This table illustrates the impact on accuracy when differential pressure is low and changing the orifice plate is necessary.

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Let's suppose our meter has a DP range of 100" WC and is running normally at about 2 roots, or 4" WC. We notice at downtime the differential is above zero at about 1" WC.

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Referring to the top chart, this means when the meter is running at 2 roots the run time error is roughly 12% too high. Referring to the bottom chart, if the downtime error is -1" WC the run time error would be roughly -13%.

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Referring back to the top table, notice the 1% error drops as recording differential increases. Conclusion - keep the recording differential high (smaller orifice, lower DP range or smaller tube).

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