Calibration and IEPE Sensors Sold Through National InstrumentsHardware: Modular Instruments>>Dynamic Signal Acquisition and Analysis (DSA)
Problem: How often should I calibrate IEPE sensors? Solution: Routine calibration of sensors and data acquisition equipment is recommended to maintain measurement quality. Understanding the reasons for calibration and the factors that cause a measurement device to go "out of calibration" is essential for determining the calibration interval appropriate for your application. Though the term calibration applies to both measurement equipment and sensors, the actual calibration procedures and the reasons for calibration are quite different between the two. Before we begin the discussion we must clarify two important concept related terms. Strictly speaking, calibration is nothing more than a validation of the measurement device. This validation is performed by taking a known signal, measuring it with the device under question, and verifying the measurement is within expected limits. Once a calibration is performed, some devices have the ability to adjust themselves to improve the measurement accuracy. An adjustment actually stores measurement correction information locally so that subsequent measurements will more accurate. Calibrating Data Acquisition Hardware / Instrument: Every measurement instrument advertises a certain level of accuracy. An instrument cannot perform up to its published specifications forever, so the manufacturer will typically specify a calibration interval (See Figure 1). This "calibration" performs both a verification and an adjustment. Most calibration facilities generate reports that show the accuracy of the measurement device both before and after the adjustment.  Figure1: Excerpt from NI 4070 User Manual
Another strategy that is often used to improve measurement accuracy is a procedure called self-calibration. Not all devices support self-calibration but those that do use a very stable on-board voltage reference to correct for minor errors that may have been introduced by temperature and/or time. These on-board voltage references however also drift over time so even devices that support self-calibration require regular external calibration (i.e. verification and adjustment). This calibration interval is a function of the on-board reference and is specified by the manufacturer. At any rate the important point here is that for measurement instruments, regular verification and adjustment is essential to meeting the device's published accuracy specifications over the life of the product. Calibrating Sensors: One critical difference between sensors and instruments is that sensors may be calibrated but they cannot be adjusted. Calibration for sensors is still recommended but only to verify its current measurement condition. Any adjustment to the signals reported by the sensor must be done in software after the data has been digitized. Consequently sensor vendors generally advertise a wide range of accuracy for their products (or none at all). PCB Accelerometer Model Number 352C33 for instance specifies its sensitivity in this way:  Figure 2: Excerpt from PCB Accel 352C33 This spec sheet is telling you that the sensor you purchase will have a sensitivity that is somewhere between 90mV/g and 110 mV/g. +/- 10% of accuracy is hardly adequate for most applications which is why each unit also comes with a calibration certificate. This certificate lists a much more specific sensitivity (i.e. 98.7 mV/g) that was taken at a specific temperature. The key difference between a sensor and instrument is this: sensor calibration is recommended to determine the sensitivity of an individual sensor under a specific temperature and mounting condition. It is not recommended so that the sensor will meet published specifications. As a result the calibration interval for a sensor is not a hard requirement and is largely left up to the end user. Summary: There are many factors which affect sensor sensitivity but temperature, use and mounting conditions are the most common. For some applications a good "rule of thumb" is to verify the quality of your sensor on an annual basis, but it is more important to verify the sensitivity after exposure to extreme environmental conditions or prior to a critical test. Related Links: Attachments:
Report Date: 06/18/2009 Last Updated: 06/18/2009 Document ID: 4YHD5N7E |
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