PerfectFlite Pnut User Manual

Pnut Users Manual

PO Box 29
Andover, NH 03216 URL: www.perfectflite.com
Voice [phone] Sales: [email protected]

FAX [phone] Support: [email protected]

A miniature, high accuracy, data-logging altimeter

for advanced model rocketry research.

Pnut Users Manual

Contents

Introduction................................................................................................. 1

Parts Identification Diagram.............................................................. 3

Installation

Payload Bay Construction........................................................................................... 4

Static Pressure Sampling Ports................................................................................. 6

Operation

Charging Battery............................................................................................................. 7

Power Switches............................................................................................................... 7

Numerical Reporting..................................................................................................... 8

Powerup Sequence........................................................................................................ 9

Downloading Data.......................................................................................................... 9

Changeable Settings....................................................................................................... 9

Tips for Achieving Optimal Accuracy.................................................................. 14

Telemetry Information.............................................................................................. 16

Testing............................................................................................................................... 18

Cautions............................................................................................................................ 19

Specifications............................................................................................ 20

Warranty.................................................................................... Back Cover

1

Introduction

The Pnut is a high-quality, high-precision, full-featured rocket

altimeter that acquires flight data continuously during your
rocket’s flight for later download and inspection. A large
onboard memory stores altitude, temperature, and battery
voltage data sampled 20 times per second for up to 9 minutes
per flight, for the last 31 flights. It also provides immediate
post-flight reporting of peak altitude and maximum velocity
without the need for a computer. It is ideal for intermediate to
advanced rocket education, advanced science fair projects, and
contest use.

The Pnut is installed inside your rocket and activated prior to

launch. When you retrieve your rocket, the Pnut will report
the apogee altitude (how high your rocket went, up to 100,000
feet above ground) and the maximum velocity (how fast your
rocket went, in miles per hour). It uses a convenient, easy to
understand audio reporting method that is audible from
outside your rocket, so removal or inspection of the altimeter
is not necessary. The reported altitude and velocity are
alternated with a 10 second siren sound that helps to locate
your rocket even if it is hidden in tall grass or a tree.

Using the optional USB Data Transfer kit, the full flight data
(altitude, temperature, voltage) can be downloaded to a
computer for storage, graphing, and additional manipulation.
Because of the generous memory size, it is not necessary to
bring the download equipment to the launch – 31 flights can be
made before download of the data is necessary.

If you want to step up to electronic parachute deployment, you
may want to consider the PerfectFlite StratoLogger, which has
the Pnut’s flight data download features and adds two event

2

electronic parachute deployment for advanced users.
Electronic deployment provides the ultimate in control over
when your parachute is ejected – no more guessing about
which ejection delay length to use, the altimeter will always
fire precisely at apogee.

The Pnut utilizes a precision pressure sensor and 24 bit delta

sigma analog to digital converter to obtain an extremely
accurate measurement of the air pressure surrounding your
rocket. When turned on, the altimeter “tracks” the ambient
pressure surrounding your rocket to get an up-to-the-second
reading of the barometric pressure at ground level. As the
rocket rises, the pressure decreases, and the altimeter converts
the pressure differential to a precise measurement of altitude
above launch point according to the US Standard Atmosphere
model. All of the calculations are done inside the altimeter,
with the results reported simply as “altitude above ground
level”. No conversion or adjustment is necessary.

3

Parts Identification

Refer to the top of the altimeter to locate the following items:

A) Beeper: Audibly reports altitude and velocity after flight.

B) Battery: Built-in rechargeable Lithium Polymer battery.

C) Power Pins and Jumper Shunt. Shunt shown in storage (“off”) position.

Place shunt across both pins to turn altimeter “on”.

D) Charge Connector: Supplied charging cable connects here.

E) Data Connector: Connect to optional data transfer kit and computer for

post-flight download of flight data. Can also be used for access to in­flight telemetry data.

