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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