Sunday, December 29, 2013

Pitot test and calibration part 2

This is a follow up of a precedent post
Here is examined some practical aspect of the proposed test rig layout.
Refer to the following DIY calibration rig layout.
Figure P27.1 Calibration rig layout scheme

Previously we've talked about pitot test, neglecting all the practical aspects. 
A brief description of operation is examined. The connections to reference instrument and to pitot are based on a wide used layout, the five ways manifold or instrumentation manifold.

A typical test operation is here examined. Is quite clear that procedure can be further automatized, here is considered the use of a basic hardware DIY setup. A basic IAS test is examined, in a quite similar manner also a TAS pitot measurement can be tested.

At the beginning of test procedure all valves are closed.
The first operation to be accomplished is the connection of the reference differential pressure manometer and the pressure ports of pitot. The total pressure port should be connected to the (+) line, the instrument high pressure port should be connected to the (+) line too.
Commonly used sensors have a very little internal volume. The connection of a pressure line, practically speaking the connection of a plastic tube to a instrument nipple, will cause a sudden rise in the internal pressure. To avoid the pressure ram rise the “bypass” valve should be left open during the whole operation. If present condensate should be removed opening valves “(+)” and” (-)”.
Then open “(+)” and “reservoir” valves and unscrew the piston knob to the full back position.
Close “(+)” valve.
Connect instruments and close the valves ”(+)””(-)” and “bypass” , open the “air” valve.
Connection of instruments shall respect
Now is time to zero your reference instrument, it is a function available on almost all digital instruments , refer to manometer manual for details. After zeroing the differential pressure reading should be exactly zero.
Turn the piston knob clockwise to reach the desired test differential pressure.

P27.2 Piston with knob
Once the test pressure have been reached close the “reservoir” valve, this operation is essential to avoid problems with piston leaks.
If your reference manometer is capable of take “average” measurements than only wait for the desired number of samples is collected and note the average value. If your reference manometer takes the pressure instant value than carefully note the desired number of samples and compute the average.
Do the same to get IAS measurements.
At this point just move to the next test pressure point, open the “reservoir” valve and turn clockwise the piston knob to increase the pressure.

As you note the procedure is quite simple, anyway some precautions should be taken during the design and use of the test rig.

Pressure lines cannot be made of flexible material, that will avoid the risk to have a pressure reading modified by the variation of line volume. A short flexible pressure line segment can be acceptable, anyway use a high thickness material. Good construction materials are metal and rigid plastics, do not use silicon tubing.

The drain lines exit must be below any other point of the pneumatic circuit. Drain lines routing should avoid any obstruction to the flow of condensate. A minimum slope of 5° is to be used.

P27.2 DIY Calibration rig
Pressure lines connections of the single instruments should lie on the same plane.

All the components for test, testing equipment and tested equipment should be placed in the test room some hours before the test.

Turn on the pitot electronics and the reference manometer at least one hour before the test measurements.

The electronic used to power and reading the pitot should be the same of those that will be used during normal operation.

Work in a room with a mild temperature, around 20 °C is a good value.

Record the room temperature during the test

As soon as the prototype is ready I will post a review and off course other information to allow the construction of your own DIY pitot test rig.

Next part post
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