Discard outrageous sensor readings

Setup, Configuration and Use Scripts & Rules
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4

So finally managed to put in it:

rule "TemperatureFilter_gf_cupboard1 filter rule"
when
        Item gf_cupboard1_temperature_sensor changed
then
  var Number difference
  if (gf_cupboard1_temperature.state != Uninitialized) {
        difference = Math::abs((gf_cupboard1_temperature.state as DecimalType).floatValue) - ((gf_cupboard1_temperature_sensor.state as DecimalType).floatValue)
        logWarn("gf_cupboard1_temperature", "sensor value difference = "+  difference)
        if (difference < 5)
        {
                logWarn("gf_cupboard1_temperature", "Posting update as difference < 5")
                gf_cupboard1_temperature.postUpdate(gf_cupboard1_temperature_sensor.state)
        }
        else
                logWarn("gf_cupboard1_temperature", "Discarding Difference as >  5")
  }
  else {
        logWarn("gf_cupboard1_temperature", "gf_cupboard1_temperature.state is null, posting  " + gf_cupboard1_temperature_sensor.state)
        gf_cupboard1_temperature.postUpdate(gf_cupboard1_temperature_sensor.state)
  }
end

Interestingly though I log the diffrence variable, and it still seems to post negatives. I thought that was the point of the Math::abs?

2016-08-03 22:33:58.850 [WARN ] [o.m.s.gf_cupboard1_temperature] - sensor value difference = -0.10000038
2016-08-03 22:33:58.860 [WARN ] [o.m.s.gf_cupboard1_temperature] - Posting update as difference < 5
2016-08-03 22:35:36.064 [WARN ] [o.m.s.gf_cupboard1_temperature] - sensor value difference = -0.10000038
2016-08-03 22:35:36.067 [WARN ] [o.m.s.gf_cupboard1_temperature] - Posting update as difference < 5