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Piezometer FAQ

Question: I've read about piezometers in fully grouted boreholes. Can you tell me more about that?

Answer: The grout-in method is a faster and easier way to install piezometers. It eliminates the conventional sand filter and the bentonite seal. Instead, the entire borehole is filled with a non-shinking, low permeability grout.

Visit the References and Papers page to learn about the grout-in method.

Download the VW Piezometer Manual and Multi-Level VW Piezometer manuals for instructions.

Question: What grout mix do you suggest for standpipe piezometers?

Answer: The rule of thumb is that the grout should have a lower permeability than the surrounding soil so that water does not migrate through the grout to the intake area of your standpipe piezometer. The standard practice is to place a bentonite seal above the sand intake zone and then place grout above the seal. Any lag time between changes in pore-water pressure in the ground and changes in the water level in your standpipe is really from the volume of your standpipe and the permeability of the surrounding ground. The grout itself has little influence so long as it prevents migration of water from above. We sometimes recommend the same grout mixes that we use for vw piezometers.

You might also be interested in a technote on grouting-in diaphragm-type piezometers (eliminating the sand intake zone altogether). One of the references listed at the bottom is Dr Vaughn, who reports results from his experiments in grouting in standpipe piezometers. .

Question: How do I saturate piezometer filters?

Answer: There are two types of piezometers in common use today: standpipe piezometers and diaphragm piezometers (VW or pneumatic). There is no need to saturate standpipe filter tips. Water flows into them easily. VW and pneumatic piezometers do not require saturation either, but there is some air in front of the diaphragm that should be displaced, as explained below:

Diaphragm piezometers, whether VW or pneumatic, contain air between the diaphragm and the filter. For best results, you should displace this air with water. VW piezometers have a removable filter. Pull on the knurled ring to remove the filter. Fill the cavity with water and then replace the filter. Pneumatic piezometers don't have a removable filter. In this case, you simply direct a slow stream of water into the piezometer or submerge it in a bucket of water and tap the bubbles out.

Some people install the VW piezometer with its filter end up, and our instructions for grouting-in piezometers use this technique. This generally isn't possible with a pneumatic piezo, since tubing is so stiff. In any case, not much water will drain out of the piezometer so long as it isn't knocked about during installation.

What happens if there is a small bubble of air? The air will slow the response of the piezo, since the air bubble must deform before it can transmit the pressure of the water. More water must flow into the piezometer to make that happen. Eventually, the air bubble reaches equilibrium and transmits the full pressure of the water. If there is plenty of water available, the slowed response is unlikely to cause much of a problem, and over time the air bubble will disappear. If the piezometer is installed in a low-permeability soil, where less water is available, this process will take longer.

Question: What's the difference between high-air and low-air entry filters? Sometimes high-air entry filters are written into specifications for vibrating wire piezometers. What is the purpose of these filters? Do they alter the performance of the piezometer?

Answer: High air entry filters, are generally not appropriate for standpipe or diaphragm piezometers. The high air entry filter relies on the surface tension of water in its pores to sustain a pressure difference between air and water on the filter surface. This keeps air out of the measuring system and allows measurement of matrix soil suction (negative pore-water pressure) that is present in non-saturated, cohesive soils, such as clays in embankments. The high air entry effect is operative only when the filter is saturated with water. When water drains out of the filter, the high-air entry effect disappears.

Generally speaking, only one type of piezometer, the hydraulic piezometer, is capable of maintaining saturation of the filter in non-saturated soils. Diaphragm piezometers and standpipe piezometers do not normally have this capability, and therefore should not be specified with high air entry filters. In addition, most diaphragm piezometers are not calibrated to read high negative pressures and thus, even if the filter properties were intact, would not be able to read those pressure.

Question: Your manual used to contain instructions on saturating a high-air entry filter. How is this done?

Answer: A high-air entry filter can be saturated as follows: Remove the filter. Saturate it in deaired water. Replace the filter while holding the piezometer underwater. Bag the piezometer in deaired water to maintain saturation until installation.

To prepare deaired water, boil the water and apply a vacuum or use a device such as a Nold DeAerator, which combines propeller cavitation with a vacuum to deair the water rapidly.

To saturate the filter, place stoppers in the top and bottom of the filter. Place a vent tube through the stopper at the top of the filter. Immerse the filter in deaired water. Water flowing through the filter will displace the air inside the filter, which flows out the vent tube. Allow the filter to remain in the deaired water for 24 hours.

Question: Push-In Piezometers - Will filters get clogged over time? We intend to drive the push-in VW tips directly into soft marine muds prior to placing fill. These piezometers are required to function fully over the six year project life. Is there serious potential for filter clogging and loss of performance in such an environment? Would a sand filter zone provide only minor additional protection?

