Electrical noise, also known as electromagnetic interference, can destroy a clean signal from your measurement instrumentation (such as level sensors and pressure transducers). Protecting your sensors from electrical noise is a top priority. Luckily, this is a fairly simple task in most cases.
There are a couple of ways to keep electromagnetic interference (EMI) at bay. You need to use properly shielded cable, you must be careful when routing your cables, and your sensors need to be grounded properly. Finally, it’s important that the signal output is appropriate for the electrical environment.
Using shielded cable may seem like a no-brainer. However, it is overlooked when budget is tight, or when the risks are not understood. Part of this task is making sure the right personnel are installing your instrumentation.
Recently, we found one of our sensors installed with standard electrical wire, the kind buildings are wired with. This happened because our customer asked a standard electrician to install the sensors. Instrumentation wiring is a specialty, and may not be well understood by any electrician – especially one whose trade is light fixtures, appliances, and power outlets.
Good wire for instrumentation is usually twisted and shielded – though the specific cable varies depending on the sensor. Make sure you stick to the sensor manufacturer’s recommendations.
Routing the cable properly is very important. Unfortunately, this is often forgotten. The image shows signal cable routed right next to an electrical outlet. In no uncertain terms, this is wrong! Signal cable should be routed away from electrical power cables, power sources, electric motors, and any other electrical fixture.
Higher voltages increase electrical noise. Electric motors are especially bad, as they produce a magnetic field.
How far away do you need to route the cable? That depends on things like voltage and the size of any magnetic fields. However, even a few inches can make a huge difference. The point: do the best you can to keep them separate. If you notice a noisy signal, then you need to do a better job.
Your sensors should be connected to a good ground. Avoid ground loops by using a single ground, or by ensuring that multiple grounds have the same potential. What does this have to do with EMI? Ground is the path that carries electrical noise away from your signal wires.
The shield in your cable should have a good contact with a drain wire, which should be connected to ground. This provides a path for electrical noise to travel to ground, instead of using your signal wire. If the connection to ground is insufficient, the shield will be ineffective. If a ground loop exists, a current will flow through the cable shield and will not stop EMI.
An improper ground is bad. Not only will you be prone to EMI, but you’ll also loose your protection from power surges. In addition, an electrical circuit without a ground increases the risk of shock. Proper grounding should be a top priority.
In contrast, a good ground, proper cable routing, and the right signal cable will stop electromagnetic interference – keeping your measurement signals clean.
A NOTE ABOUT SENSOR OUTPUTS
If you are still experiencing EMI, note that some outputs are more prone to interference than others. For example, sensors with millivolt output may be very economical, but due to the low-power output, these signals are exposed to EMI. Sensors with voltage output are better because the signal is amplified.
However, a milliamp output is the best for EMI avoidance. A 4-20mA signal, for example, is well suited for long distance cable runs because it is not susceptible to electrical noise and resistance. Unlike voltage output, a milliamp signal has the same strong signal at any point in the circuit.
Ignore any one of these critical tips and the signal from your level and pressure sensors may be left unprotected. Electromagnetic interference will compromise the integrity of your signal and ruin your ability to measure your key levels and processes.
Follow these important tips to ensure you don’t suffer from EMI. You’re already investing in measurement instrumentation and expecting a certain accuracy rating. Protect that investment with proper cable, smart cable routing, thoughtful grounding, and the right signal outputs.