F) Charge LED: Lights during charging, goes out when charge is complete.

4

You should perform initial testing on your rocket without the

altimeter installed. Make sure that the parachute is ejected
and opens properly so that you have a slow and safe landing. If

Installation

For best results, your altimeter should be installed in a
separate payload compartment, sealed from the pressure and
heat of the ejection charge gasses. While other alternatives are
possible, isolating the altimeter in a protected compartment
will provide the most precise readings and will keep high
temperature and pressure from affecting the long-term
accuracy of the instrument.

A typical payload compartment consists of a section of body

tube behind the nosecone with a sealed tube coupler
connecting it to the main body tube (see illustration below).
Some rockets (e.g. Estes “Nova Payloader”, Quest “Zenith II”)
already have such a payload section, and one can be added
easily if yours does not. Use pieces of foam rubber in front of
and behind the altimeter to prevent it from shifting under
acceleration and deceleration and to protect it in the event of a
crash. The altimeter will slide into 18mm/BT20 size body
tubes, and a “sleeve” made out of standard foam pipe
insulation can be used for larger size tubes. Your payload
section should close securely so that the altimeter is not

“ejected” upon motor burnout deceleration or chute

deployment shock.

5

you conduct your preliminary tests with the altimeter installed
and the chute doesn’t eject, the resulting high speed ballistic
descent will likely damage the altimeter (and your rocket!).

When installing the Pnut in larger rockets it may be easier to

add a short (~3” long) section of BT20 tube with padded end
plugs for the altimeter to ride in. The short BT20 tube could be
glued to the inside of the larger airframe or to a plywood
mounting plate. A static pressure sampling hole can be drilled
through the main airframe and into the inner tube to allow
external air pressure to get to the altimeter.

As a last resort, if accuracy isn’t of paramount importance, you

can simply tie the altimeter to the rocket’s shock cord and pack
it in along with the chute. If you must do this, observe the
following precautions:

1. Use plenty of wadding between the ejection charge and the
parachute.

2. Position the parachute between the wadding and the
altimeter to provide additional protection from the hot
ejection charge gasses.

3. Make sure the altimeter is securely tied to the shock cord
so that it doesn’t separate and free-fall.

4. Add a wrap of tape around the jumper shunt so it doesn’t
get dislodged at ejection. Note: Make sure the tape doesn’t

cover the pressure sensor (small white rectangle marked

“U3”) on the bottom of the altimeter.

6

Static Pressure Sampling Holes

You must drill one or more clean-edged holes in the payload

compartment to allow outside air pressure to be sampled by
the altimeter (see table below for recommended sizes). These
holes should be as far away from the nosecone shoulder and
other body tube irregularities as possible (at least 3 times the
body tube diameter or more) to minimize pressure
disturbances being created by turbulent airflow over the body
tube. Sand the area around the hole as necessary to eliminate
flashing or raised edges.

Best performance and greatest accuracy will be achieved by
using four smaller holes distributed at 90 degree intervals
around the body tube’s circumference instead of a single larger
hole. When using four holes, each hole should be ½ the size of
a single hole as noted in the table. This will minimize the
pressure variations due to wind currents perpendicular to the
rocket’s direction of travel.

Payload Payload Single Four
Diameter Length Hole Size Hole Size

<1.5” 6” .024” .012” (small pinholes)

1.6” 6” .024” .012” (small pinholes)

2.1” 6” .042” .021”

3.0” 8” .113” .057”

3.0” 12” .170” .085”

3.9” 8” .202” .101”

3.9” 12” .302” .151”

Other sizes:

Single hole size = Diameter * Diameter * Length * 0.0016
Four holes, each hole = Diameter * Diameter * Length * 0.0008

7

Operation

Battery

The altimeter is powered by a built-in Lithium Polymer

rechargeable battery that should never need replacement. The
battery is charged using the supplied charging cable and a
standard 9V rectangular battery. When the cable is connected
to the altimeter and a 9V battery, the “charge” LED on top of
the altimeter will light. When the charge is complete the LED
will go out. A full charge takes 2-3 hours if the onboard battery
is fully depleted. As long as the altimeter is turned off between
uses, a full charge can easily last for a year’s worth of typical
rocket launches. A single fresh 9V alkaline battery can provide
up to 6 full charges to the Pnut’s battery.

