =-0.0068 kPa
= 0 [this is a vented transducer]
= P
uncorrected
+ P
= 2.798 - 0.0068 -0 = 2.791 kPa
ΔL = +284.5 mm of water
temperature
- P
barometric
13
4.3.1 Vented Transducers
Vented transducers are designed to eliminate barometric effects. The space inside the
transducer is not hermetically sealed and evacuated (see Figure 1) but is connected via a
tube (integral with the cable) to the atmosphere. A chamber containing desiccant capsules
is attached to the end of the tube to prevent moisture from entered the transducer cavity.
As supplied, the outer end of the desiccant chamber is closed by means of a seal screw to
keep the desiccant fresh during storage and transportation. THE SEAL SCREW MUST
BE REMOVED BEFORE THE TRANSDUCER IS PUT INTO SERVICE! The
desiccant capsules are blue when fresh, they will gradually turn pink as they absorb
moisture. When they have turned light pink in color they should be replaced. Contact
Geokon for replacement capsules.
4.4 Environmental Factors
Since the purpose of the transducer installation is to monitor site conditions, factors that can
affect these conditions should always be observed and recorded. Seemingly minor effects can
have a real influence on the behavior of the structure being monitored and can give an early
indication of potential problems. Some of the factors include, but are not limited to; blasting,
rainfall, tidal levels, excavation and fill levels and sequences, traffic, temperature and barometric
changes (and other weather conditions), changes in personnel, nearby construction activities,
seasonal changes, etc.
14
5. MAINTENANCE
Vented transducers require periodic replacement of the desiccant capsules in the moisture trap.
Replace capsules when they change from blue to pink.
If the sensor is installed in water with high concentrations of silts etc. it may require cleaning
from time to time. This can be accomplished by gently flushing water through the transducer;
first through the P-1 port and then back through the seal screw hole. Repeat this several times
until the water passing through is clear. Remember not to apply excess pressure to the sensor.
15
6. TROUBLESHOOTING
Maintenance and troubleshooting of vibrating wire pressure transducers is confined to periodic
checks of cable connections and maintenance of terminals. The transducers themselves are
sealed and are not user serviceable. Gauges should not be opened in the field.
Should difficulties arise, consult the following list of problems and possible solutions. For
additional troubleshooting and support, contact Geokon.
Symptom: Transducer fails to give a reading
3) Is the cable cut or crushed? This can be checked with an ohmmeter. Nominal resistance
between the two gauge leads (usually red and black leads) is 180Ω, ±20Ω. For long cables a
correction can be applied, equal to approximately 14.7 Ω per one thousand feet (48.5Ω per
km). Multiply this factor by two to account for both directions.
. If the resistance reads infinite, or very high (megohms), a cut wire must be suspected. If the
resistance reads very low (<100Ω) a short in the cable is likely.
Does the readout or datalogger work with another transducer? If not, the readout or
datalogger may be malfunctioning. Consult the readout or datalogger manual for further
direction.
Has the transducer been overranged or shocked? Contact the factory for further direction.
Symptom: Transducer reading unstable
Is the readout box position set correctly? If using a datalogger to record readings
automatically are the swept frequency excitation settings correct? Try reading the transducer
on a different readout position. For instance, channel A of the GK-404 and GK-405 might be
able to read the sensor. To convert the Channel A period display to digits, use Equation 1 in
Section 4.1.
Is there a source of electrical noise nearby? Most probable sources of electrical noise are
motors, generators, transformers, arc welders and antennas. Make sure the shield drain wire
is connected to ground whether using a portable readout or datalogger. If using the GK-404
or GK-405 readout box connect the clip with the blue boot to the shield drain wire.
Does the readout work with another transducer? If not, the readout may have a low battery or
be malfunctioning. Consult the appropriate readout manual for charging or troubleshooting
directions.
Has the transducer been dropped or otherwise shocked? If so, it may experience a large zero
shift or be damaged.
