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measurements taken within the time specified by the response time. The expo-
nential filter simulates a 1-pole analog filter where more recent measurements
are given more weight than earlier measurements. The filter can also be dis-
abled by selecting NONE.
The response time of the filter is adjusted with RESPONSE TIME. For the
moving average filter the response time is the length of time in seconds over
which measurements are averaged. For the exponential filter the response time
is the time required for the output to reach 63% of its final value with a step
change in the input. There is a tradeoff between response time and smoothing
effectiveness. Increasing the response time smooths the measurements and im-
proves the effective resolution. However, if the response time is too long you
will not be able to accurately measure changing temperatures. The response
time of the filter should be set to the smallest value that achieves the desired
resolution and accuracy.
To help allow the filter to track changing temperatures while still removing
noise, the digital filter has an automatic reset feature. The filter will reset if the
difference between the raw measurement and the previous filtered measurement
exceeds a specified reset threshold. This limits the amount of error the filter can
cause. When the filter resets its memory of past measurements is erased. The
effect of automatic reset is illustrated in Figure 7. Notice that with reset the
measurement follows more closely the actual temperature during a sudden large
change in temperature. If the input changes continuously the output may appear
to stair-step as the filter attempts to track the input.
The reset threshold can be adjusted within the DIGITAL FILTER window. The
smaller the threshold the closer the measurements will be to the actual tempera-
1575/1590 Thermometer Readout
User’s Guide
42
without filter
filter without reset
filter with reset
reset threshold
Figure 7
Effect of Automatic Reset on Filtered Measurements
Содержание Hart Scientific 1575 Super-Thermometer
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