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작성자지구수비대 조회 4회 작성일 2021-09-16 10:56:33 댓글 0


Calibrate - Metrology Training Lab (What is Calibration?)

What to know how to calibrate your metrology tools? In this episode of Metrology Training Lab, we discuss Calibration, which is critical to maintaining the quality and accuracy of your measuring equipment and what it means to calibrate your tools.

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Instrumentation Calibration - [An Introduction]

In this video I introduce you to instrumentation calibration. I discuss why calibration is so important in industry. Go over instrumentation calibration process and also talk about how often instrumentation is calibrated.

As we learnt in the transmitter lesson, Instrumentation is measuring the real-life conditions of a plant’s process. Lets take an electronic pressure transmitter that is measuring the pressure inside a vessel. The instrument is connected to the sites control system via a 4-20mA current signal. The control system then gives a readout to our control room operator so he can monitor and know the exact state of the plant.

But how does the control room operator know that the pressure he is seeing on his screen is the exact pressure that is in the vessel, what makes him trust that this instrumentation is accurate and reliable?

In many process industries the precision of the process variables instrumentation is measuring is extremely important, for quality of the product that is being created, The safety of people and equipment and for the profitability of the business.

Regular calibration of the Instrumentation is what gives us this confidence. Calibration is the process of comparing plant instrumentation to a known quantity to check for the instrumentation’s accuracy. Adjustments are then made if the accuracy of the equipment is out of its allowed tolerance. The known quantity Instrumentation is checked against is usually in the form of test equipment that is regularly calibrated to a high degree of accuracy.

Test equipment calibration is often carried out at dedicated calibration labs that will be accredited to certain standards depending on the country you are operating in. Here in UK UKAS is an accreditation body that will ensure a calibration lab is certifying its equipment to the appropriate standard.

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What is Sensor Calibration and Why is it Important?

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When engineers design modern process plants, they specify sensors to measure important process variables, such as flow, level, pressure, and temperature.

These measurements are used to help the process control system adjust the valves, pumps and other actuators in the plant to maintain the proper values of these quantities and to ensure safe operation.

So how does a plant maintain the operation of these sensors to guarantee that the actual value of the process is sensed and passed to the control system?

In this video, you will learn that the answer to that question is: “Sensor Calibration”.

Sensor calibration is an adjustment or set of adjustments performed on a sensor or instrument to make that instrument function as accurately, or error-free, as possible.

An error is simply the algebraic difference between the indication and the actual value of the measured variable.

Errors in sensor measurement can be caused by many factors.

First, the instrument may not have a proper zero reference.

Modern sensors and transmitters are electronic devices, and the reference voltage, or signal, may drift over time due to temperature, pressure, or change in ambient conditions.

Second, the sensor’s range may shift due to the same conditions just noted, or perhaps the operating range of the process has changed.

For example, a process may currently operate in the range of 0 to 200 pounds per square inch, but changes in operation will require it to run in the range of 0 to 500 pounds per square inch.

Third, error in sensor measurement may occur because of mechanical wear, or damage. Usually, this type of error will require repair or replacement of the device.

Errors are not desirable since the control system will not have accurate data from which to make control decisions, such as adjusting the output of a control valve or setting the speed of a feed pump.

If the calibration is too far from the accurate process conditions, process safety may be jeopardized.

To perform an “as-found” check, an accurate and precise instrument is used to develop process signals corresponding to 0%, 25%, 50%, 75% and 100% of the process range of the transmitter.

The corresponding transmitter output, in milliamps, is observed and recorded. This is called a “5-point” check.

Then, in order to check for hysteresis, a phenomenon whereby the sensor output for a process value is different going 'downscale' as it is going 'upscale', the output signals corresponding to 100%, 75%, 50%, 25%, and 0% in order are recorded.

The deviations at each checkpoint are calculated and compared to the deviation maximum allowed for the device. If the deviation is greater than the maximum allowed, then a full calibration is performed. If the deviation is less than the maximum allowed, then a sensor calibration is not required.

If we have an analog transmitter, we must adjust zero and span to reduce the measurement error. With an analog transmitter, there is a ZERO and SPAN adjustment on the transmitter itself.

Zero adjustment is made to move the output to exactly 4 milliamps when a 0% process measurement is applied to the transmitter, and the Span adjustment is made to move the output to exactly 20 milliamps when a 100% process measurement is applied.

Unfortunately, with analog transmitters, the zero and span adjustments are interactive; that is, adjusting one moves the other. Therefore, the calibration is an iterative process to set zero and span, but only 2 to 3 iterations are usually required.


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