Efficient Plant July 2018 - 33
Choose the Correct
Answering eight questions on how and where an accelerometer is to be used
simplifies the selection process. Photo: IMI Sensors
1. WHAT TYPE OF DATA IS REQUIRED?
Raw or overall data: An accelerometer's VAC signal is a dynamic
signal on which frequency analysis can be performed while a vibration transmitter's continuous 4-to-20-mA signal of overall values
can connect directly with PLC, DCS, and SCADA systems.
Acceleration, velocity, or displacement: The appropriate measurement range depends on the frequency of the measured vibration.
+ displacement: 0 to 10 Hz
+ velocity: 10 to 2,000 Hz
+ acceleration: greater than 2,000 Hz.
2. WHAT IS THE VIBRATION AMPLITUDE?
The maximum amplitude of the measured vibration will determine
the sensor's measurement range.
+ common accelerometer ranges: 10, 50, 100, and 500 g
+ common vibration-transmitter ranges: 0 to 1 and 0 to 2 ips;
0 to 5 and 0 to 10 g
3. WHAT IS THE VIBRATION FREQUENCY?
Low-frequency response: Most industrial sensors will have a limit
near 0.2 Hz.
High-frequency response: Sensors have a resonant frequency at
which the signal is amplified, leading to sensitivity changes and
possible saturation. To combat saturation, most industrial accelerometers have single- or double-pole RC filters.
4. WHAT IS THE TEMPERATURE OF THE
Determine the highest and lowest temperatures to which the sensor
will be subjected.
+ -65 F to 250 F: Most general-purpose industrial sensors with
integral electronics can operate in this range.
+ 250 F to 325 F: High-temperature sensors with special integral
SELECTING ACCELEROMETERS for specific applications need
not be difficult. Meredith Christman of IMI Sensors, Depew, NY
(pcb.com), says the process basically involves answering eight questions.
- Jane Alexander, Managing Editor
electronics are available since higher temperatures can harm
built-in electronics of general-purpose units.
+ >325 F: Charge-mode designs without built-in electronics are
5. WILL THE UNIT BE SUBMERSED IN LIQUID?
Depths <1,700 ft.: Sensors with integral cables can be permanently
submersed without additional testing.
Depths >1,700 ft.: Sensor should be pressure tested for one hour.
6. WILL THE UNIT BE EXPOSED TO CHEMICALS?
Sensors: Industrial designs feature corrosion-resistant
Cables/connectors: Check a chemical-compatibility chart. With corrosive chemicals, use PTFE cable with non-corrosive connectors.
7. DO YOU WANT PRECISION OR LOW COST?
Calibration: Precision sensors receive a full calibration. Low-cost
designs receive single-point calibration.
Tolerances: Precision sensors have tighter tolerances on specifications such as sensitivity, i.e., sensitivity tolerance of ±5% versus
±10% for low-cost designs.
8. ARE SPECIAL APPROVALS REQUIRED?
Most industrial sensors can be certified as intrinsically safe for potentially explosive areas. Compare the type of approval needed with
a sensor's published approvals to ensure it meets your particular
Meredith Christman is a product-marketing manager with IMI
Sensors, a division of PCB Piezotronics, Depew, NY. Learn more about a
range of condition-monitoring solutions at pcb.com.