how are the signals acquired?

[Dallas Fuller]

I'm trying to understand the polygraph,  How are the signals acquired.  I know a blood pressure cuff is applied but to what pressure is it set, why and how from this is the signal acquired?  Have similar questions regarding respiration and cardio signals.  If anyone can answer or direct me to where I can get these answers I'd appreciate it.
Thanks

[George W. Maschke]

My understanding is that the pressure of the blood pressure cuff is inflated midway between the subject's systolic and diastolic blood pressure. I don't know about signal acquisition.

The discussion of the physiological activity monitored by the polygraph instrument found in Appendix D of the National Academy of Sciences report, The Polygraph and Lie Detection may be helpful to you.

[Drew Richardson]

Dallas Fuller,

The standard "cardio" channel is a pneumatically-operated channel with or without electrical amplification.  With regard to the pressure in the cuff, it has been my experience that polygraphers largely either do not know what they are doing or simply ignore good practice.  In reality what they do is after equilibrating the pressure in the cuff (massaging the cuff on the examinee's arm), they either seek a "low" pressure of 55 to 65 mm of Hg in the cuff or some seek a higher pressure of 70 to 80 mm Hg in the cuff.  The rationale given for either largely makes little sense and is largely based on polygraph folklore.  The correct way to determine cuff pressure would be at a given sensitivity setting to determine over a range of cuff pressures which pressure produces maximum signal coupling or tracing amplitude and quality.  The lowest pressure at which the aforementioned is accomplished will be the correct cuff pressure.  This may well differ for different subjects and thus is a parameter which needs to be determined individually for a given examinee on each day of examination.

George,

Assuming average blood pressure readings for adults to be something in the order of 120/80 mm Hg with a mean arterial pressure being something in the range of 95 to 105 mm Hg for young to middle-aged healthy adults, this pressure (or something between systolic and diastolic arterial pressures as you suggested) would undoubtedly be too high for appropriate cuff pressure and, in fact, would be quite uncomfortable for many examinees over the course of an examination.  The aforementioned methodology will generally lead to maximum coupling at 65 to 70 mm Hg, although, again, this will vary somewhat among examinees...

[orolan]

Drew,
Several posters here have recounted experiences wherein the cuff pressure was so high that it was painful. Some have also experienced numbing of the fingers, hand and forearm. With that in mind is it possible that the levels George speaks of, while not optimal for readings, are in fact standard practice for many polygraphers?

[Saidme]

You're grasping Orolan.

[orolan]

Saidme,
Grasping? At what? I'm really sorry that you are too ignorant to participate in valid discussions on polygraphy. Perhaps you should go back to school and finish the last 9 grades, because you obviously skipped them.