Understand the Basics of Calibration

 

What is calibration?

I will explain a daily life example of calibration concept.

If you are using a quartz watch to know (measure) the time, when we buy a new watch, we will adjust the time in the newly brought watch by comparing with a standard time from another precise clock.

If we did not do this adjustment; the watch will show some other time which is the incorrect time. If we simply say this comparison and adjustment of an instrument is called calibration.

Calibration is a comparison of an instrument with a highly accurate master instrument to ensure traceability to international standards. Calibration aims to reduce any measurement uncertainty by ensuring the accuracy of measuring equipment.
 
We need periodic calibration of the sensor and transmitters to ensure that the parameter measured by the sensor is correct.
 
All sensors are prone to deviation due to several factors like environmental, overloading, prolonged use, mechanical wear or damage etc.
 
Periodic calibration is used to ensure the performance, precision and accuracy of sensors and transmitters.
 
ISO/IEC 17025 standard specifies a general requirement for testing and calibrations. It covers testing and calibration quality system using standard methods and laboratory-developed or modified methods.
 

Why do we need to calibrate sensors?

A simple example in building management system; if you have 18 degree Celsius supply temperature set point in your air-handling unit and temperature sensor at the supply duct of the air-handling unit gives 20-degree Celsius instead of the actual temperature of 17 degrees Celsius,  due to sensors un-calibrated for a long time.
 
So the chilled water control valve actuator remains fully open, and fan will run at high speed even the temperature set point was actually attained, 
 
This error reading of sensor or sensor drift due to lack of calibration lead us to the energy wastage in the form of chilled water and electricity. Are you shocked? Yes, this lack of calibration will decrease the energy efficiency of our system. 
 
We have many more examples in our energy management, do you ever think about what will happen if our flow meters and electrical meters are not calibrated correctly.
 

Importance of calibration

When we design a control system, we specify different types of sensors to measure critical process variables such as temperature, pressure, flow and level etc.
 
These measured variables are used to control the process to attain the desired set point by adjusting the valves, pumps, variable speed drives and other actuators in the control system.
 
So the proper operation of these sensors to guarantee that the actual value of the process is sensed and it passed to our DDCs, PLCs or other controllers for appropriate control of the control system.
 
Calibration is adjustments performed on a sensor or instrument to make that instrument working properly within specified accuracy, without error as possible.
 
Proper sensor calibration ensures accurate measurement of the process variable, which helps to make reasonable control of the process as possible.
 
So due to the proper and periodic calibration of sensor or instruments, we can achieve higher energy efficiency, more productivity, lower cost of operation, less maintenance etc. , in our control systems, such as HVAC controls in building management systems.

Example: How to calibrate a differential pressure transmitter?

A differential pressure transmitter (DPT) is an instrument used to measure the difference of pressure in two different points in a pipeline or duct. 
 
It produces an electrical output signal like 4-20 ma current or 0-10 v voltage analogous to the input differential pressure within the calibrated pressure range.

Procedure

 
Caliberate Sensors
Isolate the pressure transmitter from the process.
 
Zero checks: Open the vent plug and vent valve to release all the trapped pressure.
 
Connect the handheld test pump with master gauge (standard pressure source) to the high-pressure transmitter port without leakage.
 
Apply 0% pressure as per the range of the transmitter(LRV- lower range value), connect the multi-meter with transmitter and ensure the output is 4 ma ( if we use 4-20 ma output signal) when zero pressure is applied. If it not, adjust zero pot in the transmitter and correct the transmitter output to 4 ma.
 
Span check: Apply 100%pressure  as per the range of the transmitter (URV- upper range value), connect the multi-meter with transmitter and ensure the output is 20 ma.If it not, adjust Span pot in the transmitter and correct the transmitter output to 20 ma.
 
Apply pressure range at  0%,25%,50%,75% and 100% and check there is any error.
 
Repeat step 2 to 5 to rectify the error.
 
An example of 5 point calibration is given below.
 
Lower range value = 0 psi 
 
Upper range value= 500 psi
 
% of Range 
Input Pressure (PSI)
Output Current (ma)- ideal
0
0
4
25
125
8
50
250
12
75
375
16
100
500
20
75
375
16
50
250
12
25
125
8
0
0
4
 

About Author: Nishad Koppilakkal

Instrumentation and Automation Engineer with experience in maintenance, operation, installation, Testing and commissioning of Process control system and Building Management System in Commercial and Industrial high raised buildings.

 

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