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Practical Questions

How a Bourdon Tube Pressure Gauge might be Adjusted for Linearity?

A pressure gauge is supposed to accurately indicate applied pressure over its full calibrated range.

Bourdon Tube Pressure Gauge

In this example, a gauge with a range of 0 to 500 PSI is subjected to five different pressures along that range, and its response is accurate at all those points:

Pressure gauge is subjected to five different pressures

Describe, by drawing a set of five meter readings such as the set shown above, how a pressure gauge accurate at 0% and 100% of applied pressure – but with a nonlinearity problem between the LRV and URV points – might respond to the same five applied pressures.

Furthermore, describe how a bourdon tube pressure gauge instrument might be adjusted for linearity. In other words, how may a non-linear pressure gauge be calibrated to become more linear?

Answer:

Here is one example of how a pressure gauge might respond in a non-linear fashion to the same five applied pressures, while still being accurate at the LRV and URV points:

non-linear pressure gauge

Here, the gauge reads high at the 25% point (125 PSI), slightly low at the 50% point (250 PSI), and low at the 75% point (375 PSI), while still accurate at 0% (0 PSI) and 100% (500 PSI).

Any adjustment that affects the traveling angle of the mechanism will have an effect on linearity.

Some (high-quality) pressure gauge mechanisms are equipped with an adjustable-length link to facilitate changes to this angle:

Traveling angle of Bourdon Tube

It is sage advice to leave all angle adjustment(s) untouched until all possible zero and span adjustments have been made to the instrument. Usually, it is possible to get a non-linear instrument to read within specified tolerance in a 5-point calibration just by adjusting the zero and span adjustments.

In many mechanical instruments, a simple linearity alignment is to apply a 50% input signal and check for link/lever perpendicularity (that all links and levers intersect at 90o angles to each other).

Questions For You:

Explain how keeping both “As-Found” and “As-Left” calibration records on instruments such as this pressure gauge make it possible to track long-term calibration drift.

Can a non-linearity error be corrected by adjusting the zero and/or span screws on an instrument? Why or why not?

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Credits: Tony R. Kuphaldt

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How a Bourdon Tube Pressure Gauge might be Adjusted for Linearity?

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