Geokon 3400 Series User manual

Instruction Manual
Model 3400 series
Semiconductor Piezometer
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon, Inc.
The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for
errors, omissions, or misinterpretation. The information herein is subject to change without notification.
Copyright © 1983-2018 by Geokon, Inc.
(REV H, 02/12/2018)

Warranty Statement
Geokon, Inc. 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, Inc. 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, Inc. or any breach of any warranty by Geokon, Inc. shall not exceed the
purchase price paid by the purchaser to Geokon, Inc. 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, Inc. 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. PRELIMINARY TESTS.........................................................................................................................................2
3. SATURATING FILTER TIPS...............................................................................................................................3
3.1 SATURATING LOW AIR ENTRY (STANDARD)FILTERS ..........................................................................................3
3.2 SATURATING HIGH AIR ENTRY CERAMIC FILTERS...............................................................................................3
3.2.1 One Bar Filters.............................................................................................................................................3
3.2.2 Two Bar and Higher Filters .........................................................................................................................4
3.3 MODEL 3400DP ...................................................................................................................................................4
4. INSTALLATION ....................................................................................................................................................5
4.1 ESTABLISHING A ZERO PRESSURE READING.........................................................................................................5
4.2 INSTALLATION IN BOREHOLES..............................................................................................................................5
4.3 INSTALLATION IN FILLS AND EMBANKMENTS ......................................................................................................7
4.4 INSTALLATION BY PUSHING OR DRIVING INTO SOFT SOILS..................................................................................9
4.5 INSTALLATION IN STANDPIPES OR WELLS ..........................................................................................................10
4.6 MODEL 3400H TRANSDUCER .............................................................................................................................11
4.7 SPLICING AND JUNCTION BOXES ........................................................................................................................11
4.8 ELECTRICAL NOISE.............................................................................................................................................12
4.9 FREEZING PROTECTION ......................................................................................................................................12
4.10 LIGHTNING PROTECTION ..................................................................................................................................12
5. READOUT PROCEDURES.................................................................................................................................14
5.1 INITIAL READINGS..............................................................................................................................................14
5.2 INPUT VOLTAGE .................................................................................................................................................14
5.3 CONVERTING TO PRESSURES ..............................................................................................................................14
5.4 MEASURING TEMPERATURES .............................................................................................................................14
5.5 CALIBRATION .....................................................................................................................................................15
6. DATA REDUCTION ............................................................................................................................................18
6.1 PRESSURE CALCULATION ...................................................................................................................................18
6.2 TEMPERATURE CORRECTION..............................................................................................................................19
6.3 BAROMETRIC CORRECTIONS ..............................................................................................................................19
7. TROUBLESHOOTING........................................................................................................................................19
APPENDIX A. SPECIFICATIONS.........................................................................................................................20
A.1 3400 SERIES SPECIFICATIONS ............................................................................................................................20
A.2 THERMISTOR (SEE APPENDIX B. ALSO) .............................................................................................................21
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION.....................................................................22
APPENDIX C. WIRING CHARTS.........................................................................................................................23
C.1 MILLIVOLTS PER VOLT OUTPUT ........................................................................................................................23
C.2 ZERO TO FIVE VOLT DC OUTPUT ......................................................................................................................23
C.3 FOUR TO 20 MILLIAMP OUTPUT.........................................................................................................................23

