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

TLH8709 – 4

FIGURE 1

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

sent the different thermal resistances from probe to ambient

respectively As a result of these temperature changes the

Hence

D

L

D

L

D

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 008 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

sible probe resistance range is from 35X 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

TLH8709 – 5

FIGURE 2

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

TLH8709 – 6

FIGURE 3

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

TLH8709 – 7

FIGURE 4

5