Piezometers

Geokon (and other manufacturers) use digits in their calculations:
digits = Hz^2 / 1000
Therefore, to convert digits to Hertz, multiply digits by 1000 and take the square root of the result.

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.

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.

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.

Where the cable exits the borehole, we’re planning to route it into PVC pipe.

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.

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.

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.

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?

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.

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.

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

Our published instructions assume that users are reading these 2K ohm RTDs using DGSI 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

Approximately 240 meters or 800 feet. The datalogger 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.

Please note that we no longer calibrate VW piezometers with this method, however the information is presented here for older piezometers that were calibrated utilizing the old method.

We’ve made an Excel 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.