20 DRÄGER REVIEW 99 | FEBRUARY 2010

BACKGROUND EXPLOSION PROTECTION

Hot Pellets Trigger Gas AlarmThe first part of our series of articles on “Detection of Flammable Liquids” (see Dräger Review 98,

pp. 20 ff.) discussed the physical and safety-relevant properties of flammable vapors. Part 2

covers their DETECTION. Two different measurement methods have proven effective here: the thermo-

catalytic method and the infrared-optical method. The latter will be covered in the next issue.

Just two tiny pellets inside the sensor protect an entire area containing a potentially explosive atmosphere.

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HOW DOES A SENSOR reliably

detect flammable gases? In principle by

the fact that they burn flamelessly and

oxidize at a heated catalyst, such as the

catalytic converter in the exhaust system

of an automobile. Here, for example, tox-

ic CO and uncombusted hydrocarbons

are oxidized to form “less harmful” CO2.

The fact that the catalyst becomes slightly

hotter is just a side-effect of this process.

The thermocatalytic method utilizes this

side-effect, however. In fact, this “heat

effect of a reaction” is the basis of mea-

surement using what the gas detection

industry typically refers to as a “catalytic

bead sensor.” But what actually goes on

inside this seemingly simple sensor?

The catalytic effect

The aggressiveness of the oxygen is ulti-

mately responsible for the heat effect

measurement principle. Oxygen mole-

cules love metallic surfaces. They cling

tightly to them, split apart (one oxygen

molecule becomes two oxygen atoms),

and briefly assume a highly reactive

state. The oxygen atoms are now just sit-

ting there with unoccupied bonds, wait-

ing for a reaction partner. And if the oxy-

gen doesn’t react directly with the metal,

it does so indirectly by first undergoing

a prior reaction with moisture — the pro-

cess we commonly refer to as “rusting”

(oxidation).

Even the surfaces of very noble met-

als such as platinum or palladium take

on a coating of oxygen. Considered on

the microscopic scale, this coating is an

equilibrium state. After all, the oxygen

atom is quite impatient. If it doesn’t find

an appropriate reaction partner relative-

ly quickly (palladium is too noble for it),

it pairs up with another oxygen atom. In

other words, it “recombines” and flies

off, making room for other oxygen mol-

ecules, which move in to take its place.

If an oxidizable molecule of a flamma-

ble, gaseous substance comes along, the

oxygen strikes: The molecule is quick-

ly converted to CO2 and H2O. However,

this is only the case if the oxygen atoms

are more strongly attracted to this mole-

cule than they are to their metallic sub-

strate. This force of attraction can be

controlled. The hotter the surface, the

easier it is for the oxygen atoms to sepa-

rate from it again. And if it is too hot, they

won’t even dock on the surface at all! A

hot metallic surface is therefore used as

a roundabout path to force a reaction

that would not normally take place at all.

The metal is unchanged by this process;

it acts as a reaction facilitator — a cata-

lyst. The location at which this reaction

takes place is generically referred to as

the catalytic center.

But what happens during a reaction

of this type? Heat of reaction is released,

heating the catalytic center and its sur-

roundings. To make this slight heat-

ing measurable, you need as many of

these catalytic centers as possible in the

smallest possible body. This is necessary,

because only small masses can be heat-

ed perceptibly with such slight amounts

of energy. A highly porous body similar

to the completely air-permeable materi-

al of a flower pot would be ideal, as fired

clay or ceramic has countless micropores

and channels. As a result, it makes good

sense to take a small ceramic pellet and

impregnate it during the production pro-

cess with catalytic material! A pellet of

this type measuring only one millimeter

What signal is produced by 10 % LEL octane?The resistance of a platinum wire as a function of its temperature T (in °C) can be calculated according to EN 60751. The resistance R0 required to do so is approximately 0.928 times the resistance measured at 20 °C. This cold resistance R20 can be measured directly with a resistance thermometer when the catalytic bead sensor is turned off. At 20 °C it is 1.6 ohms. In other words, R0 = 1.48 ohms.

