Your bug is an elementary Python error. In line 75 of the module flowMeters.py you try to read a global variable which hasn't been initialised in the module or imported. The interpreter simply can't find it to read. Consider
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>>> def foo():
... global z
... print(z)
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in foo
NameError: name 'z' is not defined
>>>