What is Calibration?
Calibration is defined by the way of measuring the instrument with the standard level at appropriate and suitable conditions.
Calibration of Instruments:
Instruments must be calibrated at some point of time due to several factors that includes the repetitiveness of the instrument for a particular period. It may cause errors in readings that leads to wrong values. It is recalibrated when it is in reusable condition.
Calibration of instruments is a process in which the measured values of an instrument is plotted against the standard values. If the measured values are same as the standard values, then it is in good condition and does not require calibration. If it has different displacement along the plot with standard values, then the instrument must be calibrated.
It is advisable that calibration to be done for an instrument from time to time. It will give the correct values. Calibration makes the instrument to achieve precession as well as accuracy at high levels.
Calibration is required when;
· Testing a new instrument.
· Repaired instrument.
· Checking in regularintervals.
· Random weather also effects the calibration.
· When observations are not accurate.
· Experiencing sudden shocks, vibrations adverse weather conditions.
It is easy to calibrate and fast.Calibration of thermometer involves 2 types of methods.
Method 1: Make a glass of Ice cubes and pour some water on it. Stir it well for a minute. Keep the thermometer inside the glass and make sure it does not touch the walls of a glass.Lowest temperature should be recorded as 32°F (0°C). Calculate the difference and balance the error reading.
Method 2:Make a glass full of boil waterand place the thermometer inside the boiling water. The temperature should be recorded as 212°F (100°C). Note down the readings and measure the error. Balance the error measurement then thermometer is calibrated.
Errors in measurement: While measuring an instrument there will be some errors caused by several factors. some of them are listed below.
· Indeterminate errors: These errors are of statistical probability.
· Systematic errors: These errors include method errors, instrumental errors, personal errors.
· Gross errors:This type of error occurred mainly of human negligence.
Calibration curve: It is used to measure the parameters of a measuring instruments indirectly and gives the values of a quantity as a function of output.
Calibration of pressure transducer: In this process of calibration the output value of a sensor is compared with the value measured by an accurate device. Based on the relation between the output and accurate gives the result.
In the following schematic diagram pressure transducer(P) is connected to the power supply (T) of 220V. The pressure transducer and manometer are connected to reservoir. The scale is attached to a glass pipe to read the height (mm) of water column in manometer (Horizontal tangent of meniscus should be calculated for height). The output signal of a transducer varies from 4mA to 20mA. It is measured by digital multimeter (D) and set to 20 mA.
Fig:1 Measurement Rig
Measurement process description.
Set the scale 0mm at manometer at height meniscus of water surface. Hand pump to be attached at valve G0 and pump air into reservoir until it reaches to 95cm.Pump should be operated with small and hard punches. Because of this the oscillations of water column are reduced in manometer. Close the G0 valve when water column reaches the required height and detach the pump. Make sure that pump is not leaked and start the process.
Readings are taken in 15 intervals by opening and closing valves L0 slowly. pressure in the reservoir should be placed equidistantly while carrying out a calibration. Read (hj, lj) values and write it on the table. The height of water column is measured with 1mm accuracy and output current shown by digital multimeter is written with 0.01mA accuracy. Pressure in the manometer can be calculated using the height of the water column and using the formula pj = ?ghjwhere ?=?water = 1000 kg/m3hjshould be in metres.Using the method of least squareswe obtain the results.
Caluculate the derivates of D
After the derivation we obtain:
p j I j
p j I j)?
(?nj=1 I j2)?
(?nj=1 I j)2
· Book: Mechanical and Industrial Measurements by R. K. Jain.