Ph meter signal smoothing

I am reading a ph probe 4-20ma signal that changes as much as .75 every 500 msec which makes it difficult to read. I would like to buffer this output to reflect a moving 3-5 second average. How would I go about doing this?

This is typical in PLC applications:

FV= Filtered value
C= constant (range 0-1) The smaller the number the more dampening.
NV= New Value