Automatic Pressure Calibration System For Pressure Sensors

Pressure sensors are devices that sense the pressure and convert it into electrical signal whose magnitude depends upon the pressure applied. These sensors are employed in ground tests during engine development for aircrafts. The output (electrical signal) provided by these sensors form part of input to airworthiness checks of the aircraft. Hence, it needs to be re-verified or re-calibrated to evaluate its performance at specified or regular intervals of measurement and control. Towards this objective, sensor calibrations are performed using comparison method at Calibration Laboratory. Manual pressure calibration using hand pumps is a tiresome and time consuming process for metrologists. Metrologists find it very difficult to generate different steps and monitor pressure. The efforts will be further magnified in a laboratory environment where there will be a need to calibrate more numbers of pressure sensors on a regular basis. The situation further gets complicated with the different types and ranges of outputs from pressure sensors. The setup consists of a Precision Pressure Controller, Data Acquisition/switch unit and a Pressure manifold. An effort is made to exploit the capabilities of these instruments by automating the pressure calibration process by developing an automation software using NI LabVIEW.


INTRODUCTION
Most of the structures which are light-weight are quite flexible but are prone to vibration.In this regard pressure sensors are used routinely to test the aero engines, wind turbine and other mechanical structures [1][2][3][4][5][6].In addition to this the pressure sensors are also used to measure vibration levels.Calibration of pressure sensors is one of the most important aspect in the field of measurement and control.It is very much necessary to conduct a performance check of these sensors at regular intervals.In calibration laboratory, the calibration of pressuresensors is carried out using comparison method [7,8].The setup consists of a Pressure Controller/Calibrator, Data Acquisition/Switch unit, pressure distribution manifold and a computer that controls all these instruments.The output of the pressure sensors under calibration (will be called DUT here on wards) is compared with the standard pressure controller/calibrator and the results are recorded.Initially the pressure calibration was done manually.Depending on the range of the pressure sensor, five set points within the Range for calibration is decided and set using the controller front panel menus one after the other and the output of the sensor (also called a Device Under Test, DUT) was measured using Data logger individually [9].The process was time consuming and prone to manual errors.
An automation of pressure calibration using pressure controller/calibrator and data acquisition/switch unitwas planned using NI LabVIEW software.Software is developed to interface pressure controller/calibrator and data acquisition/switch unit using serial port.The software is controlling thecontroller with set point generation and measure the output of the DUT at the data acquisition/switch unit.The features of the software are automatic set point generation, overpressure protection and data logging.

EXPERIMENTAL SETUP
An illustration and actual setupof pressure calibration system as shown in Figure 1 and 2 respectively consists of a personal computer interfaced with pressure controller/calibrator, PPC4 and a data acquisition/switch unit, 34970A.The instruments are controlled by a personal computer.They are connected to computer using serial interface, RS232.Software is developed using Lab VIEW.The DUTsare connected using a multi-point feeding system (pressure distribution manifold) to the pressure controller.This enables many DUTs to be connected at a time for simultaneous calibration.

PPC4, Pressure Controller/Calibrator
The pressure controller/calibrator is a stand-alone, pressure controller intended to precisely set and control gas pressure into a closed volume as is commonly needed for the calibration and testing of pressure measuring instruments and is shown in Figure 3.
PPC4 uses Quartz Reference Pressure Transducers (Q-RPTs).A reference barometer is also included.The on-board barometer is used only to measure changes in atmospheric pressure to provide dynamic compensation of the Q-RPTs atmospheric pressure offset when using an Axxx (absolute)(Q-RPT to make gauge pressure measurements.The accuracy of this controller is ±0.012% of reading.

