BIGGEST MISTAKES IN INSTRUMENTATION

BIGGEST MISTAKES IN INSTRUMENTATION

Mistake #1: Selecting the wrong sensor

Technology mismatch Although it’s generally obvious what quantity needs to be measured in a flow, temperature, or pressure control application, it’s not always obvious what kind of flow meter, temperature sensor, or pressure gauge is best suited to the job. A mismatch between the sensing technology and the material to be sensed can lead to skewed measurements and severely degraded control.

 Mistake #2: Installing sensors incorrectly

The best sensor can yield disappointing results if not installed correctly.

Mistake #3: Poor Grounding

Instruments must be grounded to provide a reference voltage for the data signals they generate. Relying on earth ground is risky since not all of the earth shares the same electrical potential. The resulting currents will interfere with the sensors’ signals.

While it’s generally a good practice to insulate a sensor from the thermodynamic effects of its surroundings it’s absolutely critical to establish electrical isolation. The most common electrical problems due to poor installation are ground loops. Ground loops occur when an extraneous current flows through the instrumentation wiring between two points that are supposed to be at the same voltage, but aren’t (see Figure ). The resulting electrical interference can cause random fluctuations in the sensors’ output and may even damage the sensors themselves. As the name implies, ground loops most often occur when instruments and their cables are grounded improperly or not at all. Interestingly, the best way to isolate a plant’s instruments from ground loop currents is to connect them together at one master grounding point. If that’s not possible, a grid of grounding points must be spread throughout the plant, making sure that all points on the grid are at the same electrical potential. Insecure connections and inadequate wires can cause a voltage imbalance in the grid and ground loops between the instruments connected to it.

Mistake #4: Generating gibberish

Noise

Ground loops are not the only source of noise that can distort a sensor’s readings. Radio frequency interference (RFI) is even more common in plants that use walkie talkies, pagers, and wireless networks extensively. RFI also results whenever a current changes, such as when an electro mechanical contact or a static discharge generates a spark.

The sources of RFI noise must be eliminated or at least kept away from the plant’s instrumentation if at all possible. Replacing electro mechanical equipment with solid state devices will eliminate arc-generated RFI.

Mistake #5: Quitting to Soon

Calibration

Most instrumentation engineers know that a sensor must be calibrated in order to associate a numerical value with the electrical signal coming out of the transmitter. Yet all too often, the instruments are calibrated just once during installation then left to operate unattended for years. The result is an insidious problem known as drift. A sensor’s output tends to creep higher and higher (or lower and lower), even if the measured variable hasn’t changed.

Deposition on the sensing surfaces, corrosion in the wiring, and long term wear on moving parts can all cause an instrument to begin generating artificially high (or low) readings. As a result, the controller will gradually increase or decrease its control efforts to compensate for a non-existent error.

Planning for the road ahead