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When an error arises, it is best to try to locate the cause first. Here's a list of all possible reasons that might cause measurement issues:
READINGS ARE NOT REPRODUCIBLE
SLOW RESPONSE (READINGS SLOWLY CHANGING)
OUT OF RANGE READING
LOW SLOPE OR NO SLOPE
NOISY RESPONSE (READINGS RANDOMLY CHANGING)
DRIFTY RESPONSE (READINGS CONTINUOUSLY CHANGING)
INACCURATE READINGS (BUT CALIBRATION IS OK)