Answer: If the soil is saturated and the piezometer is saturated, there should be no problem with clogging because there is no place for the mud to go. Clogging over time implies some kind of flow, but water does not actually flow through the filter. The piezometer measures static hydraulic pressure.

Question: What is the maximum safe tip resistance for push-in piezometers? What is the maximum tip resistance that can safely be applied to the push-in vibrating wire piezometer during installation? We would like to install several push-in VW piezometers at depths up to 80 feet. During CPT testing at the site, strata with a tip resistance of 30 mpa were encountered. Are these stiffer soils more than the VW piezometer can handle?

Answer: The Push-In Piezometer was designed to be used in very soft soils, especially soils whose structure or hydraulic characteristics would be disturb by drilling. Normal installation procedures for the push-in piezo are to drill within 2 to 5 feet of the target zone and then push the piezometer to the required elevation. Once the push rod is removed, the soil usually collapses and seals the boring. If it does not, you should fill the boring with a bentonite cement grout.

You can use the cone penetrometer rods to push the piezometer inside an existing cone penetrometer hole. The piezometer does not have the same strength as a cone penetrometer and it will be destroyed if
pushed into medium to hard soils. When pushing the piezometer, you must take piezometer readings to be sure that the pore water pressures created when pushing the cone do not exceed the pressure rating of the piezometer, and you will probably have to push very slowly and stop often to let pressures dissipate before continuing.

Question: Do I need to apply corrections for changes in atmospheric (barometric) pressure?

Answer: The VW piezometer is a sealed unit that is sensitive to any pressure on its diaphragm. It does not distinguish between pressure of the atmosphere and pressure of the water. It responds to both. However, whether this is important or not depends on your application.

Suppose you suspend the piezometer in a standpipe or water well that is open to atmosphere. The piezometer will report the combined pressure of the water and the atmosphere above the water. If your intention is to monitor the level of water in the well, you must correct for variations in atmospheric pressure.

Now suppose you seal the piezometer in a borehole to monitor pore-water pressure. In this case, the pressure acting on the diaphragm is only the water pressure at that depth. Thus you would probably not correct for variations in atmospheric pressure, even if you later found a relation between atmospheric pressure and pore-water pressure.

You can find more about this subject and how to use barometer readings to make corrections in the VW piezometer manual.

Question: I have a Geokon readout. How do I convert the units that it displays to values in Hz?

Answer: To convert the Geokon readout value to Hz: sqrt (reading x 1000). This give you a reading in Hz. Then you can apply the calibration factors normally.

Question: How much are water level readings affected by changes in atmospheric pressure? My piezometer is installed in a standpipe that is open to atmosphere.

Answer: Atmospheric pressure may vary as much as 34 millibars (0.5 psi) during the day. This is equivalent to apparent water level changes of ±150 mm ( ±6 inches). Even larger variations can occur during stormy weather.

Question: I'm having trouble getting readings. What tests can I perform?

Answer: The tests below can be performed with a handheld multimeter.

If there is no reading: Set your handheld multimeter to a low range (less than 5k ohm).

  • Measure the resistance between the two VW wires (orange and white-and-orange). A normal reading should be about 300 ohms. If the reading is very high or infinite, the coil is probably burned out (typically from lightning damage). If the reading is very low, the cable is probably damaged.
  • Measure the resistance between the temperature sensor wires (blue and white). Thermistors should read about 3000 ohms. RTDs should reading about 2000 ohms. If the reading is very high or infinite, the temperature device is burned out. If the reading is very low, the cable is probably damaged.

If the reading is unstable: Set your handheld multimeter to a high range (10 or 20 M ohm).

  • Measure the resistance between a VW wire and a Temp wire. The reading should very high or infinite.
  • Measure the resistance between any of the colored wires and the drain (shield) wire. The reading should be very high or infinite.
  • Measure the resistance between the shield wires of two VW sensors. The reading should be very high or infinite. A lower reading indicates the presence of a ground loop. Check that each instrument is isolated from the others when it is read. This may require rewiring your multiplexer. Contact the factory for suggestions.
  • Some other sources of trouble may be electrical noise from nearby power lines or motors. A over-ranged or shocked instrument may also exhibit these problems.

Question: What lightning protection is built into the VW piezometer?

Answer: We have wired a zener diode in parallel with the coil (which is used to pluck the wire). The diode is rated for 1.5 KVA peak pulse, with conduction starting at 10 volts.

