PSN-L Email List Message

Subject: $200 microbarograph first light
From: "David Saum" DSaum@............
Date: Mon, 9 Jan 2006 15:08:10 -0500

I finally got my $200 home-brew microbarograph running yesterday,
and here are some pictures:

Here is last night's data logged and plotted hourly with the seismic
freeware AMASEIS.  Time is GMT.  Bandpass 0.01 Hz to 4.5 Hz.
The microbarograph was in a cooler, outside on the 4th 
floor deck of my condo, with no spatial wind averaging array. 
I am about 10 miles from Washington Reagan airport, and
they reported 9-15mph winds, so the peaks are probably
wind and not seismic events, nuclear tests, meteors, etc. :(

Here is some fine structure of one of last night's events:
The adc pressure resolution is about 0.004 Pa, so a 200 adc 
count peak is about 0.8 Pa.  AMASEIS allows you to block any time
segment and examine it in detail, including fourier transform
and bandpass filtering.

Exterior of microbarograph with serial cable:

Interior of microbarograph:

PCB removed from box

PCB and high-pass pneumatic filter removed from box:

System specs: 

System Bandpass: 0.01 to 4 Hz.
High-Pass filter: ~100 sec or 0.01 Hz using volume/capillary tube.
Low pass filter: 4 Hz anti-alias, 8 pole Bessel filter (MAX4701)
Differential pressure sensor: silicon, bridge type, 1"wc (DUXL01D)
Pres Resolution: ~0.004 Pa - possibly lower depending offsets
Range: +/- 120 Pa (32768 times pressure resolution)
Amp: instrumentation,1000x,single supply,micropower,RTR out(INA122P) 
Power: ~2ma 5VDC, derived from PC serial port, no exernal power supply
Regulator: 5VDC, low dropout micropower, precision (LP2950-5.0)  
Amplifer Reference Voltage: 2.5VDC fixed, precision reference (LT1004-2.5)
Microprocessor: 4mhz, onboard 16 bit ADC and serial I/O (PIC14000)
Firmware: ADC sampling at ~35 SPS, zero drift tracking ~5 min TC.
Enclosure: diecast aluminum box for shielding (Hammond 1590P)
Serial Out: RS232, 9600 baud, 8N1, ascii records, -32767 to +32767
Wind Spatial Averaging: array of micro-perforated garden tubing 
Data Log/Display: freeware PC AMASEIS seismic log/analysis software

Improvements?:  Offset voltages in the instrumentation amp and pressure
sensor seem to limit the maximum gain.  I may try a variable amp reference
voltage to eliminate these offsets. I bought some pressure transmitters
from eBay to see if their noise levels are lower: an MKS Baratron 223B and
an Ashcroft XLDP.  Unfortunately these transmitters use a lot more power
and voltage than I can steal from the PC serial port.

Kits: If there is some interest I will offer a kit for this
microbarograph.  It would be similar to my seismometer kit that
is described below, including step-by-step building instructions :
The seismometer kit was recently built by an 8th grader for her
science project.


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