24 Dräger review 100 | June 2010

If you take the time to look at the huge number of flammable gases and va-pors we are familiar with, you will see

that very few of these substances are in-organic in origin. The most common of these are hydrogen, ammonia, carbon monoxide, carbon disulfide, hydrogen cy-anide, as well as the hydrides as a class, which also includes hydrogen sulfide.

All other flammable gases and va-pors, including the flammable solvents mentioned in Dräger Review 98, are or-ganic substances. Their molecules al-ways contain carbon-hydrogen bonds, which is why they are also known as hy-drocarbons. And it is precisely these car-bon-hydrogen bonds which, because of their particular infrared optical proper-ties, provide the basis for the infrared de-tection of flammable gases.

The infrared measuring principle

The measuring principle is simple. Cer-tain substances absorb particular wave-lengths when exposed to white light and thereby take on a color which is percep-tible to us in the transmitted light. The same principle applies to the near infra-red range. Gas molecules likewise absorb certain wavelengths of the incident in-frared radiation.

When the intensity of the radiation in this wavelength range is measured, it can be seen that the intensity dimin-ishes in relation to the gas concentra-tion: the greater the number of gas mol-ecules present, the “darker” the received infrared radiation (IR) is. And light and dark can be converted into an electri-cal signal using an IR detector. Without

Infrared Measurement of Gases in the first and second parts of this series, we looked at safety aspects related to the detection of flammable liquids and explained the thermocatalytic measurement technique in detail (Dräger review 98, 99). This final section is devoted to a method of measurement that is based on the infrared absorption of many gases and vapors and is widely viewed as a TechnoloGy of The fuTure.

one of the harshest workplaces in the world: reliable warning of dangerous gases is vital on a drilling platform.

Ph

oTo

gr

aP

hy

: g

eT

Ty

im

ag

es

25Dräger review 100 | June 2010

explosion protection Background

going any deeper into the details of the physics involved, it is possible to estab-lish the following laws:u The IR absorption depends on the mo-lecular structure; there are strongly and weakly absorbing gases and vapors.u The IR absorption depends on the opti-cal path – the longer the route traveled by the IR, the greater the absorption (Lam-bert’s Law).u The IR absorption depends on the number of absorbing molecules along this path, so it is related to the concen-tration of the gas (Beer’s Law).

This means that the concentration of a given gas can be measured by us-ing an IR radiation source whose inten-sity is measured after the radiation has passed through a gas-filled volume with a known absorption path. This is first done using only pure air (oxygen and nitrogen do not absorb any IR) and then with the gas-air mixture to be measured. The ab-sorption of the gas is determined by cal-culating the difference in these values. This difference is a measure of the gas concentration.

That, at least, is the theory. A prac-tical application looks somewhat dif-ferent. Unlike IR analysis equipment, the IR measuring devices associated with stationary gas detection technol-ogy are field instruments that ensure re liable concentration measurements over long periods without maintenance or service.

What’s more, they do so continu-ously, sometimes in very adverse environ-ments. In field instruments of this kind, the IR radiation intensity in air is deter-

mined only once (zero point calibration) and saved as a reference value.

The sensitivity calibration is accom-plished by a similar method. The mea-surement volume is filled with the sam-ple gas, and the measured IR radiation intensity is saved as a reference value for the sensitivity. The rest is hardware and software. Any decrease in radiation intensity measured by an IR detector is subsequently compared with character-istic curves or calculated numerical val-ues stored in the measuring instrument and converted into a gas concentration.

compensation and optimization

There is, however, a small problem. A de-cline in the radiation intensity isn’t nec-

reliabilitystationary gas detection systems are automatic machines. they are operated con tinuously and are left to themselves for long periods of time. one must therefore ensure that the required safety function is, in fact, triggered in the event of a dange- rous gas concentration and is not impeded by an unnoticed fault. From the point of view of safety engineering, detectable faults are not problematic, because they can always guide the monitored system to a safe state. in the context of a failure analysis, engineers therefore determine the average probability that a non-detectable fault will occur in a system within the inspection interval (normally a year). the ratio of the failure rates resulting from non-detectable faults in relation to all other faults also plays a major role in assessing reliability. For systems with a safety function in accor-dance with safety integrity level 2 (sil2), this ratio must be under 10 percent.

to not only largely rule out hardware faults but also software errors, the entire development of such a device must be continually monitored by an independent inspecting organization according to the specifications detailed in the standard en 61508. to date, only a few gas detection instruments in the world have been certified as conforming to the standards of reliability specified in en 61508. the Dräger pir 7000 is one of them.

