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[DPRG] Whetstone Bridge with AC excitation

Subject: [DPRG] Whetstone Bridge with AC excitation
From: Bill Boyer daweasel at swbell.net
Date: Tue Sep 2 22:24:01 CDT 2003

If these are pen-and-ink chart recorders, could you use an optical
sensor to "see" the pen when it goes above a preset level. Crude, but
will work for both.

> I always end up with the weird problems.
> I have a pressure sensor with a Whetstone bridge arrangement.  I need to 
> trigger a timer when the pressure is above a certain amount.
> Unfortunately a chart recorder is providing excitation and reading the 
> measurements of the pressure.
> More unfortunately there are two chart recorders that can be used and one 
> uses a 4V (to 10V) DC excitation, and the other a 7V peak to peak AC signal.
> The measurements of interest are 5 microvolts/V/mmHG or if I excite at 5V 
> DC the output should be 0.25 mV @ 10 mmHg, 1mV @ 40 mmHg and 1.5 mV @ 60 mmHg.
> So I have to tap onto these measurements, without messing up the chart 
> recorder accuracy (approx +/-1%) with a system powered from a 3V coin cell 
> battery.
> Any ideas how to make one system work for both AC and DC excitation? Then 
> how to do it cheaply?
> The first idea is to detect the zero crossing on the AC unit and predict 
> the peak and sample the outputs at the peak.  If there is no zero crossing 
> then it must be DC and sample at a regular interval.  The problem with AC 
> is the coupling to my measurement system. Still not sure what to do with it.

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