This paper presents a method to check the purity of gases by measuring the
thermal conductivity. Gas purity is of great importance to industry in general. Power of
Industrial lasers have increased dramatically over the past few years, therefore
maintaining a highly pure atmosphere in laser resonator is important. Similarly, there are
many other industries which look for gas purity. Technique for checking the purity of a
gas depends on the type of gas. There are a number of sensors to measure the purity of
gases but they are costly. The purity of gas can be checked by comparing it with the
thermal conductivity of a reference gas (pure). If constant electrical power if applied to a
sensor through which gas stream flows then, Thermal conductivity of a gas stream will
affect the temperature of the sensor system. The temperature can be accurately measured
by measuring electrical resistance. We use a wheatstone’s bridge and pass the pure and
impure gases through two arms. When both the gases are pure the bridge will be
balanced. If there is an imbalance in the bridge, a small voltage will be developed which
is then amplified and fed to a NI-DAQ and checked if there is any impurity. We use a NIDAQ
because we can look for the minimum purity.
Problem:
Sensors available for checking gas purity are too costly, moreover checking the
purity of gases is vital for many industries and hence an alternative should be made. For
example in fuel cells gas purity is very important.
Solution:
Using NI-DAQ ELVIS 6251 an accurate data acquisition is made to check the
purity of gases. The purity is identified on the basis of the thermal conductivity property
of gases using a wheatstone’s bridge
Introduction:
There are many industries which use pure gases hence checking the purity of gases
is very important. Sensors are available to check the purity but they are costly. In this
paper we have proposed a highly reliable sensor system- all purpose purity monitor
exploiting the ease and sensitivity of wheatstone’s bridge. We have used the principle of
thermal conductivity of gases. Thermal conductivity is different for different gases and
also mixture of gases shows different thermal conductivity. We compare the thermal
conductivities of both the pure and impure gases and the difference is generated as a
small voltage and amplified and sent to a NI-DAQ (Data Acquistition System). With the
NI-DAQ we can acquire data from different places in the industry and monitor the gas
supply system. We can then stop the process if the impurity level crosses the safety limit.
This equipment is to be mounted in the gas line( In-Line) The same gas can be
checked for purity at different locations and the place where impurity gets into the line
can be found.
Importance of gas purity:
Lasing gas purity:
The use of lasers has increased dramatically. Certain gaseous impurities can cause
a severe damage to the resonator. The impurities in lasing gases impede the performance
of the laser by reducing the output power and electrical discharge will not be uniform.
The most harmful impurities are water vapour and hydrocarbons. The gain of the lasers
will be reduced.
Tab 1:Impurities in gases
SOURCE GAS IMPURITIES
Helium(He) O2 ,N2,H2O,THC
Nitrogen(N2) O2 ,N2,H2O,THC,At
Carbondioxide(Co2) O2 ,H2O,THC
Carbonmonoxide(co) O2 ,H2O,THC
Hydrogen(H2) O2
,N2,H2O,THC,CO
Oxygen(O2) O2 ,H2O,THC,At
Xenon(Xe) O2 ,N2,H2O,THC,At
The above table gives the gases used in lasers and the possible impurities in them. It is
impossible to remove impurities from lasing gases but the impurities can be allowed to
about 5ppm level. A 99.995% pure gas has about 50ppm level of impurity. If unchecked,
this will result in operational problems. So lasing gas purity is very important.
Oxygen purity:
Elevated concentrations of oxygen are used in many industries to increase the
yield without increasing the cost. About 80 to 100% enrichment of oxygen is given in
many chemical processes. The oxygen supplied to these industries are manufactured
cryogenically and for low level consumption by pressure swing and vacuum power
adsorption method.
Hydrogen purity:
In many plants explosions can be avoided when the turbine generators perform
with optimal efficiency. The efficiency depends on hydrogen purity. A small drop in the
purity might cause windage losses which reduces generator’s efficiency. Hence, highly
accurate hydrogen monitoring is necessary for generator efficiency.
Other purity sensors:
Oxygen sensor:
Both the oxygen producers and users depend on paramagnetic type oxygen sensing
devices for measuring oxygen purity. These sensors give very accurate results at
suppressed levels of 90 to 100%. But these sensors are very costly. These sensors are also
sensitive to even small particulates, moisture, temperature, pressure and mechanical
vibrations and calibrations should be done on a daily basis
Optical spectroscopy:
In this method we measure the optical absorption, emission or scattering by the
gas. It is highly sensible and offers a direct means of measurement. The major drawback
is that the gases must have a distinct absorption, emission or scattering.