Tip: You can also use partially depleted batteries (e.g. old smoke

detector batteries) and the Pnut’s charger will extract any
remaining life out of them.

Power Switch

The jumper shunt shown in the picture on page 3 is used to

turn the altimeter on and off. Install the jumper shunt across
the two pins on the altimeter’s circuit board to turn the
altimeter on, and remove the shunt to turn it off. When the
altimeter is not in use, you can place the jumper shunt on just
one of the pins for storage.

For added convenience, you can use the optional power switch
cable to connect an externally-accessible on/off switch to the
altimeter. The cable plugs onto the two pins on the circuit
board in place of the jumper shunt, and your switch is
connected to the loose ends of the cable.

8

Numerical Reporting

Numbers are reported as a long beep (separator), followed by
a pattern of shorter beeps for the individual digits, with a
pause before the next digit. You simply count the number of
short beeps for each digit place and assemble them together to
form a number. You will hear a series of beeps for the first
digit (tens of thousands of feet), a short pause, another series
of beeps for the next digit (thousands of feet), etc.

Leading zeroes are suppressed: 1,582 feet would be
represented with four digits as in 1582, not five digits as in

01582.

Ten beeps are used to indicate the number zero (if zero beeps

were used, you would not be able to differentiate between
2200 feet and 22 feet!).

As an example, 12,560’ would be reported as:

long beep-pause-beep-pause-beep-beep-pause-beep-beep-beep­beep-beep-pause- beep-beep-beep-beep-beep-beep-pause-beep­beep-beep-beep-beep-beep- beep-beep-beep-beep-long pause

Digit Reported as:

0 beep-beep-beep-beep-beep-beep-beep-beep-beep-beep
1 beep
2 beep-beep
3 beep-beep-beep
4 beep-beep-beep-beep
5 beep-beep-beep-beep-beep
6 beep-beep-beep-beep-beep-beep
7 beep-beep-beep-beep-beep-beep-beep
8 beep-beep-beep-beep-beep-beep-beep-beep
9 beep-beep-beep-beep-beep-beep-beep-beep-beep

9

Powerup

When the altimeter is turned on, it will report the peak altitude

from the last flight and the current battery voltage before
readying itself for flight. This is what you will hear:

• A three to six digit number (range of 160 feet to 103,500
feet) representing the apogee altitude of the last flight.

Note: A warbling siren tone will sound instead of the last
flight altitude if power was lost during the last flight.

This error will clear after the next good flight.

• A two second pause, and then a three digit number
representing the battery voltage in hundredths of a volt (e.g.

3.97 volts would report as 397).

• A thirty second pause (giving you time to close up the
rocket after turning the altimeter on), and then a periodic

“chirp” approximately once per second when the altimeter is

ready to launch.

While sounding the launch ready “chirp”, the altimeter will

begin tracking ground level pressure, and will continuously
update its internal ground reading to follow fluctuations in
ground level pressure until time of launch.

The altimeter is ready to launch at this point.

Do not launch before the periodic “chirp” is heard or the
altimeter will not function properly!

After flight the altimeter will report in this sequence:

• An extra-long tone to indicate the start of the reporting
sequence.

10

• A three to six digit number representing the peak altitude
in feet.

• A long separator tone followed by a two to five digit
number representing the maximum velocity during the
flight in miles per hour. This number, and its preceding
separator, are reported in a higher pitch to differentiate it
from the peak altitude number.

• A pause of 5 seconds, and then a 10 second warbling siren
tone to aid in locating the rocket if it is hidden from sight in
a tree, tall grass, etc.