Has the pressure chamber been de-aired? See Section 2.2.3 for instructions.
Has the moisture trap seal screw been removed?
Symptom: Thermistor resistance is too high
Is there an open circuit? Check all connections, terminals and plugs. If a cut is located in the
cable, splice according to instructions in Section 2.3.
Symptom: Thermistor resistance is too low.
Is there a short? Check all connections, terminals and plugs. If a short is located in the cable,
splice according to instructions in Section 2.3.
Water may have penetrated the interior of the transducer. There is no remedial action.
16
Model:
4580-1
4580-2
4580-3
4580-4
Available Ranges:¹
0-2.5 psi, 0-15 kPa
0-10 psi, 0-80 kPa
Resolution:
0.02% FSR
Accuracy:²
0.1% FSR
Linearity:³
±0.5% FSR
Over-Range:
2 × FSR
Thermal Coefficient:
<0.02% FSR/°C
Temperature Range:
-30 to +80° C
Frequency Range
1400-3500 Hz
Diameter:
63 mm
2.5"
38 mm
1.5"
63 mm
2.5"
100 mm
4.0"
Length:
110 mm
4.33"
172 mm
6.75"
172 mm
6.75"
112 mm
4.5"
Weight:
1.3 kg.
2.8 lbs.
1.5 kg.
3.3 lbs.
2.0 kg.
4.4 lbs.
4.0 kg.
9.0 lbs.
APPENDIX A. MODEL 4580 SPECIFICATIONS
A.1 Model 4580 Pressure Transducer
0-5 psi, 0-40 kPa
Table 2 - Model 4580 Pressure Transducer Specifications
Notes:
¹ Other ranges available upon request.
² The accuracy of the calibration equipment.
³ 0.1% FS linearity available upon request. Also 0.1% accuracy can be achieved by using a
second order polynomial instead of a linear calibration factor. If the second order polynomial is
used the pressure is calculated from the readings using the equation:
AR2 + BR + C = Pressure.
A.2 Thermistor (see Appendix B also)
Range: -80 to +150 °C
Accuracy: ±0.5 °C
17
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
201.1K
-50
16.60K
-10
2417
+30
525.4
+70
153.2
+110
187.3K
-49
15.72K
-9
2317
31
507.8
71
149.0
111
174.5K
-48
14.90K
-8
2221
32
490.9
72
145.0
112
162.7K
-47
14.12K
-7
2130
33
474.7
73
141.1
113
151.7K
-46
13.39K
-6
2042
34
459.0
74
137.2
114
141.6K
-45
12.70K
-5
1959
35
444.0
75
133.6
115
132.2K
-44
12.05K
-4
1880
36
429.5
76
130.0
116
123.5K
-43
11.44K
-3
1805
37
415.6
77
126.5
117
115.4K
-42
10.86K
-2
1733
38
402.2
78
123.2
118
107.9K
-41
10.31K
-1
1664
39
389.3
79
119.9
119
101.0K
-40
9796 0 1598
40
376.9
80
116.8
120
94.48K
-39
9310
+1
1535
41
364.9
81
113.8
121
88.46K
-38
8851 2 1475
42
353.4
82
110.8
122
82.87K
-37
8417 3 1418
43
342.2
83
107.9
123
77.66K
-36
8006 4 1363
44
331.5
84
105.2
124
72.81K
-35
7618 5 1310
45
321.2
85
102.5
125
68.30K
-34
7252 6 1260
46
311.3
86
99.9
126
64.09K
-33
6905 7 1212
47
301.7
87
97.3
127
60.17K
-32
6576 8 1167
48
292.4
88
94.9
128
56.51K
-31
6265 9 1123
49
283.5
89
92.5
129
53.10K
-30
5971
10
1081
50
274.9
90
90.2
130
49.91K
-29
5692
11
1040
51
266.6
91
87.9
131
46.94K
-28
5427
12
1002
52
258.6
92
85.7
132
44.16K
-27
5177
13
965.0
53
250.9
93
83.6
133
41.56K
-26
4939
14
929.6
54
243.4
94
81.6
134
39.13K
-25
4714
15
895.8
55
236.2
95
79.6
135
36.86K
-24
4500
16
863.3
56
229.3
96
77.6
136
34.73K
-23
4297
17
832.2
57
222.6
97
75.8
137
32.74K
-22
4105
18
802.3
58
216.1
98
73.9
138
30.87K
-21
3922
19
773.7
59
209.8
99
72.2
139
29.13K
-20
3748
20
746.3
60
203.8
100
70.4
140
27.49K
-19
3583
21
719.9
61
197.9
101
68.8
141
25.95K
-18
3426
22
694.7
62
192.2
102