Question: What is the minimum radius bend for piezometer signal cable? Where the cable exits the borehole, we're planning to route it into PVC pipe.

Answer: The standard PVC sweep used by electricians in the construction industry should be fine. These typically have a 6-inch radius. The problem is not the minimum radius itself, but the friction in pulling cable through the bend. Also, it is a good idea to allow a little slack cable for settlement and other movements.

Question: What effect does cable length have on the thermistor reading?

Answer: The AWG 22 copper wire in signal cable for piezometers has a nominal resistance of 16 ohms per 1000 feet. The round trip of the wire-pair to and from the thermistor effectively doubles this value to 32 ohms.

The thermistor used in our sensors, as well as in those of most other instrument manufacturers, provides a resistance of 3,000 ohms at 25 degrees C. The thermistor is non-linear, however. (See manufacturer's chart).

At 25 degrees C, 32 ohms represents about 0.24 degree C, and at 10 degrees C, 32 ohms represents about 0.11 degree C. In other words, the effect of cable length has less effect as the temperature decreases. In either case, though, correction for cable length is probably not needed.

Question: If we cut off cable, do we need to adjust the RTD offset? After installing some VW instruments, we had to cut the cables to connect to the data logger. Do we need to adjust the RTD offset, and if so, how can we do this in the field with all the instruments installed?

Answer: For many, this question is not relevant. Our VW piezometers are almost always shipped with thermistors rather than RTDs.

For those with older VW piezometers: T he VW sensor calibration record lists an RTD offset based on the original length of the cable that was attached to your sensor. If you change the length of the cable, you must adjust the offset. Use this table to find the change in offset for a given change in cable length. If you have shortened the cable, subtract the change in offset from the original offset. If you have lengthened the cable, add the change in offset to the original offset. Note that the vibrating wire part of the signal (the pressure measurement) is not affected by cable length.

Also, take a look at the most recent version of the VW piezometer manual for a discussion about whether you really need to go through these steps.

Question: Temperature readings from my piezometer are between 120 and 125 °C. What's wrong?

Answer: You're probably using the VW Data Recorder and reading a thermistor as an RTD. At the "type" prompt, choose Hz + Thermistor. Then your temperature readings will look normal.

Question: What RTD do you use and what are its characteristics?

Answer: Unless you ask for an RTD, we now supply a thermistor.

In previous years, we used the Honeywell TD5A sensor. Here is an Acrobat datasheet listing its calibration characteristics. More general information appears below:

  • Sensor Type: 2000 Ohm Silicon RTD
  • Temperature Sensing Range: -40 °C to 150 °C (-40 °F to 302 °F)
  • Linearity % of Full Scale: ± 0.2%, Typ. -40 °C to +150 °C [-40 °F to 302 °F]
  • Supply Voltage: 10 Vdc
  • Supply Current (Recommended): 1 mA typ., 2 mA max

Question: I am not using a Slope Indicator readout. How can a obtain a reading in degrees C from the RTD used in your sensors?

Answer: Our published instructions assume that users are reading these 2K ohm RTDs using Slope Indicator readouts or the Campbell Scientific CR10X with AVW1 or AVW100 vibrating wire interface.

If you are using another device to read these sensors, obtain a reading in K ohms. Then apply either of the following polynomial coefficients to obtain degrees C:

Solution A: For a range of -10 to +30 C.
Ax2 + Bx + C, where x is the reading in K ohms, and:

A = -23.508334394
B = 227.625006633
C = -341.217356436

Solution B: For a range of -50 to 120 C.

Ax5 + Bx4 + Cx3 + Dx2 + Ex + F, where x is the reading in K ohms, and:

A = 0.95659
B = -12.65962
C = 70.30009
D = -216.32308
E = 483.85218
F = -472.87637

Question:  I'm using a CR10 to read 4-20mA titanium pressure transducers. What's the maximum cable length that I can have?

Answer: Approximately 240 meters or 800 feet. The CR10 supplies 12 volts, and the transducer requires at least 8 volts. The standard cable used with the 4-20mA transducer has 22 gauge wire so voltage drops about 0.6 volts per 1000 feet.

Question: I want to use linear factors (rather than polynomial factors) with my VW piezometer readings. How do I calculate linear factors (mx + b) using the data on my calibration record?

Answer: We've made an Excell worksheet called: vw-linear-factors-xls. Your calibration record gives frequency readings for 12 pressures. Enter these Hz readings in the frequency column, overwriting existing data. The spreadsheet converts calculates frequency squared readings and then applies Excel slope and offset functions to calculate an m factor and a b factor. These are labelled slope (m) and offset (b). You'll need WinZip or some other archiving program to unzip the file.

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