• After a 10 second period of silence, the sequence repeats
until power is disconnected. The flight’s peak altitude is
preserved when power is turned off, and will be reported
every time power is turned on until a new flight is made.

Downloading Data to a Computer

If you have the optional USB Data Transfer Kit and a USB­equipped computer, you can download full flight data from the
altimeter’s last 31 flights to your computer. Instructions for
installing the software and downloading the data are included
with the data transfer kit. The download procedure is very
simple and involves connecting a cable to the altimeter’s “Data”
port, turning the altimeter on, and selecting the flight you want
to download. After a few seconds an altitude vs. time graph
will appear with the data from your flight. You can also choose
to display graphs of velocity, temperature, and battery voltage.

Changing Settings with a Computer

The Data Transfer Kit can also be used to modify several

parameters that control how the altimeter functions.

11

The following descriptions assume that you have the software

running and the altimeter connected to the computer and
powered up. Select the “Settings” menu item from the

“Altimeter” menu. A screen will appear with the current

settings, altimeter model, altimeter serial number, firmware
revision level, and total number of flights listed. Note: You

must make the initial connection to the altimeter while the last
flight altitude and battery voltage are being reported; if you wait
until the altimeter is making the “launch-ready chirp” it will not
communicate with the computer. This prevents possible
electrical noise from interrupting data recording during flight.

Launch Detect

You can change the Launch Detect Altitude by selecting the text,

entering a new number, and hitting “update”. The Launch
Detect Altitude controls the point that the altimeter recognizes
liftoff and begins to acquire flight data. If a flight doesn’t make
it to this altitude no data will recorded and no altitude/velocity
information will be reported when you recover your rocket.

The altimeter also stores 28 data points (1.4 seconds) of pre-

liftoff data prior to the time that it reaches the launch detect
altitude. This will include data back to the point that the
rocket was stationary on the pad in most cases.

Valid settings range from 40 feet to 300 feet above ground level.

Lower numbers allow data acquisition from low-altitude flights
(e.g. smaller water rockets), while higher numbers provide
more resistance to accidental triggering on the pad from wind
gust-induced pressure fluctuations in the payload compart­ment. Factory default is 100 feet, which accommodates most
flights and also provides considerable resistance to very strong
winds (assuming that you have used properly sized Static
Pressure ports as described earlier).

12

Powerup Delay

An additional delay can be inserted between the time that the

altimeter finishes reporting battery voltage on powerup and
when the “launch-ready chirps” start as the altimeter awaits
launch. This is typically used when a power switch is not
accessible from the outside of the rocket and additional time is
needed after power is applied to provide time to close up the
altimeter compartment and allow pressure to stabilize before
the altimeter begins to look for a valid launch condition. If the
altimeter is looking for pressure changes when you close up
the payload compartment, pushing the rocket parts together
can cause enough pressure fluctuation to trigger the altimeter,
resulting in a false launch detection. If this happens, the
altimeter will begin beeping out bogus altitude and velocity
data instead of the normal “launch ready chirp” and it will not
record the flight properly. In this case, remove the altimeter,
turn it off and back on, and close the rocket back up before the
chirp starts.

There is an unchangeable base delay time of 20 seconds built-

in. The Powerup Delay adds additional time to this base delay.

Acceptable settings are 0 (no additional delay) to 60 seconds.

Factory default setting is 10 seconds additional for a total of 30
seconds.

Siren Delay

Shortly after landing, the altimeter begins its beeping sequence
to report peak altitude and maximum velocity, with a pause of
about 10 seconds before the sequence repeats. If the Siren
Delay is set to a number other than 0 (disabled), the 10 second
pause will be preceded by a delay as specified by the Siren
Delay value, followed by a 10 second warbling siren from the
beeper. This sound makes the rocket easier to locate, even if it
is hidden from view by tall grass or other obstacles. The

13

altimeter goes into a low power sleep mode while it is silent so
that with the maximum siren delay of 120 seconds a full
battery charge will continue to operate the altimeter for over
two weeks. Even if you don’t find your rocket the first day,
you’ll have many more opportunities before it goes silent!

