Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.39 Figure 8.39 spring between the grapevine trunk and the base plate (Fig. 8.40A). One end of the steel pin is glued to the trunk. As the trunk expands or shrinks, the pin moves inside the coil. This changes the voltage output which is converted to millimeter. The LVDT is connected to a logger which stores the data on an hourly basis.

0.12

0.12

0.10

0.10

0.08

0.08

0.06

0.06

0.04

Daily shrinkage (mm)

0.04

0.02

Daily shrinkage (mm)

y = -0.181 + 0.063(lnx) (R 2 = 0.9897)

0.00

0.02

0 10 20 30 40 50 60 70 80 90 100 PAW depletion (%)

y = -0.181 + 0.063(lnx) (R 2 = 0.9897)

0.00

0 10 20 30 40 50 60 70 80 90 100 PAW depletion (%)

Figure 8.40

FIGURE 8.39. The relationship between the daily shrinkage of a grapevine trunk and PAW depletion (Myburgh, 1996).

A

B

Figure 8.40

Invar shaft

Back plate

Grapevine trunk movement

Base plate

LVDT coil

Steel pin

Glue Spring

Invar shaft

Back plate

Adj usting nut

Grapevine trunk movement

Base plate

LVDT coil

FIGURE 8.40. Illustration (A) of the components of a linear variable differential transformer (B) to measure diurnal shrinkage and expansion of grapevine trunks. St el pin

272 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING

Glue Spring