Sunday, December 14, 2014

Automatic Pitot-Static Calibrator. Requirements Part 1.

If we need to assure our instrument performances it is necessary to test it. There are no shortcuts; our measurement should be compared against a reference instrument. This article is the first of a mini-series that will expose the requirements for the BasicAirData Open Pitot-Static calibrator; it is an Open project so interact freely.

A Pitot-Static probe is typically used to measure the airspeed, altitude and the climb ratio. The measurement relies on the knowledge of the total pressure and the static pressure. Total pressure is the pressure exerted by the air at the probe tip. Static pressure is the free stream pressure, referred also as barometric pressure. For further details about the Pitot-Static probe refer to the BasicAirData site.

Static pressure measurement is needed for altitude and vertical speed determination.

Total pressure measurement is necessary for airspeed calculation; often airborne instrumentation measures the differential pressure between the total pressure port and the static pressure port. In line of principle is the same of having two absolute pressure measurements, one per port. There are some practical issues with the later configuration, that aspect will not be investigated in this article.

So the first requirement for the calibrator is that it should have two pneumatic connections that can be fast fixed to the instrument under test. Both the probe and the calibrator should be accurately designed for this purpose. Commercial producers call this hardware Pitot Test Adapters and Static Test Adapters. Here below a picture from a Barfield catalog page 5



Figure 1. Barfield. Pitot Test Adapters and Static Test Adapters

This kind of adapters is not what we exactly need for the BasicAirData 8 mm probe. The 8mm probe have both the static and the total pressure ports on the tube body hence we need an adapter that can handle both the pressures at the same time. The pressure ports of the Pitot-Static Calibrator should be fitted with a removable connection hose. Same ports will be used to periodic verification of calibrator internal instrumentation.

Let's focus on the altimeter. To ensure that every physical device and software used in the altitude measurement is working properly it is necessary to check if for a known given pressure input the output is those predicted by ISA model. ISA model is a widespread used atmosphere model. The test should be carried out for a certain number of different pressures in a desired operating pressure set.

Refer to the following figure as the typical static pressure measurement equipment layout.
Figure 2. Typical static pressure measurement equipment layout

The aerodynamic behavior of the probe is not tested by a Pitot-Static calibrator. Is up to the probe designer to warrant a correct behavior, the user have the duty of carefully install the probe. By instance if the probe is not installed correctly the altitude indication during the fly can be wrong, even if the measurement equipment has been carefully tested. 
Figure 3. Typical Pitot-Static calibrator test layout

As per figure 2 the Pitot-Static calibrator should operate in closed loop. For every altitude within a test set the output of the probe is compared with the output of a reference instrument. The difference between the reference instrument output and the instrument under test output is indicated on the figure as “Error”. For each tested point the calibrator evaluates Error and if this value lies outside the required interval mark then marks as failed the test. A leading requirement is that all the test procedure should be completely automated.

Focus on the box “A”; inside this box the pressure at the static port is converted into a number. Any problem within the box “A” will be individuated by a Pitot-Static calibrator. Prior to the test all the equipment in box “A” should be checked and calibrated, otherwise the whole test will be a waste of time. In the most basic setups box “A” contains a pressure sensor connected with a pneumatic line to the static pressure port. The sensor should be calibrated to remove any treatable source of uncertainty, as can be a simple offset. I stress on the fact that if your airspeed indicator at rest on the bench top measures -3 m/s then you should take care of it prior to go further with the tests.

Given that another requirement is that Pitot-Static measurements should be available to the Pitot-Static Calibrator. With standard BAD 8mm Pitot a serial connection is available, others units may need a dedicated interface.

Now let's individuate the main requirement parameters for the Pitot-Static calibrator.
RC planes operate in the very low part of the troposphere. If you live at about 0 MSL you should be happy, check your local regulations, with an instrument calibrated within the (-50,100)m MSL range. However if you are operating from a higher altitude, for example on a hill top at 800 m MSL, then you should be able to calibrate your instrument with the same span but within the (750, 900)m MSL range. A huge part of the Earth's population lives under 4000 ASL [1 Fig, 3] 
hence it's seems adequate to set a calibration range of (-200, 4000)m MSL, using ISA model that lead to a calibration interval of (103751, 61640)Pa.
That is the pressure swing that should be managed at the static pressure port.
What are the pressures to test and the tolerances? 
FAA regulate that aspects under “FAR 43-Appendix E”

Figure 4. Tolerances table. Table I from FAR 43-Appendix E

Stated tolerance for 1000 feet (304.8m) is 30 feet (9.2m). 
We can use that tolerance values straight away, however to support GPS enhancement health and recovery systems or other high level tasks it seems more appropriate to require a tolerance of 1m (0.3048 feet). The tolerance of 1 meter is referred to the unit under test, the Pitot-Static calibrator should have and uncertainty that can handle such a requirement; an uncertainty of 0.2 m on the average of multiple measurements seems adequate. That value is the performance required to the Pitot-Static Calibrator internal static pressure reference sensor.

Using the ISA model at 0 MSL and considering an altitude variation of 1 meter we get 12 Pa of pressure variation. In the same conditions an uncertainty of 0.2m is equivalent to 2 Pa of uncertainty.

Calibration procedure should be repeated for a certain number of pressures or equivalent pressures. Best way to choose that test pressure is to fit the flight envelope of the aircraft that bear the equipment. If the aircraft is designed to operate, in respect of local regulations, between (-100,100)m MSL then is necessary to test at least the altimeter at -100m(0%), 0m(50%) and 100m(100%).If it is know that the airborne unit will operate most of the time around an altitude h then is wise to include also that altitude into the test routine.

Until now we've introduce some aspect of Pitot-Static Calibrator, with an attention to altitude calibration. Next article will cover airspeed calibration requirements.

References

[1] JOEL E. COHEN AND CHRISTOPHER SMALL (1998), Hypsographic demography: The distribution of human population by altitude, Proc. Natl. Acad. Sci. USA Vol. 95, pp. 14009–14014, November 1998. Retrieved online 9/12/2014.
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