¢ºFOR MEASURING TOTAL AND STATIC PRESSURES
OF A MOVING FLUID
United Sensor stainless steel Pitot - Static probes sense total and static pressures at the same point in a moving fluid. These measurements are often sufficient for calculating flow velocity and weight flow rate if the density is known. As a fluid's density is often a function of its temperature, it may be necessary to measure temperature in addition to velocity pressure. Pitot - Static probes may standardly be equipped with a thermocouple for simultaneous temperature measurements at the same point in the fluid stream as pressure measurements.
(See Combination Pitot - Static and Temperature Probes)

The shape of the probe measuring tip determines the sensitivity of the Pitot - Static probe to flow angularity (yaw and pitch angle error) caused by flow not parallel to the head. The length of the head governs sensitivity to Mach Number errors; the longer the head, the more accurate the reading over a wide flow range. The two most popular designs for aerodynamic measurements are the modified Prandtl type with a head 14 probe diameters long and the NPL with a head 24 diameters long. There is very little difference in calibration of the two designs, and because the Prandtl type is more compact, it is more generally used. United Sensor Pitot - Static probes are of the modified Prandtl type.


How to Order:

To order standard Pitot - Static probes, select the probe Type (e.g. Type PA, PB, etc.) best matching your application requirements. Then order from the specification chart the ordering Part Number (e.g. PAA-8-KL), which covers the exact specifications shown.

To order non-standard probes (e.g. special lengths, configurations, constructions) consult United Sensor.


SIX BASIC TYPES


TYPE PA
Fixed Take-Offs, Attached
Mounting Chuck, No Rein-
forcement Tubing.
TYPE PB
Fixed Take-Offs, Attached
Mounting Chuck, Rein-
forcement Tubing added.
TYPE PC
Removable Take-Offs, No
Mounting Chuck, No Rein-
forcement Tubing.
TYPE PD
Removable Take-Offs, No
Mounting Chuck, Rein-
forcement Tubing added.
TYPE PS
Senses Static Pressure Only.
TYPE PT
Senses Total Pressure Only.



Properties and Characteristics

MACH NUMBER RANGE

The lower usable limit for Pitot-Static probesdepends on the sensitivity of the readout deviceused with the probe. A differential pressure of 1"of water, for example is about the minimum thatcan be measured with 1% accuracy with ordinaryslant gauges, so the lower limit is approximately ata Mach Number of 0.06 or velocity of 70 ft/sec forair at standard atmospheric conditions. Whilethere is no minimum Mach Number for the probeitself, there are viscous drag effects that should beconsidered when using a probe in a low velocityfluid field. (See Reynolds Number Range). Theupper limit is at about Mach 0.95 for the totalpressure reading and 0.70 for the static as shownin Figure 1. The static reading is accurate to 0.5%to a Mach Number of 0.50 and to 1.5% up to Mach0.70. At this point the calibration becomes erraticdue to the formation of local shock waves on andaround the tip of the probe and the reading canvary as much as 10% with slight changes in flowconditions or proximity to solid boundaries.
Above Mach 1.0 both the total and static readings
vary considerably from true stream values butthey can be corrected theoretically.


Figure 1. Mach Number Range.
Pt Total pressure (impact / stagnation pressure)

BOUNDARY EFFECTS

The static pressure indication is sensitive to
distance from solid boundaries. Figure 3 shows
how this error increases the indicated velocity
pressure at a Mach Number 0.25. The probe andboundary form a Venturi passage, which
accelerates the flow and decreases the static
pressure on one side. The curve shows that staticreadings should not be taken closer than 5 tubediameters from a boundary for 1% accuracy and 10tube diameters is safer.

Figure 3. Boundary Effects

REYNOLDS NUMBER RANGE


Pitot-Static probes are not directly affected by
Reynolds Number except at very low velocities.
Therefore, in liquids where compressibility effectsare absent, their calibration is substantiallyconstant at all velocities.

The minimum Reynolds Number for the total
pressure measurement is about 30 where the
characteristic length is the diameter of the impacthole. Below this value the indicated impactpressure becomes higher than the stream impactpressure due to viscosity effects. This error is onlynoticeable in air under standard atmosphericconditions for velocities under 12 ft/sec with impactholes 0.010" diameter or less.


Figure 5. Thick wall installation.



YAW AND PITCH ANGLE RANGE

If the fluid stream is not parallel to the probe head, errorsoccur in both total and static readings. These are the mostimportant errors in this type of instrument because theycannot be corrected without taking independentreadingswith another type of probe.

Figure 2. shows the errors in total and static pressure, velocity, and weight flow at various yaw and pitch angles.


Figure 2. Yaw and Pitch Angle Error.
VP Indicated velocity calculated from Ptp and
Psp using standard equations.
W Weight flow rate - lbs. sec x ft?BR> Wp Indicated weight flow rate from Ptp and Psp


Note that yaw and pitch angle affect the readings exactlythe same. The errors in total and static pressure increase
quite rapidly for angles of attack higher than 5? but theytend to compensate each other so the probe yieldsvelocity and weight flow readings accurate to 2% up

TURBULENCE ERRORS

Pitot-Static tubes appear to be insensitive to isotropicturbulence, which is the most common type. Under someconditions of high intensity, large scale turbulence,could make the angle of attack at a probe vary over awide range. This probe would presumably have an errorcorresponding to the average yaw or pitch angleproduced by the turbulence.

TIME CONSTANT


The speed of reading depends on the length and
diameter of the pressure passages inside the probe, thesize of the pressure tubes to the manometer, and thedisplacement volume of the manometer. The timeconstant is very short for any of the standard tubesdown to 1/8" diameter; however, it increases rapidly forsmaller diameters. For this reason 1/16" OD is thesmallest recommended size for ordinary use - this willtake 15 to 60 seconds to reach equilibrium pressure withordinary manometer hook-ups. These tubes have beenmade as small as 1/32" OD, but their time constant is as
long as 15 minutes and they clog up very easily with finedirt in the flow stream. If very small tubes are required, itis preferable to use separate total and static tubes ratherthan the combined total-static type. Where reinforcing
stems are specified on small sizes, the inner tubes areenlarged at the same point to ensure minimum timeconstant.

INSTALLATION INFORMATION

Probes are installed in the
fluid stream with the impacthole facing upstream, the head parallel to
the flowdirection and the stem perpendicular. Types PA and PB(Fig. 4)
are well suited to mounting on thin - walled ductszhere the probe is to
be inserted from the outside.
Types PC and PD (Fig. 5) are designed with removablepressure take-offs.
This allows for installation fromwithin the duct where it is not practical
to make aninsertion hole diameter equal to the length of the probetip.
Figure 6 shows a correlation between probe diameter
and minimum wall insertion dimensions for a probe withfixed take-offs.


The dimensions a, b, and d should satisfy the
following:
d <= 5.8 x a - 2.9 x b
The smaller d is than the result of 5.8 x a - 2.9 x b,
the easier the insertion thru a.
Figure 6. Limiting Lengths and Diameters


Choose from a broad selection of standard pitot-static models:



Type PA:


Type PB:



Type PC:

Type PD:

Type PS:

Type PT:
Copyright ¨Ï 2002 JungJin Pressure Systems All Rights Reserved.