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The maximum length of load cell and communication cables depends on many parameters. Some are internal because they depend on the signal type (analog, digital, mV / V, 4 … 20 mA, RS-232, USB, etc.); others are external, such as the electromagnetic and thermal environments of the facility.
- Do not route measurement cables near power cables
- Always use long-distance (shielded) cable and connect the shield to ground.
- Preferably twisted pairs
- Adjust the cable section according to the cable length.
1. Analog signals
There are mainly 3 types of analog signals for strain gauge sensors:
Non-amplified signal (Wheatstone bridge – mV/V)
This type of signal is not normally intended to be carried over long distances. Therefore, taking into account the environmental conditions mentioned above, it should not exceed a length of 20 m.
The use of sensing lines at the power supply level eliminates the effects of temperature on the power cables.
A larger cable cross-section (for example: 0.5 mm² instead of 0.14 mm²) also makes it possible to limit voltage losses and thermal drifts
Amplified signal 4..20 mA
This type of analog 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 the signal over several hundred metres.
It can also detect any broken wire since the signal cannot be zero.
There are 2 different connection types:
The measurement cycle of the load cell supply
3 cables where the load cell supply is provided by a 3rd cable.
However, the voltages at the inputs of the reading devices and the sum of the current supply voltage should be taken into account.
Amplified signal 0 … 10V and -10..0..+ 10V
This type of signal has the advantage of being able to connect reading devices in parallel and therefore without interrupting the measurement loop as in the case of 4..20 mA. This is particularly useful during checks.
It also makes it possible to hold the zero at 0 V, which allows positive and negative values to be read, especially on bidirectional load cells (e.g. compression + tension)
We recommend 50m as a maximum length, taking into account the above-mentioned precautions.
2. Digital signals
▪️RS-232
This type of connection is not designed for long distances.
Maximum cable length depends on the transmission speed
For example, 19 200 In case of transmission in baud it will be limited to 5 m.
▪️RS-485
This type of connection makes it possible to transmit signals over longer distances:
In addition, it allows the connection of 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 1200 m is allowed for up to a 100 kbps transmission rate.
This maximum value is specified by equipment manufacturers.