Selecting voltage-based pressure sensors
While the bulk of process instrumentation has standardized on 4-20 mA current loops, this approach depends on having 24 Vdc with a few amperes available. Where power is on short supply, pressure sensors that use one of a variety of voltage output options can reduce consumption significantly. While the differences between the various voltages may seem minor, each option offers its own unique features and benefits for specific use cases and different user groups. The key is to match output with available system operating voltage.
Lower-power systems are becoming more common, particularly in wireless environments thanks to lower installation cost features, along with remote device clusters that depend on solar panels and/or lithium batteries. In these situations, the need to conserve power takes on paramount importance. In such systems, voltage availability typically ranges from 7-12 Vdc, with currents in the 2-3 mA range to power the transducer. The only solution here is to use a voltage output approach, such as 0.5-4.5 V ratiometric with 5 Vdc supply, or 1-5 V sensors with 7-12 Vdc systems. Upstream and midstream oil and gas applications are driving this today where power is not freely available in remote areas.
For battery-powered and pulsed systems, the sensor unit is often energized for a short time between sleep periods for monitoring, such as a tank level application. Such systems operate at 3.3 V, so the sensor uses less than 1 mA excitation with 0.5-3 V output. Here, the need to maintain battery life for two years or more is essential.
The next most popular powering approaches are fixed 5 V or unregulated 6-12 V systems. These are typical with lithium and solar cell combinations. For these, sensors use 0.5-4.5 V outputs with maximum current limited to 2 mA or less so that the system can work for many years. This is particularly important in higher northern or southern latitudes where sunlight is limited for several months of the year.
Finally, land-based systems with generator power or some other permanent supply run at 8-28 V. These situations allow several options for output signals, including 0-5 V, 1-5 V, 1-6 V, and 0-10 V. Here current consumption is less than 10 mA, so it is well below a 4-20 mA loop. The downside of a voltage loop is the limit on cable length between the transducer and controller such as a plc or computer.
Low level or no signal?
The main disadvantage of any zero-based output signal is that there is no signal with zero pressure. If the transducer has a cut wire, broken sensing element, or electronics that received an over-voltage, the sensor will produce no signal, thus, no way to provide an output. The controller can’t tell if pressure is actually zero or if the unit is simply inoperative.
For example, if used in water pressure measurement, the controller might signal a pump to act when the sensor detects pressure has crossed a threshold. If there is no pressure in the line, the transducer will produce a 0 V signal. Similarly, at fault conditions, the sensor continuously provides a 0 V signal. Since the reading is the same at actual zero pressure and fault conditions, there is no way for the controller to distinguish between the two. In a worst-case scenario, the pump would not know to run and could cause a flooding condition.
As industrial pressure transducers become smarter with advances in electronics and microprocessors, sensors are available with a factory-set fault condition. Transducers can be programmed to rail or send output below the lowest point or above the highest point to indicate to the controller that there is an issue. For example, if a pressure spike in the system causes the sensor diaphragm to break, the output signal on a 1-5 V output signal can be programmed to drop the output below 1 V or above 5 V by about 10%. In a pump application, it can help prevent flooding, the pump from running dry, or extra wear.
Karmjit Sidhu is vice president of business development and Greg Montrose is marketing manager for American Sensor Technologies.
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