Valid settings are 0 (siren disabled) to 120 seconds. Factory

default setting is 5 seconds.

Telemetry

The telemetry option, when enabled, sends altitude infor-

mation to the data port in real time during the flight. This
information can be sent to a ground receiver via a user­supplied RF modem so you can see how high the rocket is at
any point from the ground. It can also be used by other
custom payload devices so that various functions can be
controlled at different altitudes (air sampling, payload release,
etc.).

The settings are controlled by a popup menu with three

options:

Never: Telemetry is disabled, and no information is sent

to the data port in real time.

OnPad: Telemetry is enabled. Altitude information is sent

beginning at the same time as the start of the
continuity check beeps and continues until the
rocket has landed.

OnLaunch: Telemetry is enabled. Altitude information is sent

beginning at the launch detect point and continues
until the rocket has landed.

More information about the serial port settings and the
telemetry data format is available on page 16.

14

Tips for Achieving Best Accuracy

• Use four static sampling ports instead of just one. Make
sure they are sized and positioned according to the
instructions in the previous section. All barometric
altimeters base their altitude measurements on the air
pressure surrounding the rocket, so getting a clean,
turbulence-free sample is essential. A single hole,
especially if it is over-sized, will introduce pressure
fluctuations whenever the rocket deviates from its normal
trajectory. Four evenly-spaced holes will minimize this
effect.

• With a properly designed rocket and motor combination,
the parachute should eject at apogee (peak altitude), when
the rocket is nearly stationary. This will guarantee a
minimum of turbulent airflow around the rocket, and hence
the cleanest, most accurate data. If you eject your
parachute substantially before apogee, the rocket will still
be traveling at a high rate of speed, which will degrade the
accuracy of any possible measurements due to the massive
fluctuations in pressure. In addition, deploying the chute
while the rocket is traveling at high speed can potentially
damage your rocket due to a zippered body tube, stripped
chute, or broken shock cord.

Ejecting at apogee is best, slightly after apogee is OK, but

never before apogee if you can avoid it. Ejecting before
apogee will always guarantee a loss in potential altitude. It
will also introduce significant degradation in altitude
repeatability since the final altitude will then be
determined by the (in)consistency of the motor’s ejection
delay.

15

• Use a long shock cord. This will allow the ejected payload
section and nose cone to slow gradually rather then being
jerked to a stop when the cord comes to full extension.

Again, minimizing abrupt changes in the rocket’s trajectory

will result in the smoothest, most accurate data.

16

Telemetry Information

The data connector pinout is shown below. All signals are 3.3V

CMOS logic level, and the lines idle “high”. Please double-check

your cables before connecting anything to the data port; make
sure that the input to your device is connected to the
altimeter’s output. Do not connect anything to the pins marked

“NC”. Do not ground any pins other than pin 5, and never apply

an external voltage to any of the pins.

Pin# Function

1 N/C
2 +3.3V (do not use)
3 RX data input (unused)
4 TX data output
5 GND

Serial port settings should be configured for 9,600BPS, 8 data
bits, no parity, and 1 stop bit. Output data format from the
altimeter is ASCII text based, with a <CR> and <LF> character
appended to the end of each line.

