In aerospace applications, pressure transducers and switches play critical roles in monitoring many systems through out the aircraft – from hydraulics and electromechanical systems to avionics. Breaking, propulsion, wing flap control, cabin pressure and countless other functions rely on pressure transducers and switches to ensure that the process is happening correctly. Accuracy is critical.
Because these devices are constantly subjected to extreme vibration, frequent ambient temperature changes, and shock, regular calibration is essential to ensure accurate performance. Traditionally, many FAA-approved repair centers perform aircraft calibration with a hydraulic test media, manual screw press pressure generator, and a collection of pressure reference standards. This gets the job done but is extremely labour intensive.
Each time a technician moves to calibrating a new device or a new pressure range they have to uninstall one reference standard, reinstall and secure another standard, and then purge all the air out of the system. Time is a limited commodity for these service centers, so they are always looking for ways to streamline their processes.
Fluke recently heard from an aerospace service center that was looking at automating its pressure transducer calibration. To help them choose the best solution, we visited their facility to see the scope of their operation and determine key requirements. In this specific application the customer was looking for a solution to calibrate pressure transducers with ranges as high as 6000psi.
Automation reduces labour time
They had a choice of three automated, pneumatic pressure controllers, the Fluke Calibration 6720A, 8270A and the 8370A which all provide an integrated pressure calibration solution with multiple measurement modules in the same box. Based on their findings Fluke recommended the Fluke 8270A Pressure Controller/Calibrator with an optimal configuration of PM600 pressure measurement modules that matched the required pressure ranges and performance measurement specifications.
The 8270A controller chassis houses up to five measurement modules at the same time so the customer is now able to handle their entire workload of transducers without having to change standards. Rather than manually adjusting a screw press, waiting for the pressure to stabilize, and waiting for thermal equilibrium, they simply connect the transducer to the test stand, push a button on the front of the controller, and enter the pressure they want it to set.
The controller chooses the best reference standard from the installed modules, drives the pressure up to the target value, and stabilizes so the technician can take readings. In addition to saving time during the actual testing process, a pneumatic pressure controller eliminates much of the setup and cleanup time required when using a hydraulic test setup.
Multitasking made easy
To further automate the process, this customer chose to add COMPASS for pressure software. This PC software allows an operator to set up and initiate a multi-point calibration test of transducers, and then let the test run unattended while they complete other tasks.
A digital multimeter is also connected to the transducer to read its voltage or current output. The calibration software communicates with both the modular pressure controller and the multimeter to record all the data and stores it in a computer data file for further analysis and reporting. If an aircraft or aerospace repair center receives multiple pressure transducers of the same range, the test set up can include a multiplexer to allow automated testing of multiple transducers at the same time.
At the end of the test, the operator evaluates the collected data and manually adjusts the device under test (DUT) if necessary. If adjustments are made, they are followed by a second automated test to confirm that the adjustments brought the DUT into tolerance.
Automatically collecting the data eliminates potential mistakes that can occur with hand-written or results manually entered into a computer. When the calibration process is complete, the software produces a completely customized calibration document or certificate that includes:
- Information on the DUT and the pressure standard
- All the data that was collected, with indications of where it is in our out of tolerance
- Graphs, if desired
Test pressure switches too
Repair centers can also use a modular pressure controller to test and validate pressure switches, including programmable ramp rates to easily capture state changes.
Keep it clean
Because the 8270A is a pneumatic pressure controller, there is no calibration fluid to clean out of the pressure device when calibration is complete. To further prevent contamination from residual fluids from pressure devices used in hydraulics applications, the client opted to add the Fluke Contamination Prevention System (CPS). This accessory integrates seamlessly with the 8270A controller to ensure that clean, dry gas flows from the calibrator to the test stand and contaminated gas flows away from the calibrator when the system pressure is exhausting. This protects the reference standards and eliminates another potential source of errors.
The repair center bought the 8270A to replace its legacy calibration solution and realized immediate productivity and quality benefits. They have seen a significant reduction in pressure calibration labour, freeing up a person for at least half of their day, while using their 8270A on a daily basis.
Written by: Jeff Grossman, Fluke Corporation
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