Chilled Mirror Dew Point Sensor Optics

Improvements in optical dew detection have established the chilled mirror dew point sensor an accurate and reliable method of continuous humidity measurement.

as

efore describing the optical sys- tems tha t can be used in t h e B chilled mirror sensor, we will first

review the way this type of sensor meas- ures dew point. D e w point can be defined as the temperature at which a plane surface of water (or ice) experi- ences n o net change in mass due to evaporation or condensation. This tem- perature is entirely dependent on the water vapor pressure of the surrounding gas. In keeping with t h e definit ion above, the chilled mirror sensor acquires a small amount of water on its mirror by condensation and then maintains its mirror temperature at the dew point by detecting small changes in that amount.

ANALOG CONTROL LOOP Figure 1 shows how the chilled mirror

sensor and associated electronics join to form a closed-loop dew point measure- ment system. Because the intensity of light reflected off the mirror is, ideally, inversely proportional to the water mass on it, it is possible to design a propor- tional controller with finite gain such that every optical intensity value is mapped to a particular value of Peltier cooler power. With an optical system capable of reporting changes in water mass on the mirror, the loop will control at a particular reduction in optical signal intensity. This intensity is, theoretically,

18 SENS0R.S October 1994

dependent only on the dew point of the gas stream passing over the mirror. The mirror temperature measurement is made by a platinum RTD embedded beneath the mirror and is independent of the control loop just described.

OPTICAL SYSTEM FIGURES OF MERIT

A major contributor to phase lag in the analog control loop just described is that the mirror temperature must drop below the dew point in order to acquire its ini- tial dew layer by condensation. The large- signal optical sensitivity will directly affect the controller response time, and can be defined as:

O.S. = %RED (1) '20 mass

where:

HzOmass = mass of condensation on chilled mirror

O.S. = optical sensitivity

%RED = (1 - Iwet / IdrJ 100

where: Iwet = reflected light intensity,

Idry = reflected light intensity, wet mirror

dry mirror

From Equation (l), it is seen that the larger the percent reduction for a given mass of water on the mirror, the better the optical sensitivity. It follows that the better the sensitivity, the shorter the waiting time to reach a particular reduc- tion in signal (for a given dew growth rate). In terms of small-signal sensor response, we want the sensor to generate as large an error signal as possible when responding to a dew point disturbance. High sensitivity accomplishes this.

Figure 1. The analog feedback loop in the chilled mirror dew point sensor is essentially a thermal servo system. Proportional and rate feedback, along with the integrating nature o f dew growth, form a com- plete PID controller. Mirror temperature is measured separately, and can be reported in digital and analog formats.

The optical S/N ratio defines the min- imum resolvable change in water mass on the mirror. Noise, in the case of the chilled mirror sensor, can be broadly defined as any detected optical signal from a source other than the mirror. Noise can be further subclassified into AC and DC noise. The primary sources of AC noise in the bandwidth of interest ( e 10 Hz) are photodetector thermal noise and stray capacitive pickup, which can be passed to the amplifier outputs and falsely reported as a dew point change. DC noise sources include pho- todetector leakage currents and source- to-detector path crosstalk. D C noise reduces the dynamic range of signal reduction, but does not affect the mini- mum resolvable small signal. A large optical S/N is always desirable, and a typical value for recently developed chilled mirror sensors is about 500: 1.

TYPICAL OPTICAL SYSTEMS Most chilled mirror optical arrange-

ments share certain characteristics. All possess a light source, reflecting mirror,

Stephen M. Tobin, General Eastern Instruments

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Figure 2. When a chilled mirror dew point sensor with unfocused optics grows excess water, the de- tected signal generally increases with water mass after coalescence. The result is an inflection point in the plot o f water mass vs. detected signal, and ulti- mately a sensor "crash.

and light intensity detector. The mecha- nism of water mass detection is as follows:

The system is usually balanced when the mirror is dry, i.e., when the mirror temperature is above the dew point. When the control loop drives the mirror temperature below the dew point, water condenses on the mirror surface in small droplets. The surface tension of water mated against the mirror surface mate- rial (typically rhodium or gold) produces highly spherical droplet shapes that dis- turb the normal light path. The distur- bance arises either from first-surface reflection off the droplet (scatter) or from refraction. Both phenomena usu-

ally result in a reduction of light signal intensity.

