What is the maximum length of measurement cables ?
The maximum length of the measurement cables depends on many parameters. Some are internal because they depend on the type of signal (analogue, digital, mV / V, 4 ... 20 mA, RS-232, USB, etc.); others are external such as the electromagnetic and thermal environments of the installation.
In practice, simple precautions will easily prevent some pitfalls:
- Do not route measurement cables near power cables
- Always use faradised (shielded) cable and connect the shield to the ground.
- Preferably, use twisted pairs.
- Adapt the wire section to the length of the wiring.
A. Analogue signals
There are mainly 3 types of analogue signals for strain gage sensors:
- Non-amplified signal (Wheatstone bridge - mV / V)
This type of signal is not normally intended to be transported over long distance. It will therefore be limited to a length of 20 m taking into account the environmental conditions mentioned hereabove.
The use of sense lines at the power supply level eliminates the effects of temperature on power cables.
A larger section of cable (eg: 0.5 mm² instead of 0.14 mm²) also makes it possible to limit voltage losses and their thermal drift.
- Amplified signal 4..20 mA
This kind of analogue signal is an industrial standard.
It has the advantage of not being sensitive to the length of the cable and therefore being able to transmit a signal over several hundred meters.
In addition, it can detect any cut wire because the signal cannot be zero.
It exists in 2 versions:
- 2 wires where the sensor supply is provided by the measuring loop
- 3 wires where the sensor supply is provided by a 3rd wire.
However, the sum of the voltages at the inputs of the reading devices and the available supply voltage must be considered.
- Amplified signal 0 ... 10V and -10..0..+ 10V
This type of signal has the advantage of being able to connect the reading devices in parallel and thus without interrupting the measurement loop as is the case of 4..20 mA. This is particularly useful during checks.
It also makes it possible to keep the zero at 0 V, which gives an image closer to the measured quantity, particularly in the case of bidirectional sensors (eg tension + compression).
Considering the precautions mentioned above, we recommend 50m as maximum length.
B. Digital signals
This kind of connection is not intended for long distances.
The maximum cable length depends on the transmission speed
For example, it will be limited to 5 m in the case of a transmission to 19 200 Baud.
This kind of connection makes it possible to transmit signals over greater distances:
In addition, it allows to connect several devices on the same bus.
The maximum cable length also depends on the transmission speed.
Typically, with the use of line matching resistors, a length of 1200m is allowed up to a transmission rate of 100 kbps.
This maximum value may be limited by equipment manufacturers.
The USB standard does not allow a cable longer than 5m.
Note: There are repeaters that can extend this length up to 15m.