C-THERM TCi - Contact Agent

TCi Operator Manual
99
The table below summarizes the material groups and their attributes.
Material group
Eff range
W

s/m
2
K
k range
Wm/k
Temp. range,

C
Power
level
Comments
Liquids & powders

Air

PDMS

Distilled Water
0
-
1660
0
-
0.6
0

T

100
Low
No contact
agent
Foams

2704

2710

2720*
50
-
200
0.04
-
0.09
0

T

60
Low
No contact
agent
Polymers

CVPC

Lexan

HDPE

Pyrex*
420
-
1450
0.
13
-
1.
2
-
50

T

80
Low
Ceramics

Pyrex

Steatite

Pyroceram
*

Alumina
1400
-
8000
1.1
-
29
-
50

T

200
Low
Metals

Titanium

S
tainless
S
teel

Sapphire

Phosphor B
ronze*

Brass
4,500
-
19,000
6
-
1
20
-
50

T

200
High
Notes:
1.
* means reference material for that
group.
2.
Low power, heater and guard 90 mA each.
3.
High power, heater 105 mA and guard 125 mA.
4.
All groups use start time 0.3s and stop time 0.8s.
9.3
Contact agent
The TCi sensor has a solid and hard surface. When measuring solids, there will always be some
con
tact resistance, which may significantly affect the results. The quality of heat transfer depends
on many parameters such as type of material, surface quality, etc.
To overcome this problem we use a contact agent (CA). The best CA available is water, sinc
e it
has relatively high thermal conductivity (~0.6 W/mK), low viscosity, and is easy to apply and
clean. Water can be used in a limited temperature range though, from ~5

C to ~70

C.
Calibrations of solids (except foams) are all done with water.
At temper
atures lower than 5

C and higher than 70

C a different CA is needed. Any such
material will have a lower thermal conductivity than water. And since calibrations are done only at
room temperature with water (and automatically corrected at other temperatures
), there is a need
to correct for the contact resistance. This correction factor (CF) may be constant over the entire
range of materials, or may have a simple relationship with regard to the measured
1/m
.