Standards of reliability: The german Technical

Inspectorate TÜV certifies that the standards were

applied during the develop-ment of the dräger PIr 7000.

essarily due to the presence of a gas. Sig-nals of this kind that are not caused by gases can be compensated for using the double-beam technique. Here, the IR ra-diation is divided into two wavelength ranges by a beam splitter. These are cho-sen in such a way that the gases in ques-tion only absorb radiation in one of the ranges. If, however, the IR detectors al-located to both wavelength ranges simul-taneously detect a decrease in intensity, this cannot be caused by one or more of these gases but only by contamination, or by a decrease in the intensity of the radi-ation source. The circuitry that performs the analysis therefore takes the quotient of the two signals so that influences of that kind are simply canceled out. >

26 Dräger review 100 | June 2010

ment quality can be achieved for certain gases and vapors.u As a purely physics-based measure-ment technique, the measurement can be made without the presence of oxygen. As a consequence, inerted atmospheres can also be monitored. For certain gases, such as methane, concentration mea-surements of up to 100 percent by vol-ume are possible. u The measurement signal is fail-safe, because a failure of the radiation source or contamination of the optics exceed-ing a predefined tolerance (in general, the “non-ready status of the measuring instrument”) can be quickly detected through appropriate electronic means. This increases reliability (“Safety Integ-rity”; see Dräger Review 93), as the prob-ability of undetected failures is substan-tially reduced (see box).

Unfortunately, however, there is no rule of thumb, and sensitivities cannot be predicted for substances not yet mea-

sured. The IR spectra of such substances can, at best, be used to make qualitative judgments. An IR measuring instrument can only be characterized metrologically by performing measurements with de-fined concentrations of these substances. At the Dräger applications lab, measure-ment data of this kind has been obtained for well over a hundred different gases and vapors. And, of course, this fund of metrological experience grows with ev-ery new request and measurement.

Various measuring wavelengths

If, for example, the objective is to detect many different vapors in a facility where solvents are stored, it is very important to know what substances are involved and how the intended IR measuring de-vice will react. The reason is simple: As a rule, whenever a whole group of dif-ferent substances is involved, the mea-suring instrument must be calibrated to handle the substance to which it reacts

InfraredThe wavelength of viwsible light ranges from around 0.4 (blue) to 0.8 micrometers (red). The LeDs in the remote controls of consumer electronics devices emit radiation of an only slightly longer wavelength – about 0.9 to 1 micrometer – which is already invisible, however. The wavelength range of interest for gas-measurement technology is almost four times as long, at around 3.3 to 3.5 micrometers. For these wavelengths, it is still (just) possible to use conventional radiation sources (incandescent bulbs), whereas the breath alcohol measurement techniques in the range of 10 micrometers have to use special sources. As infrared measurement technology obeys the laws of optics, one often hears of “dirty optics,” mirrors and ir optical mea-suring devices.

With the “four-beam method,” it is even possible to compensate for a decrease in the sensitivity of the two IR detectors as a result of age.

In combination with non-imaging optics and heated reflectors, modern IR measuring instruments like the Dräger PIR 7000 are equipped with many fea-tures to ensure a stable measurement signal over long periods. They also detect a large number of different substances, whose data is stored in a small “gas li-brary” database inside the device. Sim-ply switching to the specified settings for a gas in the library will both linearize the relevant characteristic curves and opti-mize the measuring features of the IR measuring instrument for this substance in many respects.

