10.4.3 Hourly And Daily Trend Computations; Table 10-5 - Evolution Of The Hourly Trend; Table 10-6 - Evolution Of The Daily Trend - GE Hydran 201 i Instruction Manual

Chapter 10 • Alarms
5. The longer the trend period, the higher the trend value in ppm is for a given steady
increase in gas level reading. For example, a constant gas level increase of
10 ppm/24 hours results in a 10 ppm/period reading if the period is set to 24 hours,
but in a 20 ppm/period reading if the period is set to 48 hours.
6. The longer the period, the smoother the trend readings are. However, it takes longer
for the trend to stabilize ("taper off ") to its final value. In the preceding example, it
would take two days to obtain the final value of 10 ppm/24 hours; in the second case,
it would take four days to reach the final value of 20 ppm/48 hours (for details on
computations, see Section 10.4.3 on page 10-12).

10.4.3 Hourly and Daily Trend Computations

A unique computation method is used for both the hourly and daily trends. In these compu-
tations, the gas level reading is processed through a first-order, digital, low-pass filter; the
trend reading (slope) is then extracted using a first-order, digital, high-pass filter. The time
constants for both filters are set to 33 % of the trend period for optimum results.
Table 10-5 on page 10-12 and Table 10-6 on page 10-12 show two examples of trend
computations.
Example 1: The gas level starts to increase at a rate of 10 ppm/24 hours. The hourly trend
period is set to 24 hours.
Initial
0 ppm/24 h +4 ppm/24 h
Example 2: The gas level starts to increase at a rate of 50 ppm/30 days. The daily trend
period is set to 30 days.
Initial
0 ppm/30 d +20 ppm/30 d
10-12

Table 10-5 - Evolution of the Hourly Trend

After 8 hours
+9 ppm/24 h

Table 10-6 - Evolution of the Daily Trend

After 10 days
Rev. 6, March 2011
After 24 hours
After 30 days
+45 ppm/30 d
After 48 hours
+10 ppm/24 h
After 60 days
+50 ppm/30 d
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