The liner is a critical component of the milking machine. It serves to support the teat from radial expansion and cyclically massages the teat by the application of a compressive force (3). This retards fluid accumulation in the teat tissue, facilitating blood and lymph circulation under conditions of negative pressure. Liner design is also important in udder health, especially as influenced by liner slip (7). Shortened liners increase new infections and result in a form of pulsation failure (6). Pulsation failure consistently results in increased mastitis incidence (12). Pulsation is the cyclic opening and closing of the liner (ISO 3918) (1). Pulsation occurs because of the admission and evacuation of air in the space between the rigid shell and flexible liner known as the teat cup chamber. The liner opening and closing is a consequence of the differential pressures across the walls of the flexible liner barrel (2). The wave form or a graph of pressure changes of the pulsation chamber vacuum is divided into four phases: ‘a’ the increasing vacuum phase, ‘b’ the maximum vacuum phase, ‘c’ the decreasing vacuum phase, and ‘d’ the minimum vacuum phase (1). The pressure changes within the teat cup, including the chamber vacuum and internal liner vacuum cause the liner to open and close below the inserted teat. The vacuum inside the liner is not consistent and varies with the internal volume and relative amounts of air and milk inside the cluster (2). Thus, liner wall movement can be expected to be different during milking, in contrast to a static test. In addition, teat size affects the ‘buckling pressure’ or the point where the liner begins to collapse (3). Buckling pressure is sometimes referred to as the critical collapsing pressure difference (CCPD). A repeatable measure at the point where the liner walls touch is called the touch point pressure difference (TPPD) (10). From the touch point, the liner closes further on the teat, the extent to which depends on the liner material, dimensions, tension, teat size and skin thickness, and vacuum conditions. Due to a continuum of collapse and rapidly changing physical dynamics, measurement is difficult. No standards or methods exist to characterize the forces or extent of liner movement.
Attempts to scientifically characterize liner wall movement have been tried by numerous methods. Thiel and Akam (13) developed an electrical switching device to measure pulsator ratio as defined by liner wallmovement. This report makes clear that pulsation graphs and liner wall position may differ extensively. Caruolo (4) stitched electrodes through the opposing liner walls to determine the closure and separation point of the liner walls. These methods were unable to take measurements during actual milking because of fluid or equipment interference. Reitsma and Breckman (8) fitted a teat cup shell with a linear transducer and measured the liner wall movement in relation to chamber vacuum. They conclude that liner composition and design affect the liner’s response. In addition, the rise and fall times of the liner (opening and closing) are considerably shorter than the rise and fall times of chamber vacuum. They suggest that liner wall movement provides a much better indication of teat cup operation than the pulsation chamber vacuum recordings commonly used in testing milking systems (8). Butler and Adley (2) developed amethod of measuring liner wall movement using video recordings.
Measuring the width of the liner by video camera and the pressure difference across the walls allowed an estimate of the degree and duration of liner collapse. Thiel and Mein (14) used high-speed cine photography in a transparent shell and liner to characterize the milk flow in relation to liner wall movement. They noted that the declining vacuum phase (‘c’ phase), when allowed to decline slowly or rapidly, had little influence (0.01 s) on the rate of liner collapse. Schuiling et al. (11) developed a device using a series of eight light beams that were converted to a light sensitive cell. They contend that measuring liner action gives much more information than measuring pulsation vacuum.

Figure 1. The Wenglor analogue laser sensor YP 06 MGV-P24 (Tettnang, Germany) works with a visible red pulsed laser beam by the triangulation method. The unit measures 50 ◊ 50 ◊ 20 mm. The pictured laser is shown beside a ruler in inches. Supply voltage is 18-30 V DC and analogue output is 0-10.

Figure 2. The laser of the Wenglor analogue laser sensor (Tettnang, Germany) is held in place to the transparent teat cup shell by a stainless steel case. The case has adjustable slots to keep the laser within a working range of 40 to 60 mm. Vacuum is measured on the short pulse tube (not shown).

They suggest that too short ‘a’ and ‘c’ phases may cause uncomfortable milking and mastitis. No data are presented in this regard. Schlaiss (10) utilized fiber optics, which gives excellent response characteristics of the liner in relationship to chamber vacuum.
Although teat cup liner performance is critical to machine performance and cow health, data on various liner designs and vacuum conditions in the United States are lacking. The rate of liner collapse is said to influence peak flow (15). Mein (5) and Rosen et al. (9) did not, however, substantiate this observation. The objective of this study was to measure and characterize the rate of liner opening and collapse.