Wisdom 2 and Altitude Measurement

The manual for the Wisdom 2 computer says this on page 30:

Calculations within the Wisdom change based upon the Pressure Sensor's measurement of Barometric Pressure. Changes are made when a Pressure is sensed that is equivalent to 1 foot above or below each increment of 1,000 feet. Due to the +/- 1 foot (+/- 1,000 feet altitude) Tolerance of the Sensor, it is possible at any Altitude for the Wisdom to 'display' a LEVEL and perform calculations based upon a measured Pressure value that differs from the true vertical altitude.

What, exactly, does that mean?

I understand the part about other variables possibly affecting the calculation at a particular altitude because the instrument is sensing PRESSURE, not really measuring vertical altitude.

But I don't understand the relationship between the 1 foot and 1,000 feet. What does that mean?

What I really want to know is this: What is the level of accuracy that the instrument measures altitude? Is it one foot, or is it 1,000 feet?

The LEVELS that it displays are in 2,000 foot increments, but in the Altitude NDL chart on page 108, the increments are 1,000 feet.

I'm fine if it rounds to the closest 1,000 feet, but what is this +/- 1 foot statement all about? Is the barometric pressure sensitivity really so good that it can sense the equivalent of a 1 foot change in altitude, but that it doesn't change a computation or LEVEL display until it crosses a 1,000 foot line?

I don't understand the relationship between the 1 foot and 1,000 feet. What does that mean?

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I think what that poorly written manual is trying to say is that the sensor has an accuracy of about +/1' of depth (in water) or about +/-1000' of altitude.

Going from sea level to 1000' altitude reduces the air pressure by around 4%. 4% of 1 atmosphere or 33fsw is about 1.3fsw.

Charlie99:

I think what that poorly written manual is trying to say is that the sensor has an accuracy of about +/1' of depth (in water) or about +/-1000' of altitude.

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That's what I initially thought. Others elsewhere have had alternate explanations, but I like your answer -- you are the first to actually put some math to it. I'd like to see Sherwood confirm, though.

By the way, I agree with you about these manuals. Not only this one, but other manuals for other things seem pretty sketchy for stuff that our lives may depend upon. Maybe they assume we won't read them, or that we don't care about details. Perhaps most don't. But it would be nice if one could get access (at least online) to more technical information. So many sites simply say, "Ask your dealer."

Sorry guys,

I have been out of town the last couple of days.

Charlie99's answer is correct. The pressure transducer's tollerance is +/- 1 fsw, which equates roughly to 1,000 ft of altitude. So when it is measuring barometric pressure there can be a deviation of +/- 1,000 ft.

One thing to point out. Altitude zones are displayed for reference only. Calcualtions for decompression are done on a continuous basis and are not effected by zone definitions.

Hope this clears things up.

Sherwood Rep.

Yep. That helps a bunch.

I'm glad you clarified the point about the altitude levels. The more I thought about it, the more I figured that the computer worked off actual measurements rather than empiric cut-offs.

Thanks!