Description of a calibration application – in this case the calibration of a fully automatic blood pressure gauges: pressure gauge test with automatic point reading.
Introduction:
The compulsion to rationalize the production processes and therefore to produce cheaper products was in this case the reason to develope a fully auotmatic calibration unit for blood pressure gauges, as they are used thousands of times in hospitals and doctors’ surgeries.
Problem:
A manufacturer of standard analogues blood pressure gauges was foreced to change his cost-intensive manual calibration system into a fully automatic final test. Important criteria – in addition to the above metioned business considerations – was the traceability of the quality, the assignability of the single specimens to the test data and the absolute necessity – within the framework of the ISO 9000ff – to document the calibration data for the case of complaints or product liability claims.
At the time when the unit was designed approximately 20 samples were adapted simultaneously to a pressure supply by the operator. The pressure had to be set manually with pressure reducers and the pointer position had to be checked visually. When these were within the tolerances the specimens had passed the quality control and were placed in a prepared basket. Defective gauges were separated in another coloured basket and had to be forwarded to the postprocessing.
The disadvantages of this method were the impossibility of assignment, the subjectivity of each operator concerning his scale reading and the high expenditure of time for the parallelt adaption and sorting.
Measuring concept:
- Specimen mounting device with fixation
- Reading of the manometer scale by image processing
- Fast and stable pressure control
- Identification of the specimen; automatic reading of the serial number
- Assignability of the obtained calibration data for the specimen
- Creating an ambiguity for the specimens that passed the quality control
A modern workplace designed according to ergonomic aspects was created. The operator puts the specimen into a prepared adaptor and set the pressure on it by mounting a soft rubber hose end on the connection spout of the manometer.
On one part of the PC monitor the zoomed testing scale is shown – in real time. In the middle of the PC screen is a reticle. The operator moves the specimen until its pointer axis is on the reticle. By the operation of a switch the specimen is clamped in the device and fixed, at the same time the pneumatic test is started.
The pneumatic test parameters are retrievable as type definitions. Now the different pressure levels of 0, 50, 100, 150, 200, 250 and 300 mmHg are driven up and down again. At each pressure level the pressure controller makes a short break (for more technical details have a look at the electro pneumatic calibration units – EPK) – the so called pressure holding time – this offers a possibility for the camera to read the pointer position.
The camera reads the pointer position (interpolated) with the precision of 1/20 of a graduation mark, which is 0.1 mmHg.
When all calibration points are passed and all actual values are within the tolerance limits, the camera reads the graved serial number from the scale. The software saves after that the data set and prints a sticker label with barcode and plain text serial number.
The operator takes this – already half detached – sticker from the printer and put it on the backside of the specimen after removing it from the test unit.
With this lable a “good” sample is marked and can be placed in a provided transportation box.
Pressure control:
For a quick calibration an EPK 0.4 is used for the pressure control. When setting the control properties, big emphasis was put on the speed of the pressure increase up to the pressure levels. This goal could be achieved by using big control valve cross- sections.
The operation of the EPK 0.4 and the reading of the actual reference pressure is regulated by a PC with an add- on card.
Image processing:
The reading of the manometer scale is realized via CCD- matrix camera. The camera with corresponding lens is placed perpendicular to the specimen clamping device. It is placed in a darkened box equipped with black non-reflective walls. Non-reflective illumination of the pressure gauge allows the reading of the pointer position and the serial number through the glass pane.
Software:
The PC coordinates the operation or data recording of the pressure controller and the camera and gives the operator instructions during adaptation or for error handling. All “good” samples are recorded and their data is saved to hard disk. Due to the possibility that measuring data can be exported in ASCII formate, the data can also be transfered to other programs.
Summary:
The complete system consists of:
- PC with monitor and keyboard
- CCD matrix camera, lens and light
- Precision pressure controller EPK II 0.4
- Laboratory table with capacity for 19″ inputs (pressure controller, PC)
- Lable printer
- Inkjet printer and
- Specimen adaptor
This application was optimized with:
- Measuring window 80×60 mm
- Manometer scale cross-section < 56 mm
- EPK II 0.4: Measuring range 0…300mmHg, accuracy 0.05% of full scale
- Total measuring time 30 sec.
- Dimensions: appr. 1000 x 1000 x 1300 mm
- Weight: appr. 70 kg
- Power supply: 230VAC, < 500W; air pressure 6 bar, dry and oil free
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