Haefely 2830, User Manual, Page: 24 / 124

16 Theory
The resistivity of a given specimen my be influenced by the applied stress. For results to be
comparable, measurement shall be made with approximately the same electrical stress and
polarity.
c. Time of electrification
Upon the application of DC voltage, the current flow through the specimen decrease due to the
sweep of charge carriers to the electrodes. The conventional arbitrary time of electrification is 1
min. Variation in the time of electrification can result in appreciable variation in the results. (Some
high viscosity fluids may required considerably longer electrification time).
Calculation of the resistivity in ohmmeters by means of:
Liquid Test Cell 2903 Solid Test Cell 2914
 
I
U
K m
 = 

 
I
U
l
A
m
 = 

U DC test voltage
I DC test current
K Liquid cell constant in meters
C
air
Measured capacity in air (Tettex Test Cell 2903 approx. 60pF

20%)

0
Vacuum permittivity

r
Relative permittivity of air or liquid
A Surface of the measurement electrode (Tettex Test Cell 2903 approx. 0.0134m 2 / 2914
approx. 0.002 m
2
)
I Distance between electrodes (Tettex Test Cell 2903 approx. 0.002m / 2914 depends on
sample thickness)
   




 =   =
pF C pF C m K
air air
113 . 0
1
10
0
12
