Sunday, December 15, 2013

Pitot test and calibration

This is the first part of a mini-series. The aim is to provide an introduction to the pitot test topic and to present a DIY oriented free calibration rig.

If a basic air data system is used then calibration and test of pitot tube is a routine operation, nevertheless is a compulsory procedure if a third party pitot is used.

Also for small RC/FPV/UAS test and calibration is fundamental, in this post we will examine general preliminary requirements for test equipment.


Figure P26.1 Pitot test equipment fitted on a plane, http://www.dfwinstruments.com

During test pitot static and total pressure ports are pneumatically connected to a test instrument and then a sequence of pressures are applied, at the same time the pitot speed readings are recorded.

Video P26.1 Video shot of static and airspeed test with an analog portable device 2:30

The IAS speed is defined 

Equation 26.1
$$IAS=\sqrt{\frac{2q_c}{\rho_{base}}}$$
with \(\rho_{base}=1,225 kg/m^3\) hence it is indipendent of current air density

After a period of time from initial calibration it is necessary to ensure that our sensor is reading the correct pressure value over the full operative range. Is to be noted that to test the pitot under true operating condition is necessary to mimic the static pressure and air temperature variations with altitude. To minimize test equipment cost and complexity, in a DIY vision, only the really basic tests will be considered.

To focus on the topic we examine a pitot test that uses a differential sensor with a pressure \(q_c\) range of 12,5 mBar or 1250 Pa.
Table P26.1 Ideal, no uncertainty, input-output characteristic of a Pitot

If the sensor input range is divided into five parts we obtain the following input-output correspondence. To personalize that table you can use the provided “experimental.ods” spreadsheet.

To increase test accuracy we record \(n\) multiple samples of input differential pressure and pitot IAS . Mean of pressure measurement and IAS measurements will be reported on a table and used during the acceptance test.
After a complete test run we get a table similar to the following.


Table P26.2 Example measurements results and IAS calculated values

The uncertainty calculation must be carried out and recorded, details will be shown in a dedicated post.
The proposed test allow to evaluate, at the same time, the correct functionality of the software and of the hardware

An important part of testing is to dispose of a stable platform that allows us to work in a comfortable way.

At this stage is possible to define some requirements for the hardware that will be used during tests.

Calibration rig general requirements :
-DIY technology and cost compatible
-Portable
-Variable calibration range
-Pitot and altimeter test capable
-Usable on the field and in laboratory
-Reduced maintenance
Based on precedent test and calibration experience the followings technical requirements have been individuated.

- Manual operated, to reduce overall cost.
- Static pressure range 130 000 Pa to 90 000 Pa.
-Differential pressure range 5000 Pa
-Pneumatic connections should avoid condensate to reach reference manometer/s
-The rig should allow the operator to :
    Connect static with total port
    Insulate pressure ports
    Drain condensate
- Reference manometer/manometers requirements are linked to pitot ranges and required accuracy, more detailed description in a successive post

Refer to the following scheme to visualize a preliminary layout.
Figure P26.2 Calibration rig layout scheme

Figure P26.3 Picture of piston prototype 

All the coments are welcome, prototype is under building an testing.


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