67.1
142
24.51K
-17
3277
23
670.4
63
186.8
103
65.5
143
23.16K
-16
3135
24
647.1
64
181.5
104
64.0
144
21.89K
-15
3000
25
624.7
65
176.4
105
62.5
145
20.70K
-14
2872
26
603.3
66
171.4
106
61.1
146
19.58K
-13
2750
27
582.6
67
166.7
107
59.6
147
18.52K
-12
2633
28
562.8
68
162.0
108
58.3
148
17.53K
-11
2523
29
543.7
69
157.6
109
56.8
149
Table 3 - Thermistor Resistance Versus Temperature
55.6
150
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
1
T=
A+B(LnR)+C(LnR)
Equation 6 - Resistance to Temperature
Where;
T = Temperature in °C.
LnR = Natural Log of Thermistor Resistance
A = 1.4051 × 10-3
B = 2.369 × 10-4
C = 1.019 × 10-7
Note: Coefficients calculated over the −50 to +150° C. span.
-273.15 °C
3
18
APPENDIX C. USE OF SECOND ORDER POLYNOMIAL TO IMPROVE ACCURACY
Most calibrations use linear coefficient to calculate pressure, loads strains etc. from measured
readout “digits”. If the output of the sensor is not truly linear then there will be some inaccuracy
introduced into the calculated value. Thus, even though the calibration accuracy may be 0.1% FS
(because this is the accuracy of the calibration apparatus) yet the linearity may only be 0.5% F.S.
so that the calculated value may differ from the true value by this larger amount.
To recapture the 0.1% FS accuracy inherent in the calibration data it makes sense to use a second
order polynomial to fit the calculated values to the actual calibration curve. The second order
polynomial has the following characteristics. Calculated value = AR2 + BR + C where R is the
observed reading on the readout box and A, B, C, are the three coefficients shown on the
calibration report. Note that, unlike the linear method, this equation does not contain the item
(R1 – R0) and thus requires a different treatment for establishing zero conditions. The coefficients
shown on the calibration report are those developed under the conditions of temperature and
barometric pressure experienced at the time of the calibration and which are shown on the
calibration report. Theoretically, these could be used as is, but it is always good practice to
establish and use zero conditions at the site since the zero may have shifted slightly due to rough
handling during shipment and/or installation; and hence a slight adjustment of the “C”
coefficient may be called for. Therefor it is recommended that the value of C be determined at
the site for the conditions of temperature and barometric pressure experienced at that time.
For example, using the data from the sample calibration report shown in Appendix D:
At the site, at a temperature of T0 = 30°C and a barometric pressure of 1000 mbar, the observed
zero pressure reading on a GK-404 readout (channel B) is R1 =7420. Using the polynomial
coefficient to calculate a new value of C.
(7420)2 x 3.303E-09 + (7420) x 0.001813 + C = O
gives C = -13.634
Note that the transducer shown is of a vented type, and hence no barometric correction is
required. However, it is essential for vented types, that the moisture trap seal screw be removed
before taking the zero reading (or any reading) so that the vent line is open to the atmosphere.
19
APPENDIX D. TYPICAL CALIBRATION REPORT
Figure 7 - Sample Model 4580 Calibration Report
Loading...