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
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
(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
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