FIGURES
FIGURE 1-MODEL 3400 PIEZOMETER ASSEMBLY......................................................................................................... 1
FIGURE 2-TYPICAL BOREHOLE INSTALLATIONS .......................................................................................................... 6
FIGURE 3-HIGH AIR ENTRY FILTER ............................................................................................................................. 8
FIGURE 4-LOW AIR ENTRY FILTERS ONLY................................................................................................................. 8
FIGURE 5-TYPICAL SOFT SOILS INSTALLATION ........................................................................................................... 9
FIGURE 6-TYPICAL LEVEL MONITORING INSTALLATION ............................................................................................10
FIGURE 7-TYPICAL MULTI-PIEZOMETER INSTALLATION ............................................................................................11
FIGURE 8-RECOMMENDED LIGHTNING PROTECTION SCHEME ....................................................................................13
FIGURE 9-TYPICAL CALIBRATION REPORT FOR MODEL 3400-1 WITH 100MVOUTPUT ..............................................15
FIGURE 10 -TYPICAL CALIBRATION REPORT FOR MODEL 3400-2 WITH 0TO 5VOLT OUTPUT ....................................16
FIGURE 11 -TYPICAL CALIBRATION REPORT FOR MODEL 3400-3 WITH 4TO 20MAOUTPUT ......................................17
TABLES
TABLE 1-CEMENT/BENTONITE/WATER RATIOS........................................................................................................... 7
TABLE 2-ENGINEERING UNITS MULTIPLICATION FACTORS ........................................................................................18
TABLE 3-MODEL 3400 SPECIFICATIONS......................................................................................................................20
TABLE 4-OUTPUT UNITS SPECIFICATIONS ..................................................................................................................21
TABLE 5-THERMISTOR RESISTANCE VERSUS TEMPERATURE .....................................................................................22
TABLE 6-MV/V OUTPUT WIRING................................................................................................................................23
TABLE 7-0-5VDC OUTPUT WIRING............................................................................................................................23
TABLE 8-4-20MAOUTPUT WIRING ............................................................................................................................23
EQUATIONS
EQUATION 1-CONVERT DIGITS TO PRESSURE .............................................................................................................18
EQUATION 2-RESISTANCE TO TEMPERATURE .............................................................................................................22

1
1. THEORY OF OPERATION
Geokon Model 3400 Piezometers are intended for dynamic measurements of fluid and/or pore
water pressures in standpipes, boreholes, embankments, pipelines, pressure vessels, reservoirs,
etc. They are also used for static pressure movement where the readout system is incompatible
with vibrating wire type transducers. The piezometer assembly is shown in Figure 1.
Figure 1 - Model 3400 Piezometer Assembly
The basic pressure transducer is semiconductor based. The output from the transducer may be
100 mV/volt, 0 to 5 volts, or 4 to 20 mA at the option of the user. The transducer is packaged
inside a 1 1/4" (32 mm) diameter stainless steel tube (standard 304 SS or optional 316 SS for
aggressive environments). At one end of this tube is a filter housing to allow the passage of water
while preventing the entry of soil particles. At the other end is located a bulkhead seal and cable
entry seal to prevent water from reaching the backside of the transducer. A thermistor included
inside the main housing allows the measurement of temperature.
The output cable is multi conductor with from two to four shielded pairs depending on the
transducer output. Voltage types are generally read using remote sensing techniques. Low-
pressure models may also be vented to the atmosphere through a vent tube inside the cable.
Venting of the transducer is necessary if the effects of barometric pressure changes on the
transducer are to be eliminated.
Where venting is used, the outer end of the vent tube is connected to a desiccant chamber to
prevent moisture from migrating to the transducer interior.

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2. PRELIMINARY TESTS
•Upon receipt of the piezometer, connect it to the readout using the wiring charts shown in
Appendix C, and check that the zero pressure reading is within 1% F.S. of the value shown
on the calibration report after due correction for barometric pressure, elevation above sea
level and temperature.
•Apply a pressure or vacuum to the piezometer and check that the readout response is
reasonable.
•Check the insulation resistance. Use an ohmmeter to measure the resistance between any
conductor and the shield. The resistance should be greater than 50 megohms.
•If an attempt is made to check the calibration, make sure that the applied pressure is accurate.
Be aware that calibrations performed by raising and lowering the piezometer inside a
borehole or well can be compromised by a displacement of the water level caused by
changing volumes of immersed cable.
Calibrations performed in this way should be done with the filter housing removed. If the filter is
left in place, be sure that it is completely saturated and that the space between filter and
transducer is filled with water. Be sure to allow sufficient time (15 to 20 minutes) for the
piezometer to reach thermal equilibrium before beginning the test.

