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Doc's Corner Article 101

May 26, 2003

“My Opinion on how often is your automatic milking cluster removed from the udder under a vacuum load? Possibly more often than you realize!”

One must be aware that the milking unit must not be removed under normal milking vacuum levels. There is no recent data, that I am aware of, that suggests how low the vacuum should be at the time of removal. My present opinion is that the milking vacuum should be very close to
atmospheric pressure prior to removal.

One must be concerned about two factors:

(1) What is the decay time of your milking unit?
(2) What is the retract time of your retract cylinder?

Decay time could be defined as the time it takes the vacuum within the milking cluster and milk hose to return to atmospheric pressure after the closure of the milk valve to the milk pipe line or vacuum source.

Retract time is the time it takes the retract cylinder to retract the milking cluster after the milk flow sensor says it is time for removal.

Several years ago, this was not a major problem. Then, most dairymen were using 5/8 or even 9/16 inlet milking claws along with four to five feet of matching milk hose. Under those conditions the average decay and retract time was in the 3 second range. When the milk flow sensor
indicated time for removal and retract cylinder responded, the vacuum had pretty well dissipated from milking claw and no problems occurred.

Today, with the development of 3/4 and 7/8 inlet milking claws with enlarged milk hoses, we often find decay time to be in the 5 to 7 seconds and up to 14 seconds in some basement barns.

Vacuum within cluster and hose is dissipated in three ways after closure of milk valve:

(1) Through the cluster vent
(2) Through cluster and liner vents
(3) From around the teats and liner mouthpiece - this often happens, but not a dependable release of vacuum that can be depended upon
(4) Some back-flush units admit atmospheric pressure as soon as the slide valve moves from milk to sanitize

To the best of my knowledge, no company has developed an automatic release that removes the vacuum in question from the cluster after milk valve closure.

One must be aware of the vent diameter in any claw in question. My present opinion is that the claw vent should be in the range of 0.040 to 0.042 inches in diameter. Obviously the larger the vent, the greater the flow of air. This diameter admits approximately 0.4 to 0.5 CFM depending upon temperature and atmospheric pressure. If one can not control decay by vent enlargement or slowing retract time, the option could well be vented liners which allows approximately 0.4 CFM into the claw. It is my opinion, that claw vent should be left open because of the
difficulty of always maintaining open liner vents. My observations are that this additional air causes no problems other than perhaps a lowering of the vacuum level by 0.1 to 0.2 CFM depending upon number of milking claws within the milking parlor. In any event, one must be certain that decay and retract time are such that the milking unit is not
removed under an elevated vacuum load.

Evaluation of this problem is easily accomplished with a vacuum recorder with graphing capabilities. Insert a long needle into top of milking cluster. Before the end of milking, graph at least two seconds of normal milking, then activate removal of claw, graphing the period of decay and actual removal of milking unit. Quickly you will be able to see a normal decay pattern as apposed to an abrupt removal of milking unit. It is my opinion that this factor can play an extremely important role in mastitis control.

Doc's Corner is a contribution of Lionel H. Brazil, DVM (1928-2007). Dr. Brazil offered dairy consultation services worldwide for many years. His services covered the following areas: milking management, vacuum system evaluation; and all management procedures relating to mastitis control and SCC reduction.



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