17

When telemetry data is selected to start “OnPad”, the first value

sent will be the approximate ground elevation, and all
subsequent data will be AGL (Above Ground Level) altitudes:

880<CR><LF> {first data point is launch elevation MSL}
0<CR><LF> {all subsequent data points are AGL altitude}
0<CR><LF>
0<CR><LF>
5<CR><LF>
25<CR><LF>
56<CR><LF>
99<CR><LF>
149<CR><LF>
198<CR><LF>
248<CR><LF>
300<CR><LF>
349<CR><LF>
398<CR><LF>
447<CR><LF>
497<CR><LF>

When telemetry data is selected to start “OnLaunch”, all data

will be AGL altitudes:

198<CR><LF> {all data points are AGL altitude}
248<CR><LF>
300<CR><LF>
349<CR><LF>
398<CR><LF>
447<CR><LF>
497<CR><LF>

18

Testing

A simple apparatus for testing the altimeter can be made with a

small jar and a length of plastic hose. Drill a hole in the center
of the jar’s lid and insert one end of the plastic hose. Glue hose
in place to achieve a tight seal (hot melt glue works well).

Turn on the altimeter and place it in the jar. Tighten the lid and

wait until you can hear the periodic beep from the altimeter
indicating launch readiness. Suck on the free end of the plastic
hose to create a vacuum within the jar. The altimeter will
sense this as a launch condition and the beeping will stop.

When you stop sucking on the hose, the altimeter will sense

apogee as the pressure stabilizes. Open the hose and allow air
to bleed back into the jar and the altimeter will sense descent.

The altimeter will then beep out the “altitude” that your

vacuum was able to create within the jar.

19

Cautions

• Do not touch circuit board traces or components or allow
metallic objects to touch them when the altimeter is
powered on. This could cause damage to your altimeter.

• Charge the battery using the supplied charging cable; never
exceed 15 volts charging voltage.

• Do not charge the battery when the altimeter is colder than
32F or hotter than 115F.

• Do not charge the battery or use the altimeter if the battery
has been punctured or shows signs of damage; contact
PerfectFlite for details on obtaining a replacement battery.

• Provide adequate padding fore and aft of the altimeter for
protection in the event of a crash or excessively hard
landing.

• Do not allow the altimeter to get wet. Only operate the
altimeter within the environmental limits listed in the
specifications section.

• Do not rupture pressure sensor diaphragm with excessive
pressure or sharp object.

20

Specifications:

Power: 3.5V to 4.2V, built-in 3.7V LiPo battery

Current consumption: 1.5 ma

Battery life: 100 hours (40% pre-flight, 60% post-flight)

Launch detect: 40’ to 300’ AGL, default is 100’

Maximum altitude: 100,000’ MSL

Altitude resolution: 1’ up to 38,000’MSL

< 2’ to 52,000’MSL

< 5’ to 72,000’MSL

Analog to Digital Converter: 24 bit Delta Sigma

Calibration accuracy: +/- 0.05% typical

Measurement precision: +/- (0.1% reading + 1 foot) typical

Sample rate: 20 samples per second

Operational temperature: -10C to +60C (14F to +140F)

Charging temperature: 0C to +45C (32F to +113F)

Dimensions: 2.50”L x 0.59”W x 0.45”H

Weight: 0.26 oz. including battery

Warranty

All PerfectFlite products include a full three year/36 month

warranty against defects in parts and workmanship. Should
your PerfectFlite product fail during this period, call or email
our Customer Service department for information about
returning your product. The warranty applies to the altimeter
only, and does not cover the rocket, motor, or other equipment.

This warranty does not cover damage due to misuse, abuse,

alteration, or operation outside of the recommended operating
conditions included with your product. Broken pressure
sensor diaphragms due to puncture or exposure to ejection
charge pressure/hot gasses are NOT covered under this
warranty.

Liability

Due care has been employed in the design and construction of
this product so as to minimize the dangers inherent in its use.

As the installation, setup, preparation, maintenance, and use of

this equipment is beyond the control of the manufacturer, the
purchaser and user accept sole responsibility for the safe and
proper use of this product. The principals, employees, and
vendors of the manufacturer shall not be held liable for any
damage or claims resulting from any application of this
product. If the purchaser and user are not confident in their
ability to use the product in a safe manner it should be
returned to the point of purchase immediately. Any use of this
product signifies acceptance of the above terms by the
purchaser and user.