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3. SATURATING FILTER TIPS
Warning! Do not allow the piezometer to freeze once the filter stone has been saturated!
See Section 4.9 for information about protecting the piezometer from freezing.
Most filter tips can be removed for saturation and then reassembled. To maintain saturation, the
unit should be kept underwater until installation. If the piezometer is used in a standpipe where it
will be raised and lowered frequently, the filter housing may loosen over time, and a permanent
filter assembly may be required. The removable filter may be fixed permanently by prick
punching the piezometer tube approximately 1/16" to 1/8" behind the filter assembly joint.
Salts in the water can be deposited into the filter stone causing it to become clogged if it is
allowed to dry out completely. Filter stones may be replaced with screens for standpipe
installations. Screens available from Geokon are less likely than standard filters to collect salt
and become clogged.
3.1 Saturating Low Air Entry (Standard) Filters
For accurate results, total saturation of the filter is necessary. As the piezometer is lowered into
the water, water is forced into the filter, compressing the air in the space between the filter stone
and the pressure sensitive diaphragm. After a period of time, this air will dissolve into the water,
filling the filter and the space above it entirely with water.
To speed up the saturation process, remove the filter from the piezometer by carefully twisting
and pulling on the filter housing assembly (or unscrewing the point of the piezometer for model
3400DP). Hold the piezometer with the filter facing up and fill the space above the diaphragm
with water. Slowly replace the filter housing, allowing the water to squeeze through the filter
stone as it is installed. For piezometers with a range of less than 10 psi, take readings with a
readout box while reinstalling the filter housing to ensure the piezometer is not overranged.
3.2 Saturating High Air Entry Ceramic Filters
Because of the high air entry characteristics of the ceramic filter, de-airing is particularly
important. Different air entry values require different saturation procedures.
3.2.1 One Bar Filters
1) Remove the filter from the piezometer by carefully twisting and pulling on the filter
housing assembly.
2) Boil the filter assembly in de-aired water.
3) Reassemble the piezometer under the surface of a container of de-aired water. Use a
readout box while installing the filter to monitor the diaphragm pressure. If the
piezometer begins to overrange, allow the pressure to dissipate before pushing further.
4) Be sure that no air is trapped in the transducer cavity.

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3.2.2 Two Bar and Higher Filters
The proper procedure for de-airing and saturating these filters is somewhat complex;
therefore, it is recommended that saturation be done at the factory by Geokon. If
saturation must be done in the field, carefully follow the instructions below:
1) Place the assembled piezometer, filter down, in a vacuum chamber that has an inlet
port at the bottom for de-aired water.
2) Close off the water inlet and evacuate the chamber. The transducer should be
monitored while the chamber is being evacuated.
3) When maximum vacuum has been achieved, allow de-aired water to enter the
chamber until it reaches an elevation a few inches above the piezometer filter.
4) Close off the inlet port.
5) Release the vacuum.
6) Observe the transducer output. It may take up to 24 hours for the filter to completely
saturate and the pressure to rise to zero.
7) After saturation, the transducer should be kept in a container of de-aired water until
installation. If de-aired at the factory a special cap is applied to the piezometer to
maintain saturation.
3.3 Model 3400DP
The 3400DP Drive Point Piezometer is de-aired in the same way as the 3400 models by first
unscrewing the point of the piezometer assembly and then following the instruction for the 3400.