The hot resistance R is measured during operation: At a filament current of 270 mA, the voltage is 1,200 mV, i.e., R = 1,200/270 = 4.440 ohms. The temperature can be calculated using the formula given in EN 60571 and is 558 °C. Upon exposure to 10 % LEL octane (= 800 ppm), the voltage increases from 1,200 mV to 1,204 mV, and the hot resistance increases to 1,204/270 = 4.459 ohms. The increase in resis- tance is only 0.019 ohm! Using the cited formula, one can then compute that the tempera-ture of the platinum coil increases to 562 °C. The temperature increases by only 4 °C when exposed to 10 % LEL octane.

Explosion-proof design: The gas enters the robust stainless steel housing through a sinter disk (left). The platinum spiral (right) is a coil barely 1 mm wide with a wire diameter of only around 0.05 mm.

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22 DRÄGER REVIEW 99 | FEBRUARY 2010

in diameter has an enormous catalytical-

ly active surface area (> 0,1 m2), and its

countless pores are flooded with air, i.e.,

all of the catalytic centers are saturated

with highly reactive oxygen.

The finely distributed metallic cata-

lyst gives the pellet its grey-black color. A

small platinum spiral heating element

is embedded in the pellet to optimize its

temperature. An electrical current of

around 270 mA is sufficient to heat the

pellet to over 500 °C. Should any flam-

mable gases enter the pores, they will

further heat the catalytic centers and

thus heat the pellet. The platinum heat-

ing element becomes hotter in turn, and

its electrical resistance increases slight-

ly. Ultimately, the whole process comes

down to a resistance measurement in

the milli ohm range that makes the gas

concentration measurable. As long as the

heating current through the platinum

remains constant, a voltage measure-

ment in the millivolt range is enough to

achieve this (see box). The combination

of pellet and resistor gives the sensor type

its technical name: a pellistor.

Explosion protection

The temperature of such a pellistor only

increases by a few degrees when a flam-

mable gas is present. Variations in the

ambient temperature can be much great-

er, and must therefore be compensated

for. This can be accomplished using a

completely identically structured pelli-

stor — a “compensator,” which, howev-

er, does not contain any catalyst, and so

appears white and is insensitive to gas.

The difference signal between the two

is the only quantity that is measured. If

the temperature of both pellistors chang-

es, the measurement signal remains

unchanged. This setup works well. How-

ever a black pellistor radiates heat away

more strongly than a white one, and that

in turn leads to an asymmetry between

the resistances and a loss of measure-

ment quality. The results are significantly

better when two black pellistors are used,

and a net effect is achieved by encapsu-

lating one of the pellistors so that it only

has access to the outside world via a pin-

hole. Only the unencapsulated pellistor

serves as a measuring element for gas-

es, the encapsulated one serves as the

compensator.

There are a large number of flam-

mable gases and vapors which could

be ignited, should their concentration

exceed 100 percent of their lower explo-

sive limit (LEL) when they came into

contact with a pair of pellistors at 400–

500 °C. To prevent a catalytic bead sen-

sor becoming a source of ignition, it is

necessary to ensure that the sensor hous-

ing can resist any ignition that occurs

within it, and that any flame cannot

flash back to the outside world.

The internal volume of the sen-

sor — which is scarcely the size of a thim-

ble — must therefore be encapsulated

so that it is pressure-proof and so that

gas can only enter via a flame barrier.

Such flame barriers — in the form of a

metal sinter disk, for example, or a wire

mesh — are on the one hand, both fully

gas-permeable and, on the other, extin-

guish any possible flame thanks to their

extremely good thermal conductivity. It

is this property that cools the flame tem-

perature to below the ignition tempera-

ture of the air-gas mixture.

From the point of view of the mea-

surement process, it is just as important

that such a flame barrier also functions

as a barrier to diffusion. This is because

the individual molecules must first pass

Catalyst poisonsThe measurement sensitivity of a catalytic bead sensor can change. In addition to aging effects (e.g. desintering of the ceramic pellet, which leads to reduced porosity and fewer catalytic centers) and contamination of the flame barrier (reduced permeability, which in turn leads to lower diffusion speeds), some volatile substances can also render the catalyst unusable. Lead and sulfur compounds are not only poisonous to automotive catalytic converters (which is why these materials are no longer present in fuels), but also have a similar affect on catalytic bead sensors. In general, it is the volatile organic metal compounds such as silicone which block the catalyst for oxygen, while corrosive gases attack it directly and make it unusable. Many “harmless” cooling agents are also considered corrosive, because they release aggressive chlorine or fluorine compounds when burned.

