Vacuum Levels - Draw milk from teats

The milking machine works on the principal of providing a vacuum that alternates with atmospheric pressure to draw milk from the teats. A continuous vacuum would soon become painful for the cow and cause teat damage and so the pulsating system used to milk provides a cycle that can, when correctly set, remove milk efficiently without damage to the teat or udder.

The correct vacuum and pulsation levels are essential to this process and are checked when a milking machine test is performed. However, even on a day-to-day basis, the milking staff should be aware of how important these correct levels are, what indicates that they are not correct and how to perform a basic test to ensure the correct vacuum characteristics. A low vacuum level would result in under-milked udders and liner slippage and a high vacuum level would quickly lead to teat damage.

When the milking machine is switched on and vacuum permitted to rise to working levels before milking commences, the vacuum gauge in the parlour should read 45-48 kPa for high-line parlours and between 40-44 kPa for low-line systems. Some gauges will have a mark at which the needle should rest when the parlour is functioning at the correct vacuum level. Newer parlours may have digital read-outs.

Vacuum reserve is important as it maintains vacuum levels by allowing for air escaping into the system when cluster units are attached and removed, for example, and can be measured by milking staff quite simply:

  • When the parlour is switched-on and air is leaked through a single cluster, there should be no discernible drop in vacuum level.
  • If air is allowed to leak through two clusters, there should be a drop of no more than 2 kPa.
  • When all air leakages are stopped, vacuum should return to the normal level in less than three seconds.

The regulator also helps to maintain a stable amount of vacuum, as the requirement for vacuum within the system varies important function. The regulator is an automatically-functioning air valve that should 'hiss' continuously during milking. It is usually mounted high in the parlour and to ensure effective function it needs to be kept clean.

Vacuum fluctuations that may occur due to malfunctions, incorrect settings or liner problems can lead to reverse milk flow and teat end impacts, which can be responsible for teat end damage and the introduction into the teat canal of environmental mastitis pathogens.


The pulsators in the parlour regulate the alternating vacuum and atmospheric pressure cycle so that milk is drawn efficiently from the teat without teat damage occurring. It has four cycles, two of which are of extreme importance to its optimal function; the milking phase and the resting (or massage) phase.

The pulsation ratio between these two phases describes the length of time each should last: it should be between 50:50 and 70:30, with the ideal being 60:40 in the favour of the milking phase. Ratios above 70:30 should be a cause for concern as teat damage is more likely to occur; below 50:50 cows may be undermilked and milking times lengthened.

A simple test can be performed to test the presence and degree of effective vacuum and pulsation levels in individual teatcups. By blocking up three teatcups in the cluster and inserting a thumb into the fourth, after 20 seconds the effect is similar to the effect felt by the cows' teats. If this becomes uncomfortable then the pulsation ratios are likely to need adjustment.

Many milkers are also aware of audible changes in the pulsation cycle that may indicate a pulsation malfunction. They should also be aware of the air bleed holes, which must be kept clear to let milk flow through the plant properly and not fill cluster unit clawpieces, 'bathing' teat ends in potentially-contaminated milk.