Electricity + Control July 2018

FLOW MEASUREMENT + INSTRUMENTATION

is kind of like electrical lag – where a voltage, for example, takes time to reduce again after it rises.

0,1 mA will accurate represent a change in 18,75 psi of pressure change.

Because pressure is dynamic in range and rate of change, the accuracy and responsive measurement as is provided by an electronic device is required.

Measuring more than just pressure A transducer can observe many physical proper- ties and provide a suitable analogue output. Tem- perature, flow, fluid level, position, water satura- tion and particle contamination are all common, as previously mentioned. Although most are still available with the same 0-5 V, 0-10 V or 4-20 mA output signals (or others such as CAN), how they arrive at those ranges can vary vastly. A temperature transducer can read the changes in resistance as caused by a change in tempera- ture, and it uses on-board electronics to improve accuracy, resolution and response time. However, a flow sensor uses a completely different principle altogether. The Hall Effect uses a solid state pickup to measure the intensity of a magnetic field. An inline impeller can be made from magnetic mate- rial – which, as it spins, produces ever increasing voltage in the sensor, called the Hall Voltage. The downside of this sensor is the response time for the impeller to speed up and slow down, making it accurate for only steady-state flow. A flow sen- sor using a set of positive displacement gears (like a gear pump), is much more accurate, but is also much more costly. One of the more interesting transducers is the particle contamination measuring transducer. Rather than using some sort of change in resist- ance, inductance or capacitance, it uses a laser to measure fluid passing across its orifice. As parti- cles break the laser beam, the time and frequency of the broken beam identify the size and quantity of the passing particles. Although these units can be rather expensive they make use of exceptional technology that allows technicians to measure flu- id cleanliness on site. As electronics advance, they continue to en- sure that fluid power stays relevant. Productivity is rising more rapidly than what can be achieved with log splitter-era technology, and we in the industry owe that to the sophisticated electronics perme- ating fluid power. Transducers are a piece of the electrification puzzle, and are key to the growth of fluid power as a whole.

When all is said and done, a pressure transducer will take input electrical power, read its load cell and then output something that a PLC or digital display can use to extrapolate pressure. In most hydraulic applications, output signal can be varia- ble voltage or variable amperage. Typical voltages are 0-5 and 0-10, and variable amperage outputs are 4-20 mA. The latter starts at 4 mA to avoid the dead zone interpreted as a failure or broken wire. In applications with long runs of wires, var- iable amperage can have an advantage in that it avoids background interference that creates volt- age spikes. What a 0-5, 0-10 or 4-20 output means depends on the pressure range of the transducer it is in- stalled in. Pressure can be vacuum to 10 000 psi or more, so the output signal must provide a linear representation of the desired scale. For example, if there is a 15-psi transducer on the filter’s by- pass indicator, and the output is 0-5 V, a 1 V signal equals 3 pounds of pressure. A 2.5 V signal would equal a 7.5-psi level, and so on. With a variable amp output, any signal less than or equal to 4 mA is considered 0 psi of pressure. If a 3 000 psi transducer is used as an example, simple maths will tell the PLC what the incoming signal means. There is 16 mA worth of usable res- olution to spread amongst the 3 000 psi of pres- sure to be measured. With 4 mA being zero, and 20 mA being 3 000 psi, everywhere in between is just a factor of both. Each milliamp of energy translates to 187,5 psi of observed pressure, and even each fractional milliamp is further broken down. If the transducer has an accuracy of 0.5% (which high quality ones do), this means each

Paul J Heney is the Vice President, Editorial Director for Design World/ WTWH Media and expert technical writer for Global Industries.

Electricity + Control

JULY 2018

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