Data Acquisition / SwitchUnit
The data acquisition/switch unit is a 6½ digit Multimeter which offers good stability and noise rejection designed to measure thermocouples, resistance temperature detectors, AC/DC voltage, AC/DC current, resistance and frequency which is shown in Figure 4.The instrument has multiplexer cards (maximum of 3 upto 20 numbers) where DC measurements per multiplexer card can be made.'Z' electrical signals to be measured are connected to these multiplexer cards and are addressed by the channel numbers.In this program this instrument is used to measure DC voltage, DC current & frequency with an accuracy of ±40ppm in DC voltage.

Pressure Distribution Manifold
The pressure distribution manifold pneumatic is a compact and easy to use system for connecting maximum of 6 sensors simultaneously which is required to be calibrated.This manifold has one input port where pressure standard is connected.It also has a port to connect indicating device like pressure gauge.The manifold (as shown in Figure 5) has built in valves to isolate the port which are to be blanked during use.The pressure distribution manifold pneumatic has a working range between Vacuum and 3.5 MPa.

Front panel
The front panel of the program is given in Figure 6.The front panel is the main menu which interacts with the user.The user has to initially configure PPC4 for pressure units, pressure mode and control mode from the front panel buttons.The user has to configure the data acquisition/switch unit also for voltage/current /frequency measurement, as per the sensor output.The channel numbers where the output of the sensors are connected in the multiplexer card of data acquisition /switch unit also has to be entered along with the details of the DUT, i.e., manufactured by, model number, Serial number, range and accuracy of sensors.
Once the configuration is completed the operator can initiate the calibration process.The software will step by step perform calibration and give the results and generate report automatically which is saved in an excel format.After calibration the program vents the system automatically and puts it into the safe mode.Front panel of the program is given in Figure 6.

RESULTS AND DISCUSSION
The calibration result generated from the software for one pressure transmitter (Manufactured by: Druck, Model No.: PTX511)is shown in Table 1.The calibration graph for the pressure transmitter is shown in Figure 7.The calibration result generated from the software for one pressure transducer (Manufactured by: Druck, Model No.: PTX511)done is shown in Table 2. Taking the calibration result from the Table 2, the calibration graph is plotted for the pressure transducer and is shown in Figure 8.The accuracy is calculated based on the following formulae: Conversion of pressure to voltage in pressure transducers with voltage output is given by following expression [10], Conversion of pressure to Current in pressure transmitter with current output (4 -20 mA) is given by following expression [10], ( ) Full scale pressure For % of reading we have [10], ( ) For % of FS we have [10], ( ) where IP is input pressure.The calibration of one pressure sensor and one pressure transmitter is presented here.The result is automatically generated and provided in the form of report by the software referenceTables 1 -6.The accuracy deviation is listed in the right most column indicating the accuracy status whether the sensor is within or out of accuracy.The calibration graph gives a visual inspection of calibration deviation immediately.The results are obtained quickly in a tabular and graphical format [11].The proposed facility is a new development in the field of calibration.Such a setup is not available commercially.The proposed automation has reduced human errors and speeded the calibration activity.The errors of operation are also reduced.It is possible to calibrate many sensors at the same time.It also provides at most safety to the user and the equipment.This has reduced the calibration time from 18 hours to 1 hour.

CONCLUSIONS
The automation of sensor calibration carried out using PPC4 and software is helpful in calibration of 60 sensors at a time.This system has made the calibration work faster and simpler by making the calibration process fully automatic.This system is capable of generating the report of observations along with the plot of pressure verses electrical output automatically.The measurement system capability is fully utilized by using the software program.Apart from enhancing productivity, this calibration system has brought in repeatability and consistency by making the calibration process operator independent.Hence, routine calibration work load of pressure sensors can be achieved more effectively.

Figure 9 .
Figure 9. Calibration graph for pressure transducer

Figure 11 .
Figure 11.Calibration graph for pressure transmitter

Table 1 .Calibration result of pressure transmitter
Figure 7.

Calibration graph for pressure transmitterTable 2 . Calibration result of pressure transmitter
(* Indicates out of tolerance data.)

Table 4 . Calibration result of pressure transducer
Figure 10.Calibration graph for pressure transducer