Archive for June 2011

The Physics of Measurements

June 13, 2011

An NPR report this morning addressed the new rotary-action mechanical heart. Instead of pulsing, it pumps like a conventional pump. (KUHF News Article) As interesting as this mechanical marvel is, an obscure quote discussing the process of measurement deserves some recognition.

Dr. Billy Cohn at the Texas Heart Institute says “If you listened to [the cow’s] chest with a stethoscope, you wouldn’t hear a heartbeat. If you examined her arteries, there’s no pulse. If you hooked her up to an EKG, she’d be flat-lined. By every metric we have to analyze patients, she’s not living. But here you can see she’s a vigorous, happy, playful calf licking my hand.”

The point to be made here is that neither the process of measurement, nor the device performing the measurement, is the process or event being measured; they are intrusive and will affect the reading. In fact, the measurement will always impact, in some way, the activity or event being measured. For example, electronic voltage measurement must withdraw a small stream of electrons to perform that measurement. This small stream represents power – measurable, demonstrable power that can, and does, modify the electronics being measured. Likewise in health, blood pressure cuffs, in a very real way, will alter the blood flow and resultant blood pressure reading. In fact, users of blood pressure cuffs are told that, after two failures at getting a clear reading, one should stop trying because the results will be skewed.

Generally measurements are performed as true real-time and as post real-time. Electrical voltage in the previous example is performed in real-time. But the speed of a vehicle is actually performed after-the-fact by measuring either the distance travelled over a specific interval or measuring the number of related events (magnetic actuator readings from a rotating wheel). Similarly, blood pressure may be an instantaneous measurement, but blood pulse rate is actually the number of pulses detected over a period of time (ie 15 seconds) or the time between pulses (a reciprocal measurement).

Having said that, most physical world measurements, including voltage, blood pressure, vehicle speed, etc.) are actually filtered and processed so that random events or mis-measurements are effectively removed from the resulting display. For example, a doctor may read 14 beats during a 15 second period leading him to declare a pulse rate of 14×4 or 56 (beats per minute). But what if he barely missed a pulse before starting the 15 second window, and barely missed the next pulse at the end? Perhaps the correct reading should be 59! Further complicate this error by mis-counting the number of pulses or be misreading the second-hand of the watch.

Typically normal measurement inaccuracy is compensated through various corrections, filters, gap fillers, and spurious removal. In this heart rate example, the actual reading can be improved by

  • removing outliers (measured events that do not cluster) typically the ‘top 5%’ and ‘bottom 5%’ of events are removed as extraneous
  • filtering the results (more complex than simple averages, but with the same goal in mind)
  • gap-filling (inserting an imagined pulse that was not sensed while a patient fidgets)
  • spurious removal (ignoring an unexpected beat)
  • increasing the size of the sample set

Discounting the fact that an unexpected beat should not be ignored by the doctor’s staff, the above illustrates how measurements are routinely processed by post measurement processes.

Finally, adjustments must be made to compensate for the impact of the measurement action, or the environment. In the case of the voltage measurement, the engineer may have to mathematically adjust the actual reading to reflect a true, un metered, performance. The experimental container may also require compensation. Youth swimming does this all of the time – some swim meets occur in pools that are 25 yards long while others occur in pools that are 25 meters long. Although this does not change the outcome of any individual race, it does affect the coach’s tracking of contestant improvement and the league’s tracking of record setting – both seasonal and over time.

So the cow in the NPR article may be clinically dead while grazing in the meadow, neither are in dispute – the cow is alive, and the cow is clinically dead. The fault here lies in the measurement. And once again, the valid and reliable scientific principles and techniques are suddenly no longer either valid or reliable.

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