PSN-L Email List Message
Subject: Re: $200 microbarograph first light
From: ChrisAtUpw@.......
Date: Mon, 9 Jan 2006 22:56:18 EST
In a message dated 09/01/2006, DSaum@............ writes:
Improvements? Offset voltages in the instrumentation amp and pressure
sensor seem to limit the maximum gain.
If you use a low noise differential input amplifier, but keep the gain
at a level at which it does not saturate, you can add a high pass filter and
then additional amplification and filtering. It can be a bit difficult to zero
the sensor without increasing it's noise.
In the equipment shown, the reference pressure is a very small volume
connected to atmosphere with a leak tube. The temperature of the reference
volume usually needs to be stabilised, to reduce ambient drift, especially when
using a fairly long time constant, like 100 sec. A half pint thermos flask
filled with the lightest grade of polyurethane foam makes quite a good
reference. This allows the pressure sensing to be ~isothermal rather than semi
adiabatic and increases the sensitivity. The ambient thermal response time of an
empty flask is usually over 30 mins. The 'thermal mass' can be increased by
adding candle wax inside the flask.
The sensor shown will pick up wide band ambient noise from the wind,
passing aircraft, helicopters, vehicles..... Unless you want to observe these
sorts of signal, you can use a single stage pneumatic low pass filter, with a
leak tube and a reference volume, to reduce the input noise. This may be quite
helpful in keeping the output 'on scale', particularly in very windy / noisy
conditions.
Wind noise can be reduced by using a large sensor array of porous
irrigation hose. This may be protected from rain to keep the acoustic properties
fairly constant. A large circle of hose connected to a sensor at the centre by
four to eight hard wall connecting tubes has been used for infrasound arrays,
with diameters up to several hundred feet. Radial spokes of porous hose are
also used, usually connected to the sensor with lengths of solid tube.
It is also possible to use a single sensor head with a multi hole entry
in between horizontal disks. This can significantly reduce the wind noise.
Buildings, isolated trees, water towers, pylons etc will all have downwind
vortices associated with them which can greatly increase the input signal
amplitude at a 'miss-placed' sensor, usually to over twice the crosswind dimension.
The turbulence generated by a structure like a wall or a hedge may be
detected at distances over 50x the height downwind.
Regards,
Chris Chapman
In a message dated 09/01/2006, DSaum@............ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
size=3D2>Improvements? Offset voltages in the instrumentation amp and=20
pressure
sensor seem to limit the maximum=20
gain.
If you use a low noise differential input=20
amplifier, but keep the gain at a level at which it does not saturate, you c=
an=20
add a high pass filter and then additional amplification and filtering. =
;It=20
can be a bit difficult to zero the sensor without increasing it's=20
noise.
In the equipment shown, the reference pressure=20=
is a=20
very small volume connected to atmosphere with a leak tube. The tempera=
ture=20
of the reference volume usually needs to be stabilised, to reduce ambient dr=
ift,=20
especially when using a fairly long time constant, like 100 sec. A half=
=20
pint thermos flask filled with the lightest grade of polyurethane foam makes=
=20
quite a good reference. This allows the pressure sensing to be ~isother=
mal=20
rather than semi adiabatic and increases the sensitivity. The ambient therma=
l=20
response time of an empty flask is usually over 30 mins. The 'thermal m=
ass'=20
can be increased by adding candle wax inside the flask.
The sensor shown will pick up wide band ambient=
=20
noise from the wind, passing aircraft, helicopters, vehicles..... Unless you=
=20
want to observe these sorts of signal, you can use a single stage pneumatic=20=
low=20
pass filter, with a leak tube and a reference volume, to reduce the inp=
ut=20
noise. This may be quite helpful in keeping the output 'on scale', particula=
rly=20
in very windy / noisy conditions.
Wind noise can be reduced by using a large sens=
or=20
array of porous irrigation hose. This may be protected from rain to keep the=
=20
acoustic properties fairly constant. A large circle of hose connected to a=20
sensor at the centre by four to eight hard wall connecting tubes has be=
en=20
used for infrasound arrays, with diameters up to several hundred feet. Radia=
l=20
spokes of porous hose are also used, usually connected to the sensor with=20
lengths of solid tube.
It is also possible to use a single sensor head=
=20
with a multi hole entry in between horizontal disks. This can significa=
ntly=20
reduce the wind noise. Buildings, isolated trees, water towers, pylons=20=
etc=20
will all have downwind vortices associated with them which can greatly incre=
ase=20
the input signal amplitude at a 'miss-placed' sensor, usually to over twice=20=
the=20
crosswind dimension. The turbulence generated by a structure like a wal=
l or=20
a hedge may be detected at distances over 50x the height downwind.
Regards,
Chris Chapman
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