Line Simulation Circuit

VibWire-108

+ Sensor

- Sensor

47 W

55 nF

42 mH

55 nF

100 W

1/2 Km Equivalent line length

Coil Resistance

200 mS

Excitation Pulse

Range Tests

It is impractical to to test the VibWire-108 in place with physical amounts of cable due to the physical problems involved in obtaining and manipulating such large objects. To get around this problem Keynes assembled an in-line simulator that combines all the same electrical properties of cable in a small easy to use circuit. The in-line simulator was used to determine the maximum cable length that can be deployed in order that a vibrating wire sensor can reliably return signals back to the instrument.

Cable Type Model

Keynes chose Category 5 cable for the line simulator model as this cable type is available as standard product internationally and is of a lower specification than standard instrumentation cable. Any practical deployment using good quality signal cable will offer superior results to the to those obtained

Figure 5 shows the circuit for the line simulator. To aid testing, the simulator was assembled using circuit blocks representing 1/2 km cable lengths.

Figure 6 shows the excitation signal at the start and end of the line simulator. The yellow trace is the signal at the start of the simulation and the green trace at the end. Clearly shown is the distortion in the excitation signal that can occur in long cable lengths.

Measuring Gauge Temperature

Most vibrating wire sensors are susceptible to errors caused by temperature fluctuations and as such are often fitted with an embedded thermistor allowing the gauge temperature to be measured. The VibWire-108 uses an analogue data acquisition system to make the temperature reading. A precision 2.5V volt regulated supply along with a 3.3K Ohm resistor completes the bridge circuit to the sensor, see figure 4.

Any resistive temperature sensor can be used with the instrument however conversion to engineering units is undertaken is a feature of the analysis software of a logger or PC and is not part of the embedded software.

The VibWire-108 contains 8 analogue inputs suitable for digitising the temperature sensor signals or configured as analogue inputs. The instrument does not itself undertake any linearisation or display results in engineering units, it simply makes an analogue measurement and transmits the information for later processing to a logger or computer. Any conversion of the temperature measurement into engineering units is a function of the logger unit or analysis software on the computer system.

2.5 V Precision
Source

ADC

Fig 6 - Sensor Excitation Signal

Fig 5

Fig 4