Advice on hanging Resident script (suspect storage get/set)

[oab_nor]

Hi, i'm experiencing some kind of hang/blocking/locking when this script runs for a few days.
 
The script runs as a Resident script every minute, but suddenly it stops. I try to disable, re- enable but without any success. 
Then, when I alter the storage key ('3_3_15_vent_effect') to e.g. '3_3_15_vent_effect_1' it runs fine for some time (days?).
 
Should I safeguard the storage get/set in some way? I'm I missing something obvious here?
 

Code:

deltaThreshold = 0.015
t1 = grp.getvalue('3/3/10')
t2 = grp.getvalue('3/3/11')
t3 = grp.getvalue('3/3/14')

-- Calc. effect
effect = (t1 - t3) / (t2 - t3)

prev_effect = storage.get('3_3_15_vent_effect', 0)

effectDelta = math.abs(prev_effect - effect)

-- Write to KNX buss if Delta > Treshold
if effectDelta > deltaThreshold then
 grp.write('3/3/15', effect)
 
 -- Store Calculated effect
 storage.set('3_3_15_vent_effect', effect)  
 log("New reading - Effect prev.=" .. prev_effect .. " New = " .. effect)
end

About the script: Calculate the effect of a heat recovery unit. Input values are collected from ModBus interface on LM (no need to do a grp.read()). I don't want to flood the KNX buss by writing every calculation, therefore the delta calculation. 

Alternative approaches are welcome...


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Ole A.