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4. INSTALLATION
Before attempting an installation be sure that the filter stone is completely saturated (see Section
3) and that the space between the filter stone and the transducer diaphragm is filled with water.
Warning! Do not allow the piezometer to freeze once the filter stone has been saturated!
4.1 Establishing a Zero Pressure Reading
It is essential, in many cases, to establish an accurate zero pressure reading at the job site under
known conditions of barometric pressure and temperature. The following procedures are
important.
1) Either remove the filter housing completely (preferred) or make sure that the filter stone is
saturated and that the space between the filter and transducer diaphragm is completely filled
with water.
2) Lower the piezometer into the borehole or well until it is just above the water level.
3) Allow 15 to 20 minutes for the temperature to stabilize before taking the reading.
4.2 Installation in Boreholes
Geokon piezometers can be installed in cased or uncased boreholes, in either single or multiple
piezometer configurations. If pore pressures in a particular zone are to be monitored, careful
attention must be paid to the borehole sealing technique.
The borehole should extend 6 to 12 inches below the proposed piezometer location. Boreholes
should be drilled without using drilling mud, or by using a material that degrades rapidly with
time, such as Revert. Wash the borehole clean of drill cuttings. Backfill the borehole with
clean fine sand to a point six inches below the desired piezometer tip location. The piezometer
can then be lowered into position. (Preferably, the piezometer will be encapsulated in a canvas
bag containing clean, saturated sand.) While holding the instrument in position, (a mark on the
cable is helpful) fill the borehole with clean fine sand to a point six inches above the piezometer.
Three different methods of isolating the zone to be monitored are detailed below.
Installation A:
Immediately above the area filled with clean fine sand, known as the “collection zone”, the
borehole should be sealed by an impermeable bentonite cement grout mix, or with alternating
layers of bentonite and sand backfill, tamped in place for approximately one foot, followed by
common backfill. (See Figure 2.)
If multiple piezometers are to be used in a single hole, the bentonite and sand should be tamped
in place below and above the upper piezometers, as well as at interval between the piezometer
zones. When using tamping tools special care should be taken to ensure that the piezometer cable
jackets are not cut during installation, as this could introduce a possible pressure leak in the
cable.

6
Installation B:
The borehole is filled from the “collection zone” upwards with an impermeable bentonite grout.
(See Figure 2.)
Figure 2 - Typical Borehole Installations
Installation C:
It should be noted that since the piezometer is essentially a no flow instrument, collection zones
of appreciable size are not required. The piezometer can be placed directly in contact with most
materials, provided that the fines are not able to migrate through the filter. The latest thinking is
that it is not necessary to provide sand zones and that the piezometer can be grouted directly into
the borehole using a bentonite cement grout only. However, good results have been obtained by
placing the piezometer inside a canvas bag filled with sand before grouting.
The general rule for installing piezometers in this way is to use a bentonite grout that mimics the
strength of the surrounding soil. The emphasis should be on controlling the water to cement
ratio. This is accomplished by mixing the cement with the water first. The most effective way of
mixing the two substances is to use a drill rig pump to circulate the mix in a 50 to 200 gallon
barrel or tub.
Any kind of bentonite powder, combined with Type I or Type II Portland cement can be used to
make drilling mud. The exact amount of bentonite needed will vary somewhat. Table 1 shows
two possible mixes for strengths of 50 psi and 4 psi.

7
50 PSI Grout for Medium
to Hard Soils
4 PSI Grout for Soft Soils
Amount
Ratio by
Weight
Amount
Ratio by
Weight
Water 30 gallons 2.5 75 gallons 6.6
Portland
Cement
94 lb. (one sack) 1 94 lb. (one sack) 1
Bentonite 25 lb. (as required) 0.3 39 lb. (as required) 0.4
Note:
The 28-day compressive strength of this mix
is about 50 psi, similar to very stiff to hard
clay. The modulus is about 10,000 psi
The 28 day strength of this
mix is about four psi,
similar to very soft clay.
Table 1 - Cement/Bentonite/Water Ratios
Add the measured amount of clean water to the barrel then gradually add the cement in the
correct weight ratio. Slowly add the bentonite powder so that clumps do not form. Keep adding
bentonite until the watery mix turns to an oily/slimy consistency. Let the grout thicken for 5 to
10 minutes. Add more bentonite as required until it is a smooth, thick cream, similar to pancake
batter. It is now as heavy as it is feasible to pump.
When pumping grout (unless the tremie pipe is to be left in place,) withdraw the tremie pipe after
each batch, by an amount corresponding to the grout level in the borehole.
CAUTION! If the grout is pumped into the hole, rather than tremie piped, there is a
danger that the piezometer will be overranged and damaged. Pumping directly into the
bottom of the borehole should be avoided. It is good practice to read the piezometer while
pumping.
For more details on grouting, refer to “Piezometers in Fully Grouted Boreholes” by Mikkelson
and Green, FMGM proceedings Oslo 2003. Copies are available from Geokon.
4.3 Installation in Fills and Embankments
Geokon piezometers are normally supplied with direct burial cable suitable for placement in fills
such as highway embankments and dams, both in the core and in the surrounding materials.
For installations in non-cohesive fill materials, the piezometer may be placed directly in the fill,
or, if large aggregate sizes are present, in a saturated sand pocket in the fill. If installed in large
aggregate, additional measures may be necessary to protect the cable from damage.
In fills such as impervious dam cores, where subatmospheric pore water pressure may need to be
measured, (as opposed to the pore air pressure,) a ceramic tip with a high air entry value is often
used. This type of filter should be carefully placed in direct contact with the compacted fill
material. (See Figure 3).
Cables are normally installed inside shallow trenches with the fill material consisting of smaller
size aggregate. This fill is carefully hand compacted around the cable. Bentonite plugs are placed
at regular intervals to prevent migration of water along the cable path. In high traffic areas and in
materials that exhibit pronounced “weaving”, heavy-duty armored cable should be used.

