E-I Matched Bead Thermistor, 115 Series
One thermistor is mounted in a static area to provide for tempera-
ture compensation while the other is placed in the medium to be
measured. Any change in the thermal conductivity of this medium
will change the rate at which heat is dissipated from sensing
thermistor, thus changing its temperature. This results in bridge
unbalance, which can be calibrated in appropriate units.
Using thermistors in thermal conductivity instruments
Many physical phenomena such as liquid or gas flow, vacuum
measurements, liquid level, changes in gas composition, etc.,
which involve changes in the thermal conductivity of a medium,
can be accurately measured and monitored by means of thermis-
tors.
In a typical application, two thermistors, connected in a bridge
circuit, are placed in separate cavities in a brass block. With air in
both cavities the bridge is balanced. When the air in one cavity is
replaced by carbon dioxide, the bridge will be unstable because
the carbon dioxide has a low thermal conductivity than air and
that thermistor becomes hotter and lower in resistance. The
amount of unbalance represents 100 % CO2 in the analyzer.
50 % CO2 gives just have to meter reading and the instrument
may therefore be calibrated with a linear scale to read in % CO2
in the air. Similar calibration may be made for any of the mixture
of two gases. A specific circuit for such an instrument is shown in
Figure 3. Its output is shown in Figure 2.
Thermistors are uniquely qualified as transducers in such appli-
cations because of two inherent characteristics. The first is their
high sensitivity to small variations in their own temperature. The
second is their ability to operate in a “self- heated” mode. When
a current of significant magnitude is passed through a thermis-
tor, the temperature of the latter increases until the power in the
circuit is balanced by the heat dissipated from the thermistor.
Gas Chromatography
When used in thermally conductivity measurements, two thermis-
tors are normally connected to adjacent legs of the Wheatstone
bridge with the voltage applied at their junction. Enough voltage
is applied to heat both units considerably above ambient (typi-
cally to about 150 °C [302 °F]).
Airflow
With slight changes in physical design such a device may be
used as a flow-meter, which can measure flow rates as low as
0.001 c.c./minute and can cover a range of 100,000 to 1 or more.
If the same bridge is made with one thermistors sealed in a cavity
in a brass block and the other mounted in a small pipe, it may be
used as a flow meter. When no air is flowing through the pipe, the
bridge may be balanced. When air flows through the pipe, the
thermistor is cooled and its resistance increases which unbal-
ances the bridge. The amount of cooling is proportional to the
rate of flow of the air and the meter may be calibrated in terms
of flow rate in the pipe. The same instrument may be used for
measuring flow rate of any gas or liquid. Such instruments have
been made to measure flow rates as low as 0.001 c.c./minute.
One instrument can measure flow rates over ranges of 100,000 to
1 or more.
Figure 2. Percent CO vs. Millivolt Deflection
Each of the other bridge legs must be greater in resistance
than the negative resistance of the thermistor at their operating
point. This is determined by the slope of the thermistor EI curve.
(∆E/∆I). Moreover, the impedance of the readout device must be
greater than the sum of thermistors negative resistance.
If the same instrument is made with the sensing thermistor half
in free air, it becomes an anemometer capable of measuring air
velocity from the slightest breeze to gale and can be calibrated in
terms of miles per hour of wind velocity.
Vacuum
Figure 3. Typical Thermistors Thermal Conductivity Measurement
Circuit
Similarly, thermistors are used as broad range vacuum gages
which are useful from about 10 mm to 10-5 mm of Hg. If one of
the thermistors is mounted in a sealed, evacuated bulb, and the
other is mounted in a camber connected to a vacuum pump, it
may be calibrated as a vacuum gage in terms of mm of mercury.
By pumping the chamber down to a high vacuum and balancing
the bridge, output will be obtained when the chamber is not at
high vacuum because the presence of air will cool the thermistor
etc.
Honeywell Sensing and Productivity Solutions
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