Definition and Need for Calibration
According to Webster’s Dictionary, calibrate means “to check, adjust, or standardize systematically the graduations of a quantitative measuring instrument.” As noted in Inter-Range Instrumentation Group (IRIG) 118-98 Volume I, “a telemetry system measurement begins with the sensing of a measurand by a transducer located either on a test vehicle or at a remote test site.It ends at a data storage or display device located at a receiving test site, radio links, direct wiring, electro-optical methods, or other combinations can interconnect telemetry systems To ensure that data is of the highest possible quality, engineers test and calibrate individual components in a suitable laboratory before installing the system. Subsequently, the engineer subjects the telemetry system to carefully conducted end-to-end calibration check just before, during, and after the current test. “This chapter provides guidance to the instrumentation engineer on how to address general calibration methodologies, techniques, and cases.The instrumentation engineer uses many types of transducers to make many physical measurements. Examples of these measurements include acceleration, velocity, displacement. pressure, acoustics, flow, strain, humidity, corrosion, images, video, temperature, heat flux, position, torque, shaft power, angular rate, time, frequency, and phase angle. Each measurement may require a different calibration technique. For example, shakers and sound generators can provide known mechanical inputs to accelerometers and microphones and to voltmeter could measure the output voltage. Inexpensive transducers may require expensive calibration techniques and procedures to ensure data validity.
1. Transducer calibration which focuses on the transducer input-output output relationship
2. Data system calibration which simulates or models the input of the entire measurement system
3. Physical end-to-end calibration
Physical (also called mechanical) end-to-end calibration focuses on the relationship between the physical input and measured output throughout the entire measurement system.
The purpose of a measurement system is to accurately measure the state of the unit under test. The transducer converts one form of energy, such as force displacement and acceleration, to another form such as an electrical signal. The signal conditioner prepares the electrical signal for input to a data collection device. The transducer usually comes with a transfer characteristic(e.g. 100 mV/g), but the instrumentation engineer needs to know the final calibration through all components of the signal conditioning system.The engineer sets up the signal conditioner to receive a known input type and range and convert the signal to a standard data collection unit. Using information from the manufacturer of the transducer and signal conditioner, the engineer can calculate an output. However, whenever possible, the instrumentation engineer should perform an end-to-end calibration. Calibration involves applying known values to the unit under test under specified conditions and recording the results to improve the overall accuracy of the measurement system do this. Required data accuracy determines the need to calibrate. The cost of data acquisition is directly proportional to the data accuracy.
Usually, the transducer manufacturer performs a unit calibration in their laboratory. The instrumentation engineer should be familiar with the techniques used by the manufacturer to calibrate individual transducers. Experience recommends that the engineer perform an in-house calibration on the individual transducer to verify the accuracy of the manufacturer’s transfer function. If there are deviations from the manufacturer’s transfer function, the engineer may define a new transfer function for that unit, or in some cases, reset the device to conform to the original transfer function. Successive calibrations may indicate upcoming failures. Many engineers stop after performing a transducer calibration. They thencombine the transducer’s transfer function mathematically with the data system signal conditioner’s transfer functions. This provides a calibration estimate under the assumption that the engineer precisely knows all the transfer characteristics of the wiring and other signal conditioning between the transducer and the data storage system. The engineer assumes that all wiring and signal conditioning will function as designed, but one bad connection invalidates the data. Relying solely on transducer calibration is too risky for collection of valid data on an experiment or test.
2 Data system calibration.
For making valid engineering measurements, the most important consideration is how the transducer operates in the actual test environment with all signal conditioning attached. Although not always feasible, the transducer should be calibrated while connected to the same signal conditioning equipment in the laboratory as will be used on the actual test article. After mounting the transducer on the test article, perform a minimum of adata system calibration. This can be accomplished by simulating an excitation of the transducer such as is often accomplished for strain gages by using a shunt calibration resistor to simulate a change in resistance of the strain gage. In addition, inserting a simulated transducer signal into the system verifies all signal conditioning transfer function predictions. This calibrationsimulates transducer excitation by its physical input. Installation constraints (e.g. the transducer is inaccessible or glued onto a structure such as a strain gage) often mean that a data system calibration is the best that an instrumentation engineer can do to ensure acquisition of valid data.Data system calibration simulates the desired measurand rather than physically stimulating thethe transducer sensing device
3 Physical end-to-end calibration.
Physical end-to-end calibration is the best method of ensuring collection of valid data. As stated in the Institute of Environmental Sciences and Technology (IEST) recommended practice (RP)”, an end-to-end mechanical calibration means aa full calibration of the instrumentation from the actual physical input to the transducer to the output where the analog or digital signal will normally be analyzed.” An end-to-end calibration verifies the measurement system characteristics. Engineers perform these calibrations after installing the measurement system in the test article. A calibration source stimulates the transducer. The instrumentation engineer monitors the signal entering the data collection unit to ensure the calculated value matches the actual system transfer function. It is highly recommended that an end-to-end calibration be performed before the experiment and after the experiment is completed but before the instrumentation system is removed. The end-to-end calibration checks the measurement system (including wiring, connectors, routing, etc) installed on the test article, so that the engineer can identify and correct many potential problems early(including phasing and wiring errors).
In performing calibrations, concerns include full scale, range,output signal, linearity, resolution, cross axis sensitivity, thermal zero shifts, thermal sensitivity shifts, voltage offsets, hysteresis, repeatability, over range, bias errors, recording, record keeping, test recordings, calibration curves, and calibration reports. An instrumentation engineer should understand if the transducer manufacturer is presenting specifications as per cent of full scale or percent of reading.