Twisted/shielded application example

On a hot day last summer, in a manufacturing plant outside Philadelphia, HVAC installers were preparing to measure the cooling effect of the equipment they installed. They inserted a thermocouple probe into a new air-conditioned duct, which was part of a completely redesigned cooling system that would make the factory's stifling environment history.

The HVAC working group expects the meter reading to be much lower than the 90oF ambient temperature. However, they have exceeded the expected value. The temperature meter's LED flashes 999.9 digits.

"Surely wrong," a new employee in the working group yelled at the temperature meter. A more experienced colleague is not so confused. He soon discovered that the thermocouple wire path was too close to the air conditioner compressor motor. Temperature meters not only read the heat sensed by the probe. As the compressor hits an unshielded thermocouple wire causing electromagnetic interference, the instrument will be in a state of chaos.

Most motors generate such electromagnetic interference (EMI) or noise, called radio frequency interference (RFI). Many other electrical devices also generate electromagnetic interference more or less. At home, sources of electromagnetic interference include dimmer switches (which generate a warning buzz), roof fans, electric treadmills, and microwave ovens. (Earlier microwave ovens' electromagnetic interference leaks pose a danger to people with pacemakers, but today's strict government restrictions on leakage and the shielded wires that have become the standard for pacemakers have greatly reduced this concern.)

In the factory floor, spark-ignited machines such as spot welders, arc welders, and induction welders can generate serious electromagnetic interferences. Computer numerically controlled (CNC) lathes and milling machines, other equipment driven by large motors, high-intensity lamps, Relays and intercoms can also cause serious electromagnetic interference. As for the ubiquitous cell phone, the low-level energy it emits may not affect the type of thermal monitoring system used in the Philadelphia plant.

It is twisted/shielded thermocouple wires or cables that can prevent meter readings at the plant from being missed – they are the most cost-effective wire choices for resisting electromagnetic interference.

"Shielded twisted pair is a good start, because if the wire in the temperature meter is not protected, it will function as an antenna," said Frank Welsh, who has long been responsible for electromagnetic interference issues in the OMEGA engineering department. "The thermocouple probe is designed to sense temperature, not electromagnetic radiation. The problem is that the thermocouple circuit collects extraneous information, or noise, and transmits it to the meter."

At OMEGA's manufacturing facility in Bridgeport, New Jersey, Welsh managed the anechoic chamber, a very quiet, thick-walled laboratory with tens of thousands of sharp-edged foam acoustic panels covering the walls. The baffles absorb electromagnetic waves, allowing technicians to strictly control the testing of temperature meters, pressure sensors, and other devices made by Omega.

The Bridgeport facility is also where OMEGA produces twisted/shielded thermocouple wires – producing more than any other plant. By shielding, the point where the thermocouple wire acts as an antenna is reduced to a small point, which is the probe. In extreme cases even probes need to be shielded, although it is likely to cost the reading time from a few milliseconds to a few seconds. According to OMEGA experts, the use of twisted/shielded thermocouple wires on the sensing device can reduce electromagnetic interference by 500 to 1000 times, which can solve almost all problems in most applications.

“Whether used as a precautionary measure or as a solution to known noise problems, twisted/shielded wire represents an economical field installation solution,” said Jim Ferguson, vice president and plant manager at Bridgeport Plant. OMEGA recommends using twisted/shielded cables when the length of the sensor's wires is longer than a few feet, and when larger motors have a threat to interfere with sensor readings. "The wires used in our factory's probe assembly are all produced by us, so you can be confident that we are following the highest quality standards," Ferguson added.

OMEGA's standard twisted/shielded thermocouple wires consist of a PVC-insulated core with ANSI or IEC color code. The wire core is stranded with a tinned copper drain wire and wound with aluminized polyester tape. The wound layer is covered with a layer of PVC. Aluminized polyester tape and drain wires are used to prevent electromagnetic interference. PVC is used for moisture, chemical, abrasion and UV resistance. Some manufacturers only use polyethylene for insulation. OMEGA also offers PTFE insulation for high-end applications. For extreme durability and softness, stainless steel and tinned copper braided shields can be used to enhance electrical shielding with tinned copper. The length ranges from 25 feet of convenient spool to 10,000 feet.

“When we make cables, we pay great attention to the details,” said Robert Lesutis, who is in charge of OEM sales for the OMEGA component manufacturing division. “We twisted the ground wire in the gap between the cores. Our cable manufacturing is unique. Because the ground wire is twisted with the core instead of just attaching it to the wire core."

OMEGA also manufactures, stores, and supplies auxiliary products that shield electrical noise at the probe and instrument connections, including the HGKMQSS series of low-noise thermocouple probes and the high-temperature standard connector HFSTW-KM, which has a zinc ferrite core that suppresses electromagnetic interference. . The company also produces standard GST and miniature GMP low-noise connectors that facilitate the connection of wires to probes and extension cords.

The greatest demand for twisted/shielded wire may come from high-tech manufacturers using large automation equipment, such as computer chip makers or medical device manufacturers. These operations require stable feedback of real-time data, and they also produce high electromagnetic interference levels. In the past, manufacturing data was collected primarily in the laboratory. This was a relatively quiet environment in terms of electromagnetics. But now, for those capable of eliminating noise, the factory floor itself can become a laboratory full of information.

Sensors protected with twisted/shielded cables are also common in aerospace applications where continuous exposure to electromagnetic interference is the standard condition. For example: In space exploration and satellite communications, if these applications require advanced,

Sensitive electronic devices have no electromagnetic interference barriers and cosmic rays (very high frequency electromagnetic waves) will ravage.

The current signal is relatively resistant to electromagnetic interference. Compared with the voltage signal, the current signal can transmit a long distance. Therefore, for noisy applications, it is possible to choose to switch from voltage to current in harsh environments, such as using a twisted/shielded line with a signal conditioner, such as one of OMEGA's iDRN/iDRX series transmitters. On the 35 mm DIN rail. All models in this series are designed to work directly with various sensors and generally do not require additional components.