LM1042 Datasheet, PDF (5/8 Pages) National Semiconductor (TI) – Fluid Level Detector

Application Notes
THERMO-RESISTIVE PROBES OPERATION AND
CONSTRUCTION
These probes work on the principle that when power is dis-
sipated within the probe the rise in probe temperature is
dependent on the thermal resistance of the surrounding ma-
terial and as air and other gases are much less efficient
conductors of heat than liquids such as water and oil it is
possible to obtain a measurement of the depth of immersion
of such a probe in a liquid medium This principle is illustrat-
ed in Figure 1
FIGURE 2
TL H 8709 – 5
current with very fine wires to avoid excessive heating and
this current may be optimized to suit a particular type of
wire The temperature changes involved will give rise to no-
ticeable length changes in the wire used and more sophisti-
cated holders with tensioning devices may be devised to
allow for this
FIGURE 1
TL H 8709 – 4
During the measurement period a constant current drive I is
applied to the probe and the voltage across the probe is
sampled both at the start and just before the end of the
measurement period to give DV R_TH Air and R_TH Oil repre-
sent the different thermal resistances from probe to ambient
in air or oil giving rise fo temperature changes DT₁ and DT₂
respectively As a result of these temperature changes the
probe resistance will change by DR₁ or DR₂ and give corre-
sponding voltage changes DV₁ or DV₂ per unit length
Hence
DV
e
L_A
L
DV₁
a
(L
b
L
L_A)
DV₂
and for DV₁ ^l DV₂ R_TH Air ^l R_TH Oil DV will increase as
the probe length in air increases For best results the probe
needs to have a high temperature coefficient and low ther-
mal time constant One way to achieve this is to make use
of resistance wires held in a suitable support frame allowing
free liquid access Nickel cobalt iron alloy resistance wires
are available with resistivity 50 mXcm and 3300 ppm tem-
perature coefficient which when made up into a probe with 4
c 2 cm 0 08 mm diameter strands between supports (10
cm total) can give the voltage vs time curve shown in Figure
2 for 200 mA probe current The effect of varying the probe
current is shown in Figure 3 To avoid triggering the probe
failure detection circuits the probe voltage must be between
0 7V and 5 3V (V_REG b 6V) hence for 200 mA the permis-
sible probe resistance range is from 3 5X to 24X The ex-
ample given has a resistance at room temperature of 9X
which leaves plenty of room for increase during measure-
ments and changes in ambient temperature
Various arrangements of probe wire are possible for any
given wire gauge and probe current to suit the measurement
range required some examples are illustrated schematically
in Figure 4 Naturally it is necessary to reduce the probe
FIGURE 3
TL H 8709 – 6
Probes need not be limited to resistance wire types as any
device with a positive temperature coefficient and sufficient-
ly low thermal resistance to the encapsulation so as not to
mask the change due to the different surrounding mediums
could be used Positive temperature coefficient thermistors
are a possibility and while their thermal time constant is like-
ly to be longer than wire the measurement time may be
increased by changing C_T to suit
FIGURE 4
TL H 8709 – 7
5