The physical arrangement of the opti- cal system in a chilled mirror sensor can be classified as either unfocused or focused. Let us first examine the sim- plest of these, the unfocused system.

Figure 2A shows a layout of an unfo- cused system, which consists of a light source (modeled as a point source), a mirror, and a detector. It is easy to see from the figure that not all light incident on the mirror impinges on the detector. This simple observation has serious implications concerning the analog con- trol loop, which we will now describe.

Turning to Figure 2B, we see that our unfocused system has grown a single large droplet of water, which has de- veloped due to the coalescence of sev- eral smaller droplets. We can further observe that the light intensity reaching the detector is greater with the droplet than without it. This phenomenon is in direct violation of one of the previously noted control loop requirements, i.e., that the optical signal must continue to decrease in inverse proportion to water mass on the mirror. T h e situation in

Figure 3. This elementary focused optical system is a solution to the crash problem, since all light strik- ing the mirror is detected when the mirror is dry. Any optical disturbance at the mirror, including a large droplet, results in a detected signal reduction.

______

Figure 2B will eventually result in the sensor's toggling around the freezing point in an endless cycle known in the industry as a "crash."

Focused optical systems for chilled mirror sensors can take many forms, two of which we will discuss here.

Figure 3 is a schematic for one of these, where a focusing lens has been added into the optical path to provide convergence to the system. A crash mode is avoided because the mirror acts as an aperture in the system, and any disturbance in this aperture results in loss of focus at the detector.

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Figure 4. The classical Schlieren reflecting systei can detect small changes in refructive index betwee its mirrors due to optical wavefront distortion. long path length in this scheme improves sensitivit.

A more symmetric version of t h focused system is derived from the classi Schlieren optical arrangement, schematic for which is shown in Figure 2

Light rays traveling between mirrors M and M2 are collimated. Any disturbanc between the mirrors causes the image t be shifted from the detector. This syster has a long path length and is used t study heat transfer convection current The mirrors are sized typically at fA0.

The Schlieren arrangement can b applied to the chilled mirror sensor, a shown in Figure 5. Once again, the mil ror is acting as an aperture for coll mated light, so that any disturbanc appearing on it (in the form of liqui water or frost) results in loss of focus a the detector.

_ _

Figure 5. The Schlieren principle can be applied tc the chilled mirror dew point sensor as a refracting optical system. Essentially flat wavefronts strike the mirror, where any disturbance results in a loss of image focus.

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I I - - 1 - 4

'igure 6. When a light ray is deviated by the forma- on o f dew on the mirror, it I S desirable to avoid its etection For a given refraction angle 0, a long path v g t h will result in the required signal decrease.

3 ES I C N TR AD EO FFS The design goal in the focused chilled

nirror optical system is large signal, low ioise, and high sensitivity. As in all areas )f engineering, achieving this goal equires a series of tradeoffs: Signal. A large signal requires a tightly

ocused light beam with accurate place- nent at the detector. There is also an )bvious benefit to driving the source as lard as possible, since the output irradi- Lnce of most light sources increases with he excitation current through them. ;mall-spot diameters generally require he use of multi-element lenses and small ipertures to reduce the effect of spheri- :a1 aberration. Sources are driven as hard IS environmental conditions will allow. Noise . T h e most common noise ources in chilled mirror sensors are: #ource/detector crosstalk; source irradi- ince changes due to ambient tempera- ure changes; detector thermal noise; letector leakage current; and stray :apacitive pickup. It is a straightforward xocedure to use opaque materials to iso- ate the source body from the detector 'ield of view. General Eastern Instru- nents (GEI) has successfully imple- nented many source temperature com- lensation schemes over the years, and a hcussion of these is beyond the scope if this paper. Leakage current effects :an be eliminated in the use of silicon letectors by running them at zero DC ias. Random thermal noise is an ongo- ng concern, and its magnitude is propor- ional to detector absolute temperature. Sensitivity. In general, maximizing the

