Heat conduction through the wires is one source of thermocouple (TC) error.
Leaf temperature measurement may be affected by this error if the leaf-to-
air temperature difference is large and contact between TC and leaf is inad
equate. Selection of small diameter TCs with low thermal conductivity, as w
ell as good contact with the leaf, are possible solutions to the problem. A
laboratory experiment was carried out to determine conduction error as a f
unction of TC-type, wire diameter, insulation and length of contact between
wire and leaf. Most of the tests were conducted with a thin plastic plate,
as a leaf-substitute, to ensure repetitiveness. Some measurements were als
o made with in vivo pepper and tomato leaves, to determine a correction fac
tor Different lengths of the TC wins were glued to the plate, or leaf, to e
nsure good contact. Controllable temperature differences were established w
ith a thermo-electric cooler. A conduction error, tau, of 1% (relative to t
he leaf-to-air temperature difference) is to be expected when a length of a
bout 22 mm of AWG 40 (0.08 mm diameter), teflon-insulated type-T (copper-co
nstantan) thermocouple is glued to a leaf. Removing the insulation reduces
this length to 14 mm, and replacing the insulated type-T thermocouple by an
insulated type-K (chromel-alumel) thermocouple of the same gage, requires
about 10 mm for the same 1% error. An approximate theoretical solution was
obtained, which relates relative measurement error, tau, to normalized glue
d length, xi, in the form tau = exp{-xi}. This relationship can be used to
estimate errors with other TCs. (C) 1999 Elsevier Science B.V. All rights r
eserved.