



¢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. PAA8KL), which covers the exact specifications shown.
To order nonstandard probes (e.g. special lengths, configurations,
constructions) consult United Sensor.

SIX BASIC TYPES
TYPE
PA
Fixed TakeOffs, Attached
Mounting Chuck, No Rein
forcement Tubing.


TYPE
PB
Fixed TakeOffs, Attached
Mounting Chuck, Rein
forcement Tubing added.


TYPE
PC
Removable TakeOffs, No
Mounting Chuck, No Rein
forcement Tubing.


TYPE
PD
Removable TakeOffs, 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 PitotStatic 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
PitotStatic 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
PitotStatic 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 hookups. 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 totalstatic 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
takeoffs.
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
takeoffs.
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 pitotstatic models:
Type PA:
Type PB:
Type PC:
Type PD:
Type PS:
Type PT:




