A Twisted Tale

At a manufacturing plant one hot summer day, HVAC installers were preparing to measure the cool fruits of their labour. They inserted a thermocouple probe into a gleaming new air conditioning duct, part of a redesigned cooling system that promised to make sweaty brows on the plant floor (induced by heat rather than stress) a thing of the past.

The HVAC crew expected a meter reading much lower than the ambient temperature of 90F. Instead they got one that went off the charts. The temperature meter’s LED flashed 999.9.

“That can’t be right,” said the crew’s newest hire, staring at the meter. A more experienced co-worker wasn’t so mystified. He quickly saw that the path of the thermocouple wire went close to the air conditioner’s compressor motor. The meter wasn’t merely reading the heat picked up by the probe. It had gone haywire as electromagnetic interference (EMI) from the compressor bombarded the unshielded thermocouple wire.

Most motors generate such EMI or noise, known as radio frequency interference (RFI). To a greater or lesser degree, so do many other electrical devices. On the factory floor, spark-producing machinery such as spot, arc and induction welders generate significant EMI, as do computerized numerically controlled (CNC) lathes and milling machines, other equipment powered by large motors, high-intensity lights, relays and walkie-talkies.

What would have prevented the spurious meter readings at that plant is twisted/shielded thermocouple wire or cable — the first and most cost-effective line of defence against EMI.

“Twisted/shielded cable is a good start because a temperature meter can act like an antenna if not protected,” says Frank Welsh, Omega Engineering’s resident EMI troubleshooter. “Thermocouple probes are designed to sense temperature, not electromagnetic radiation. The problem arises because the thermocouple circuit can pick up extraneous information, or noise, and pass it along to the meter.”

At Omega’s Bridgeport, N.J., manufacturing facility, Welsh oversees the anechoic chamber, a thick-walled, eerily quiet lab with walls covered in thousands of pointed foam baffles. The baffles absorb the electromagnetic waves, allowing technicians to strictly control the testing of temperature meters, pressure transducers and other devices.

Bridgeport is also where the company produces twisted/shielded thermocouple wire. With shielding, the antenna that is the thermocouple wire shrinks to a small point, which is the probe. Even the probe may need shielding in extreme cases, though there will probably be a tradeoff in reading time, which might increase from milliseconds to seconds.

According to Omega, using twisted/shielded thermocouple wire on a sensing device can cut EMI by 500 to 1,000 times, virtually eliminating the problem in most applications.

“Whether used as a preventive measure or to solve a known noise issue, twisted/shielded wire represents a low-cost, field-installable solution,” says Jim Ferguson, vice-president and plant manager for Omega’s Bridgeport plant. “All the wire used in our in-house probe assemblies we make ourselves, so you can be sure we adhere to the highest standards of quality.”

Omega’s standard twisted/shielded thermocouple wire consists of PVC-insulated, ANSI, or IEC colour-coded conductors twisted with tinned-copper rain wire, then wrapped with aluminized polyester tape. Another PVC layer covers the wrapping. The aluminized polyester and the drain wire protect against EMI; the PVC resists moisture, chemicals, abrasion and UV light. Also offered is Teflon for high-end applications. Stainless steel and tinned-copper overbraidings are available for even more durability and flexibility. Lengths range from 25 ft to 1,000 ft.

Omega builds, stocks and supplies complementary products that shield against electrical noise at the probe and at the instrument connection, including low-noise thermocouple probes and a high-temperature standard connector. The company also makes standard GST and miniature GMP low-noise connectors, which are designed to provide easy connection of ground wires to probes and extension wires.

Factory floor like a lab

Perhaps the biggest demand for twisted/shielded wire is from high-tech manufacturers using large automation equipment, such as computer chip makers or producers of medical devices. These operations require steady feedback in the form of real-time data, yet they also tend to produce very high EMI levels. Manufacturing data used to be collected mainly in the lab, a relatively quiet environment, electromagnetically speaking. Today, however, the factory floor itself can be a lab brimming with information for those savvy enough to cut through the noise.

Sensors protected with twisted/shielded wiring are also common in aerospace applications, where constant exposure to EMI is the norm. In space exploration and satellite communications, for example, cosmic rays — electromagnetic waves of extremely high frequency — would wreak havoc if the sophisticated, sensitive electronics demanded by these applications lacked EMI barriers.

The current-signal domain is relatively immune to EMI, and compared to voltage signals, current signals can run for very long distances. An option, therefore, for the noisiest applications, would be to shift from voltage to current in the offending environment — using, for example, twisted/shielded wire in conjunction with a signal conditioner.

Reproduced with the permission of Omega Engineering Inc.