cooling circuits comprise an electrical conductivity meter preferably each cooling circuit comprises an electrical conductivity meter the conductivity meter can be situated at any point in the cooling circuit preferably it is positioned at a point in the cooling circuit where the circulating cooling medium is travelling away from the tubes 39 cooling jackets in another embodiment of the invention it is positioned at a point in the cooling circuit before the cooling medium enters the cooler the invention is particularly suitable for an ethylene copolymerisation reaction in ethylene copolymerisations at least one of the comonomers must be capable of changing the electrical conductivity of the aqueous cooling medium the comonomer can be selected from one or more of acrylic and alkacrylic compounds such as acrylic haloacrylic and methacrylic acids and esters vinyl compounds such as vinyl esters and sulphonic esters derivatives of unsaturated carboxylic acids such as maleic anhydride dialkylmaleates and dialkylfumarates and ionic vinyl compounds such as sodium vinyl sulphonate preferably the comonomer is a vinyl ester more preferably the vinyl ester is vinyl acetate monomer vam vam can change the electrical conductivity of water as it undergoes hydrolysis in water thereby forming an electrolyte acetic acid if a leak occurs vam enters the cooling circuit hydrolyses and the acetic acid triggers a sudden change in the electrical conductivity of the aqueous cooling medium monitored by the conductivity meter installed in the circuit the polymerisation reaction is started with an initiator which can be selected from the non exhaustive list of oxygen peroxides such as benzoyl peroxide acetyl peroxide and alkylhydroperoxides dioxides peracetic acid alkali metal and ammonium persulphates.