Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-22

3.3 TROUBLESHOOTING

Maintenance and troubleshooting of Pressure Cells is confined to periodic checks of cable connections. Once installed, the cells are usually inaccessible and remedial action is limited.

Symptom: Pressure Cell Readings are Unstable

1. If the readout box is not positioned correctly to record readings and the swept frequency excitation settings are not correct then the cell should be set up on a different readout position. For instance, channel A of the GK-403 might be able to read the pressure cells. To convert the Channel A period display to digits Equation 1 can be used.

2. If there a source of electrical noise nearby and most probable sources of electrical noise are motors, generators, transformers, arc welders and antennas. It should be checked that the shield drain wire is connected to ground whether using a portable readout or data logger. In case of GK-403 the clip is connected with the blue boot to the shield drain wire.

3. If the readout does not work with another pressure cell then the readout may have a low battery or be malfunctioning.

Symptom: Pressure Cell Fails to Read

1. If the cable is cut or crushed can be checked with an ohmmeter. Nominal resistance between the two gage leads (usually red and black leads) is 180Ω, +/- 5%. The cable resistance is added when checking (22 AWG stranded copper leads are approximately 14.7Ω/1000' or 48.5Ω/km, multiply by 2 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.

2. If the readout does not work with another pressure cell then the readout may have a low battery or be malfunctioning.

Figure 3.1 - Calibration Sheet

Model:	4800 Earth Pressure Cell (rectangular)	4800 Earth Pressure Cell (circular)	4810 Contact Pressure Cell	4820 Jack-Out Pressure Cell
Ranges:	350 kPa (50 psi) 700 kPa (100 psi) 1.5 MPa (250 psi) 3.5 MPa (500 psi) 5 MPa (750 psi) 7 MPa (1000 psi) 20 MPa (3000 psi)	350 kPa (50 psi) 700 kPa (100 psi) 1.5 MPa (250 psi) 2.5 MPa (500 psi) 5 MPa (750 psi) 7 MPa (1000 psi) 20 MPa (3000 psi)	2 MPa (300 psi) 3.5 MPa (500 psi) 5 MPa (750 psi)	0.3 MPa (50 psi) 0.7 MPa (100 psi) 1.5 MPa (200 psi)
Sensitivity:	0.025% FSR
Accuracy:	0.5% FSR (0.1% FSR with a polynomial expression)
Linearity:	0.5% FSR (standard) 0.1% FSR (optional)
Overrange:	200% FSR
Operating Temperature:	-30 to +70° C
Frequency range	1400-3500Hz
Cell Dimensions: (active area)	100 × 200 mm 4 × 8"	230 mm OD 9" OD	230 mm OD 9" OD	125 mm OD 5" OD
Coil Resistance:	150 Ω
Cell Material:	304 Stainless Steel
Transducer Material:	303 & 304 Stainless Steel
Weight:	2.3 kg. 5 lbs.	2.3 kg. 5 lbs.	4.7 kg. 10.3 lbs.	2.7 kg. 6 lbs.
Electrical  Cable:	2 twisted pair (4 stranded conductor), 22 AWG Foil shield (with drain wire), PVC jacket, nominal OD=6.3 mm (0.250")

Table 3.2- Earth Pressure Cell Specifications