Application

The infrared measurement technique has advantages over the method based on the heat effect:u The atmosphere to be monitored, which might well contain corrosive components, has no direct contact with the sensitive IR detectors. To ensure this is the case, the latter are separated from the gas-filled measurement chamber, the “cuvette,” by an IR-transparent window. In particular, the IR measurement technique does not suffer from sensor poisoning either, which means that maintenance and calibration intervals can be extended to a year, based on prior experience.u When the cuvette length – in other words, the absorption path – is appro-priate, full-scale readings of less than 1,000 ppm with outstanding measure-

Catalytic bead and infrared complement one another

Stainless steel nose: at left is the optical system of the IR trans mitter Dräger PIR 7000 (right), which is equipped with a splashguard when in use.

>

ST-

116

59

-20

07

ST-

116

61-

200

7

27Dräger review 100 | June 2010

explosion protection Background

with the least sensitivity. As a result, this substance moves to the 45° line in the diagram of characteristic curves, and all the other characteristic lines lie above it. In an IR measuring instrument, how-ever, the resulting sensitivity spread can be considerably greater than in a cata-lytic bead sensor (see diagram). In fact, it can be so great that an alarm thresh-old of 20 percent of the lower explosive limit (LEL) can be exceeded for what are actually much smaller concentrations.

Depending on the specific applica-tion, one should therefore use IR mea-suring systems whose measured wave-length differs in the infrared range. The median wavelength of type 334, for ex-ample, is 3.34 micrometers; that of type 340 is 3.4 micrometers. The measure-ment sensitivities of these two types are very different – there are even substances that can only be detected by one of the two types. For example, only type 334 can detect ethene, butadiene, and benzene or styrene vapors, while only type 340 can detect cyclohexane vapor.

Larger molecules

One would actually suppose that the IR absorption increases with the number of carbon-hydrogen bonds in a molecule. That is true to a certain extent. Measur-ing instruments designed to detect flam-mable gases and vapors are scaled in % LEL, and the LEL itself falls as the mol-ecule size increases. In other words, this supposition is only partially correct. But IR measuring instruments can at least detect substances which, in the case of a catalytic bead sensor, would exhibit

much too small a thermocatalytic effect. They also achieve this feat with sufficient sensitivity. For example, longer-chain hydrocarbons like n-decane and unde-cane are easily detected by IR measuring instruments (preferably type 340), while catalytic bead sensors are still unable to detect them.

catalytic bead or Infrared?

It is clear that the use of IR measuring instruments without knowledge of the measuring performance, without an ap-plication laboratory, and without cus-tomer support is often not possible. This is because it is always necessary to cal-ibrate such instruments on the basis of sound safety engineering. The initial cal-ibration, and thus the degree of safety re-quired, stands or falls with the quality of the list of stored substances. In such an application, an IR measuring system is clearly the more durable and less main-tenance-intensive product when com-pared with the catalytic bead sensor. As far as the operator is concerned, the sum of the operating and purchasing costs is likely to be roughly the same in both cases – when calculated over a certain pe-riod of time.

There is no categorical answer to the frequently discussed question, “catal-ytic bead or infrared?” Both techniques have their raison d’être; they even com plement one another. The product range of stationary gas detection equip-ment can only be complete if both tech-niques for detecting flammable gases and vapors continue to be supported and improved. dr. Wolfgang Jessel

catalytic bead and infrared complement one another

D-1

105

7-20

10-2

010

The diagrams show measuring sensitivity with respect to typical solvents for three different sensors, each calibrated for propane. at the top is a catalytic bead sensor, in the middle is a type 340 Ir transmitter, and at bottom is a type 334 Ir transmitter, which is practically iden tical except for the wavelength. red: propane (LEL = 1.7 %V/V), brown: ethanol (LEL = 3.1 %V/V), yellow: ethyl acetate (LEL = 2.0 %V/V), green: methyl isobutyl ketone (LEL = 1.2 %V/V), blue: 1-methoxy-2-propanol (LEL = 1.8 %V/V), purple: toluene (LEL = 1.1 %V/V).