Catalytic bead sensors reliably warn of the risk of explosion

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23DRÄGER REVIEW 99 | FEBRUARY 2010

EXPLOSION PROTECTION BACKGROUND

through the calm air enclosed in the

flame barrier. This means that their

speed of diffusion limits how quickly

they can affect the pellistor. A methane

molecule could cover some hundreds of

meters in a second. However, it doesn’t

manage this feat because it is hemmed

in by billions of air molecules, which

constantly force it to change speed and

direction.

Diffusion is a slow process that evens

out concentrations. It occurs essentially

because the molecules constantly tend to

move to where there are not so many of

their own kind. And when they are con-

sumed in the pellistor, there is always a

zone of low concentration there, which

attracts further molecules. The result

is a kind of “molecular suction,” which

eliminates the need for a pump with

such diffusion sensors.

A matter of calibration

A complete catalytic bead sensor is basi-

cally just a very precise apparatus for mea-

suring resistance. It’s the calibration that

establishes the relationship between gas

concentration and measuring signal. If,

for example, a sensor is exposed to a con-

centration of 0.85 volume percent of pro-

pane — which corresponds to a concen-

tration of 50 % LEL — the downstream

evaluation electronics must be set in such

a way that 50 % LEL is also displayed. If

a measurement system calibrated for

propane is exposed to different gases

and vapors, however, it will show differ-

ing measurement sensitivity. A measure-

ment system calibrated for propane will

already show full scale deflection when

exposed to methane at 50 percent of the

LEL, while exposure to 50 % LEL toluene

vapor will only produce a reading of 30 %

LEL. This is important when it comes to

safety. In order to obtain a reliable warn-

ing, the sensor must always be calibrat-

ed in accordance with the substance to

which it reacts with the least sensitivity.

Only then will the unit give a warning that

is too early, rather than too late!

The differing measurement sensitiv-

ity is, incidentally, correlated with the

molecular size: the larger the molecule,

the smaller the measurement signal. The

flash point of every flammable liquid is

also correlated with the molecular size:

the higher the flash point, the lower the

measurement signal. Extremely large

molecules can no longer be measured

using the catalytic bead sensor. However,

the temperature of the flash point of

such liquids is also well above normal

temperatures. Correspondingly, there is

no danger of ignition (see Part 1).

Reliable and economical

Correctly calibrated and properly operated,

the catalytic bead sensor is a very reliable

and economical measuring instrument.

It is capable of warning of an explosion

hazard even at very high temperatures

(up to 150 °C) — a region in which other

(electronic) measuring instruments can

no longer be used. However, the device

requires oxygen. In other words, it does

not work in inert atmospheres. But then

again there is no risk of an explosion in

such atmospheres! On the down side, the

presence of catalyst poisons can increase

the maintenance effort substantially. The

catalytic bead sensor must undergo more

frequent function testing if such substanc-

es are present. Infrared-optical sensors are

not subject to poisoning and also require

no oxygen. And even though they cannot

detect gases such as hydrogen, carbon

monoxide, acetylene, or ammonia, infra-

red sensors today have established a repu-

tation for themselves not only in safety

technology applications, but also in

process-related applications worldwide.

But more about that in Part 3 in the

next issue. Dr. Wolfgang Jessel

Methane Ethene

Propane

Toluene

n-Nonane

LEL

LEL Propane

Methane

Ethene

Propane

Toluene

n-Nonane

L LEL

LEL Toluene

Methane

Ethene

Propane

Toluenen-Nonane

L LEL

LEL n-Nonane

A measurement system calibrated for propane exposed to 50 % LEL toluene will indicate only 30 % LEL.

After calibration for toluene, the reading for toluene is correct, but 50 % LEL n-nonane is only displayed as roughly 34 % LEL.

No compromises: Only after calibration for n-nonane are the measurements for the five substances no longer lower than they should be.

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