Eliminate Signal Gibberish

Several steps can help maintain the integrity of measurement and control signals.

By Greg Livelli, ABB Measurement Products

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The best sensor can yield disappointing results if improperly installed.

For example, many flowmeters require a specific length of straight pipe before and after them to provide "fully developed" flows. Nearby bends, junctions, pumps and valves in the pipeline can adversely affect their accuracy. You must pipe an electromagnetic flowmeter so it remains full at zero flow — otherwise its output can become erratic because of electrode exposure to air. You must install a rotameter vertically plumb so the float experiences the full effect of gravity.

Placement also can affect temperature sensors. Even a highly accurate RTD only detects the temperature of its immediate vicinity. So, if it's tucked into the corner of a mixing chamber and mixing is incomplete, that local reading may not represent the temperature of material elsewhere.

With radiation thermometers, you must place the sensor at a distance where only radiation from the surface to be measured enters the lens (Figure 2). Otherwise the sensor also sees radiation from the surroundings.

Applications often employ a DP transmitter and an impulse line consisting of an isolating seal and liquid-filled capillary to measure the level of harsh liquid chemicals in tanks. If the tank is open or vented, the low-pressure transmitter port is left open, and a single impulse line from near the tank bottom connects to the high-pressure port. You can place the transmitter at any level and correct the head effect of the liquid in the capillary by the zero adjustment.

However, if the tank is closed, a second impulse line must connect to the top of the tank to cancel out vessel pressure. If that line also is a filled capillary with a remote isolating seal, the transmitter's best location is near the tank's mid-section. This location provides uniform distribution of temperature across the lengths of the two capillaries.

With a plain impulse line (no isolating seal) from the tank top, ensure condensate doesn't enter the transmitter body, which would cause measurement error. One way is to place the transmitter above the tank so any condensate flows back into it (Figure 3). For a transmitter at a lower location, fill the impulse line from the tank top with a suitable liquid of higher specific gravity to maintain a constant pressure on the low-pressure transmitter port.

Sensor location can adversely affect controller performance. Recall that a PID (proportional-integral-derivative) controller looks at the difference between the sensor signal and its setpoint. After a process event, the controller first changes the output proportionally to minimize the difference. If the difference persists, the integral (reset) component comes into play, gradually attempting to equalize the sensor signal and the setpoint.

Poor control can result if a sensor is installed too far from the associated actuator or thermal element. A distant sensor may not be able to measure the effects of the control element's last action in time for the controller to make an intelligent decision about what to do next. A case in point is a pH sensor located far from where alkali or acidic dosing to maintain the desired pH takes place.

Sensors often require protection from the environment or the process. Proper material selection and liners help flowmeters withstand corrosive and abrasive fluids. Thermowells protect temperature sensors from the process. Liquid-filled impulse lines with remote isolating seals safeguard pressure sensor diaphragms.

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