Chemfet sensors:
It is a low cost sensor which measures total exposure over time. But it is
poisonous which is undesirable and also it may consume analyte.
Pellistor gas sensor:
It is also a low cost sensor. It can detect the presence of flammable gases. But it
is poisonous and sensitive to only a group of gases and not a gas in a group. If any other
flammable gas is present it can give results lower than the lower explosive limit (LEL).
Thermal conductivity:
In general thermal conductivity of a gas is a property that indicates its ability to
conduct heat.
In other words, it is defined as the quantity of heat, Q, transmitted during time t
through a thickness x, in a direction normal to a surface of area A, due to a temperature
difference T, under steady state conditions and when the heat transfer is dependent only
on the temperature gradient.
Tab 2:List of thermal conductivities
Gases Thermal conductivity
(mW/m.K)
Hydrogen 180.5
Helium 151.3
Nitrogen 25.83
Oxygen 26.58
Fluorine 27.7
Neon 49.1
Chlorine 8.9
argon 17.72
Bromine 0.122
Krypton 9.43
Xenon 5.65
The above table gives the list of thermal conductivities of gases. We can see that
hydrogen has the highest thermal conductivity followed by helium, neon, oxygen and so
on as in the periodic table. The formula given above can be used to calculate the thermal
conductivity. When two or three gases mix together then the thermal conductivity of the
mixture changes according the percentage of the presence of each gas. The unit of
thermal conductivity expressed here is milli watt per meter Kelvin.
Block Diagram:
Wheatstone’s Bridge:
It is an instrument which is used to find the resistance of one element when the
resistance of other three elements are known. A supply voltage is connected to two
opposite ends and a galvanometer is connected to the other two ends. The formula to find
resistance is
R3/R1=Rx/R2
Fig1 :A wheatstone’s bridge
The above diagram shows a wheatstone’s bridge. We use the bridge to detect the
difference in thermal conductivities of a pure and impure gas.
Wheatstone’s bridge network:
As already discussed there are four arms in a wheatstone’s bridge. There are
two ways in connecting the thermistors and resistances.
Double thermistor:
In this we put ordinary resistances in two arms and thermistors or nickel or
platinum wires in other two arms and the supply voltage
is connected. We use a metal box in which two small holes are drilled and the filaments
such as nickel or a thermistor or platinum wires are used, to make the process cheap we
can use a thermistor, the two ends of the filament are
connected to the wheatstone’s bridge. The impure gas and the reference gases are passed
through the two holes.The diagram is shown in the figure 2.
1. filament
2. impure gas connection to bridge
3. pure gas connection to bridge
Fig 2: double thermistor bridge
Four chambered bridge:
The sensor consists of four chambers: a measurement chamber and three reference
chambers. The chamber is made up of borosilicate glass 2mm internal diameter and
40mm length. The four chambers are located inside an aluminium block which is
equipped with an electronic control to keep the block and also the sensors temperature
constant. The diagram is shown in figure3 below. Here also a thermistor or nickel or
platinum is used as the filament
Fig 3 : four chambered bridge
Working principle:
So, far we have seen the structure of the wheatstone’s bridge to be used in the
sensor. The working principle for both types of bridge is the same. If we are using
resistances it is not a problem
but we are using thermistor and nickel wires and so we have to first check whether the
bridge is balanced or not. So, we pass reference gas through all the four chambers in the
four chambered bridge and through two chambers in the double thermistor bridge. The
supply voltage is given. If a galvanometer connected between the other two ends shows
no deflection then the bridge resistances satisfy the bridge balance condition. If not then
the resistances must be changed. When the voltage is applied to the bridge, current starts
to flow through the resistances and also through the filaments. This current will heat up
all the resistances, the gases are then passed through the chambers, these gases depending
on their thermal conductivity takes away the heat of the resistances. Thus, the resistance
gets reduced due to i2r losses. The difference in resistance then results in imbalance in
the bridge which can be detected by connecting a galvanometer in between the other two
junctions that is a small voltage is developed between the terminals which is the measure
of impurity in the gas. The wheatstone’s bridge can even detect small changes in the
resistance that is the change in thermal conductivity of gases. It may be in terms of milli
or micro volts or volts. This voltage is then further processed.
Analog signal amplification:
The signal that is coming out of the wheatstone’s bridge is analog and it is very
small to be detected so we have to amplify it. The circuit here is a common emitter
amplifier and the specifications are made so as to get the required amplification. The
signal is then fed to the DAQ
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