8
Figure 3 - High Air Entry Filter
In partially saturated fills (if only the pore air pressure is to be measured,) the standard tip is
satisfactory. It should be noted that the standard coarse tip (low air entry) measures the air
pressure when there is a difference between the pore air pressure and the pore water pressure.
The difference between these two pressures is due to the capillary suction in the soil. The
consensus is that the difference is normally of no consequence to embankment stability.
The coarse tip filter is suitable for most routine measurements. Both the installation shown in
Figure 3 and the installation shown in Figure 4 may be used with the standard piezometer filter.
Figure 4 - Low Air Entry Filters ONLY

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4.4 Installation by Pushing or Driving into Soft Soils
The Model 3400DP piezometer is designed to be pushed into soft soils. In soft soils, it can be
difficult to keep a borehole open. The 3400DP may eliminate the need for a borehole altogether.
The unit is connected directly to the drill rod (AW, EW, or other) and pressed into the ground,
either by hand or by means of the hydraulics on the rig. (See Figure 5.) The units can also be
driven into the soil, but there is a possibility that the driving forces may shift the zero reading.
The ground conditions need to be relatively soft for the 3400DP to be effective. Soft soils (like
clays or silts) with SPT blow counts under 10 are ideal. In stiffer soils, it is possible to drill a
hole and then push the 3400DP only a few feet below the bottom of the hole, but if the soil is too
stiff, the sensor may overrange or break.
The piezometer should be connected to a readout box and monitored during the installation
process. If pressures reach or exceed the calibrated range, the installation should be stopped.
Allow the pressure to dissipate before continuing.
The drill rod can be left in place or it can be removed. If it is to be removed, a special five-foot
section of EW (or AW) rod with reaction wings and a left hand thread are attached directly to the
piezometer tip. This section is detached from the rest of the drill string by rotating the string
clockwise. The reaction wings prevent the EW rod from turning. A LH/RH adapter is available
from Geokon. This adapter is retrieved along with the drill string.
Figure 5 - Typical Soft Soils Installation

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4.5 Installation in Standpipes or Wells
1) Saturate the filter stone (see Section 3) and establish a zero pressure reading (see Section
4.1). (Warning! Do not allow the piezometer to freeze once the filter stone has been
saturated!)
2) Mark the cable where the top of the well or standpipe will reside once the piezometer has
reached the desired depth. (The piezometer diaphragm is located 3/4 of an inch above the tip
of the piezometer.)
3) Lower the piezometer into the standpipe/well.
4) Be sure the cable is securely fastened to prevent the piezometer from sliding further into the
well and causing an error in the readings.
Figure 6 - Typical Level Monitoring Installation
It is not recommended that piezometers be installed in wells or standpipes where an electrical
pump or cable is nearby. Electrical interference from these sources can cause unstable readings.
If unavoidable, it is recommended that the piezometer be placed inside a piece of steel pipe. In
situations where packers are used in standpipes, special care should be taken to avoid cutting the
cable jacket with the packer, as this could introduce a possible pressure leak in the cable.

