This type of service may add a nominal fee and increase the time it will take for the product to be shipped; but, it will also allow the user to quickly put the device to work, offsetting both the cost and time of factory calibration.
When a device is calibrated on sight at the location where it will be used, the device is field calibrated. This is ideal if the device will be communicating with multiple devices in an integrated system. By field calibrating the device, the user will be able to set the parameters required for the device to work properly within the system.
Field calibration is common for devices that may not be removable once installed. Some applications require a device to have a calibration certificate. A common certification for measurement devices is a NIST calibration certificate. NIST stands for the National Institute of Standards and Technology and they strive to enhance productivity, facilitate trade, and improve quality of life. It includes quality management system requirements along with technical requirements.
In general use, calibration is often regarded as including the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy however this is actually two processes: calibration and adjustment. It is important therefore to understand exactly what service you require. It is also important to understand what is being calibrated and how the calibration is being performed. As an example, consider a digital thermometer that uses an external temperature probe; some calibration service providers will perform the calibration using a simulated temperature value that is applied to the thermometer only i.
Here, a test instrument is attached to the digital thermometer and a voltage equivalent to a specific temperature is applied to the digital thermometer. The result is recorded, and the thermometer is then considered to be calibrated. Many users require, and expect, a more rigorous calibration to be performed that reflects real-world usage.
The value displayed by the system being tested is then compared against the standard the system with a known or assigned accuracy from the first paragraph. Calibration defines the accuracy and quality of measurements recorded using a piece of equipment. To be confident in the results being measured, there is an ongoing need to maintain the calibration of equipment throughout its lifetime for reliable, accurate and repeatable measurements.
The goal of calibration is to minimise any measurement uncertainty by ensuring the accuracy of test equipment. To the instrument engineer or technician, calibration is the process of determining the relationship between the values of the quantity being measured and that indicated on a measuring instrument. The calibration of an instrument can be carried out by comparing the readings on the instrument with those given by a reference instrument or calibrator.
Most instrument manufacturers have sets of reference instruments against which all instruments they produce are calibrated. As components age, they lose stability and drift from their published specifications. Even normal handling can adversely affect calibration, and rough handling can throw a piece of equipment completely out of calibration even though it may appear to be okay physically.
Continuing calibration assures the equipment continually meets the specification required at installation and it should be checked frequently thereafter. Calibration is required after any maintenance to ensure that the equipment still conforms to the required calibration data. A well designed and organized calibration program often leads to benefits in quality, productivity and increased revenue. This can vary greatly within an industry or a plant.
The manufacturer usually does the initial calibration on its equipment. Subsequent calibrations are to be done by the end user or by the manufacturer as it were. The frequency of recalibration will vary with the type of equipment and the prevailing conditions where the equipment is applied.
Deciding when to recalibrate an instrument depends mainly on how well the equipment performs in the application. As a rule, however, re-calibration should be performed at least once a year.
In more critical applications however, the frequency will be much greater. Common Terms Used in Instrument Calibration:. To range an instrument means to set the lower and upper range values so it responds with the desired sensitivity to changes in input.
Suppose we want to use a pressure transmitter to measure pressure in the range 0 bar to give an output of 4 — 20mA. To range this transmitter, we simply set:. Closely related to ranging is re-ranging which simply means resetting the lower and upper range values to a different measurement range. For example, suppose we want to re-range the above transmitter to now measure pressure in the range 50 — bar we simply reset as follows:. Zero and Span Adjustments are commonly done on analog and smart instruments.
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