Liquid Controls MA4 Meter - Manual (Page 31)

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WARNING:
When a Back Check Valve is used, an automatic safety
valve must be installed to prevent pressure build-up in
excess of rated working pressure in the meter housing.
One automatic safety valve should be installed in each
meter. Remove the pipe plug from the front cover or
rear cover and insert the appropriate automatic safety
valve.
Optional Back Check Valve Assembly (PN. 501267)