11
4.6 Model 3400H Transducer
When connecting the Model 3400H transducer to external fittings, the fitting should be tightened
into the 1/4"-NPT thread with a wrench on the flats provided on the transducer housing. Also,
avoid tightening onto a closed system since the process of tightening the fittings could overrange
and permanently damage the transducer. If in doubt, attach the gage leads to the readout box and
take readings while tightening. Teflon tape on the threads makes for easier and more positive
connection to the transducer.
4.7 Splicing and Junction Boxes
Cable splicing should be kept to a minimum since changes in cable resistance can cause changes
in calibration if remote sensing techniques or 4-20 mA output are not in use.
The Model 3400 utilizes a semiconductor transducer and, as such, has low-level output signals.
If cables are damaged or improperly spliced, the outputs can be seriously degraded.
Therefore, it is absolutely necessary to provide a high degree of cable protection. If cables
must be spliced, only recognized high quality techniques should be used. The splice should
be waterproofed completely. Geokon recommends the use of 3M Scotchcast model 82-A1; these
kits are available from the factory.
Figure 7 - Typical Multi-Piezometer Installation

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The cable used for making splices should be a high quality twisted pair type with 100% shielding
(with 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 placed around the splice
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 readout equipment and datalogging hardware are available. See Figure 7 for
examples. Contact Geokon for specific application information.
4.8 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 with AC power lines. The instrument cables
will pick up the 50 or 60 Hz (or other frequency) noise from the power cable and this will likely
cause a problem obtaining a stable reading. Contact the factory concerning filtering options
available for use with the Geokon dataloggers and readouts should difficulties arise.
4.9 Freezing Protection
If the water around the piezometer freezes this could damage the piezometer diaphragm causing
a large shift in the zero pressure reading. If the piezometer is to be used in locations that are
subject to freezing, Geokon can provide a special modification that will protect the piezometer
diaphragm.
4.10 Lightning Protection
In exposed locations, it is vital that the piezometer be protected against lightning strikes.
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 thereby protecting
the instrument.
Terminal boxes available from Geokon can be ordered with lightning protection built in. There
are two levels of protection:
•The terminal board used to make the gage connections has provision for installation of
plasma surge arrestors.
•Lightning Arrestor Boards (LAB-3) can be incorporated into the terminal box. These units
utilize surge arrestors and transzorbs to further protect the piezometer.
In the above cases, the terminal box would be connected to an earth ground.

13
Improved protection using the LAB-3 can be had by placing the board in line with the cable as
close as possible to the installed piezometer (see Figure 8). This is the recommended method of
lightning protection.
Piezometer
Piezometer Cable
To Terminal Box/Readout Equipment
Lightning Arrestor Board (LAB-3)
(in special enclosure
Ground Stake
Ground Connection
accessible from surface)
Figure 8 - Recommended Lightning Protection Scheme

14
5. READOUT PROCEDURES
Connect the piezometer to the readout instrument using the appropriate wiring chart given in
Appendix C.
5.1 Initial Readings
Initial readings must be taken and carefully recorded along with the barometric pressure
and temperature at the time of installation. Follow the instructions of Section 4.1.
5.2 Input Voltage
The Model 3400 Piezometer uses a semiconductor strain gage type transducer with an output of
either 0-100mV (Model 3400-1), 0-5 volts (Model 3400-2), or 4-20 mA (Model 3400-3).
For the 100mV type, the output voltage is directly proportioned to both pressure and input
voltage, therefore it is very important that the input voltage be accurately controlled @ 10V DC.
If any other voltage is used, the gage factor G must be adjusted accordingly in the manner shown
on the calibration report. The 0-5 volt and 4-20mA sensors require an unregulated input of 7-35
VDC.
5.3 Converting to Pressures
Formulae for converting readout voltages to pressure are shown on the calibration reports. Both
linear and polynomial expressions are shown. For better accuracy, the polynomial expression
should be used with a proviso that the value for the C coefficient be derived in the field by taking
an initial reading when the sensor is subject to atmospheric pressures only as described in
Section 4.1. Then substituting this initial value into the formula and setting the value of P to zero
will yield the correct value for C.
5.4 Measuring Temperatures
Each piezometer is equipped with a thermistor for reading temperature. The thermistor gives a
varying resistance output as the temperature changes. Appendix C shows which cable conductors
are connected to the thermistor. These conductors should be connected to a digital ohmmeter.
To read temperatures using an ohmmeter:
1) Connect an ohmmeter to the green and white thermistor leads coming from the strain gage.
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 5.