RTD cable (multicore & shielded cable)
The Anatomy of an RTD Cable
To understand why this product is
specialized, we have to look at its layers from the inside out. Unlike a
standard power cable designed to carry heavy current, an RTD cable is designed
for signal integrity.
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1.
The Cores (Multicore Configuration)
RTD cables are typically found in 2,
3, or 4-core configurations.
- 3-Core:
This is the industrial workhorse. One wire provides the excitation
current, while the other two allow the controller to measure the lead-wire
resistance and subtract it from the final calculation.
- 4-Core:
Used in high-precision metrology. It uses two wires to carry the current
and two separate wires to measure the voltage drop across the sensor.
The conductors are usually made of Stranded
Tinned Copper. Tinning prevents oxidation (which would increase resistance)
and makes the wire easier to solder or terminate into screw terminals.
2.
Primary Insulation
Each individual core is wrapped in
insulation. The material choice depends entirely on the environment:
- PVC:
Cost-effective and flexible, but limited to about 105°C.
- Teflon (PTFE/PFA):
The gold standard. It is chemically inert, moisture-resistant, and can
withstand temperatures from $-200^\circ\text{C}$ to $+260^\circ\text{C}$.
- Fiberglass:
Used in extreme heat (up to 480°C), though it lacks moisture protection.
3.
The Shielding (The "Shielded" Part)
This is the most critical layer for
preventing Electromagnetic Interference (EMI). In an industrial plant, cables
often run alongside high-voltage motors or VFDs (Variable Frequency Drives).
These devices emit "noise" that can be picked up by the RTD wire like
an antenna.
- Aluminum Mylar Tape:
A thin foil wrapped around the cores. It provides 100% coverage and is
excellent for high-frequency noise.
- Tinned Copper Braid:
A woven mesh that provides superior mechanical strength and is better at
filtering low-frequency interference.
- The Drain Wire:
A constant companion to the shield, this uninsulated wire ensures a
continuous ground path along the entire length of the cable.
4.
The Outer Jacket
The final layer protects the cable
from the "real world." If the cable will be dragged across a factory
floor, a Polyurethane (PUR) jacket is used for abrasion resistance. If
it’s going into a chemical vat, Teflon or Silicone is preferred.
Why
"Standard" Wire Won't Work
You might wonder: “Can’t I just
use a standard 3-core electrical wire?” Technically, yes, but your data
will be wrong.
The resistance of a Pt100 sensor
changes by only 0.385 Ohms per degree Celsius. If you use a cable with
high internal resistance or poor insulation, the controller will interpret that
extra resistance as a temperature increase. For example, just $0.4\ \Omega$ of
unwanted resistance in a poor-quality cable would cause a measurement error of
roughly $1^\circ\text{C}$. In a pasteurization process or a chemical reaction,
a one-degree error can result in a ruined batch of product costing thousands of
dollars.
Selection
Criteria: Choosing the Right Cable
When ordering or specifying an RTD
cable, you must balance the "Golden Triangle" of cable selection: Temperature,
Environment, and Distance.
The
Temperature Factor
If the cable is running through an
oven or a cryogenic freezer, the insulation must match. A common mistake is
using a high-temp sensor with a low-temp PVC cable. The PVC melts, the cores
short-circuit, and the system fails. Always specify the ambient temperature of
the entire cable run, not just the sensor tip.
The
Distance Factor (AWG)
The longer the distance between the
sensor and the PLC (Programmable Logic Controller), the thicker the wire should
be.
- For short runs (<10m), 24 AWG is fine.
- For long runs (>50m), 20 AWG or 18 AWG is
better to minimize the base resistance.
Flexibility
Requirements
If the RTD is mounted on a robotic
arm or a vibrating motor, you need a "High-Flex" multicore cable.
These use finer strands of copper (e.g., 40 strands of 0.08mm wire rather than
7 strands of 0.2mm wire) to prevent the copper from work-hardening and snapping
over time.
Installation
Best Practices
To get the most out of a
high-quality shielded cable, installation must be precise:
- Single-Point Grounding: Only ground the drain wire at the controller side.
Grounding both ends creates a "ground loop," which acts like a
giant loop antenna for noise—the exact thing you’re trying to avoid.
- Separation:
Never run RTD signal cables in the same conduit as high-power 480V motor
leads. Even with a shield, the "crosstalk" can be significant.
- Bend Radius:
Avoid sharp 90-degree bends. Kinking the cable can damage the Mylar
shield, creating "leaks" where interference can enter.
Summary
of Technical Specifications
|
Component |
Common Material |
Purpose |
|
Conductors |
Tinned Copper (Stranded) |
Conductivity & Corrosion Resistance |
|
No. of Cores |
2, 3, 4, or 6 |
Lead wire compensation |
|
Shielding |
Al-Mylar + Drain or Braided |
EMI/RFI Protection |
|
Jacket |
PTFE, PVC, Silicone, or PFA |
Environmental Protection |
|
Voltage Rating |
300V to 600V |
Safety (though signals are low voltage) |
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