iensitivity requires long optical path engths. Our experience has shown that l a th lengths on the order of a few nches give acceptable sensitivity with- ,ut causing excessive alignment prob- ems. Figure 6 shows t h e way pa th ength might affect sensitivity for a ;iven angular deviation 0 of a light ray.

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The lateral deviation X at the detector is given by:

(2) where: L = pathlength

In the figure, the detector D2 placed at length L2 is more sensitive than detector D1 placed at length L1.

Source Wavelength. The marketplace offers many light sourceldetector com-

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X = L sine

RELATIVE SPECTRAL SENSITIVITY S R E L ~ f ( A ) 100

80

I I 60

ap -I W

* 40

20

0 400 600 800 1000 1200

A (nm)-

rip-e 7. The radiant sensitivity o f the silicon pho- 3diode is well matched to the spectral output o f a ypical GaAlAs NIR LED, which is centered at a javelength of 880 nm. The result is improved effi- iency and S / N ratio. (Plot courtesy of Siemens Zomponents, Inc., Iselin, New jersey.)

ient energy and the short-wavelength ihotons are absorbed a t the detector urface rather than at the iunction.

tECENT DESIGN M PROVEM E NTS Many users of chilled mirror sensors re interested in visual observation of he mirror. This is because at dew points lelow O'C, the water deposit on the mir- or can take the form of supercooled liq- lid droplets or ice crystals. It is well nown that the equilibrium vapor pres- ures over ice and over water are differ- n t for the same mirror temperature. 'his means that the actual water con- ent of the gas stream can be different or the same dew point reading given by he sensor. In order to resolve this dis- iarity, it is necessary for users to know he phase of the water deposit. One way to give the user this visual

lption is to build a microscope directly n to the optical system. As shown in Tigure 8, the microscope path is simply milt into the unit on an axis orthogonal '0 the electronic sensing axis. A second, iisible light source is used and imaged lirectly onto the observer's eye. One drawback to the built-in micro-

;cope is that it requires an open aper- ure to the outside world. Light sources )utside the sensor can therefore affect ts proper function. Many schemes have ieen used in optical instrumentation to

24 SENSORS Octaber 1994 Circle 78 on Sensors RS Card

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olve this problem. GEI has successfully sed an electronic chopper technique hat involves switching the light source n and off at a particular frequency (see ’igure 9). Detec t ion circuitry syn- hronously reads the “source on” and source off” signal levels, takes the dif- Erence between these two levels, and mplifies it. The amplified signal is pro- ortional to radiation detected only from iside the sensor; all other light is re- x t e d . A practical limitation of this

method is that too large a value of out- side light irradiance can saturate the amplifier outputs and cause the usable output signal to approach zero.

Another recent improvement in chilled mirror optical design is the hermetic optical path, where the portion of the Schlieren system above the gas sample cavity is sealed from the outside world. T h e reason is that in low dew point applications it is often beneficial to cool the sensor base below ambient tempera-

Continued on page 65

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‘igure 8. As viewed from above the mirror (shown ere as a dashed hexagon), the microscope path is laced on an axis orthogonal to the dew detection ath. Light from the visible LED propagates into 4e page, strikes the mirror, and returns through the iewing aperture to the observer’s eye.

Figure 9. Synchronous detection of chopped light emitted from the LED inside the sensor results in rejection o f ambient light generated outside the sen- sor. Each light source signal is unity gain amplified with opposite polarity, sampled, and added. Finally, the summed signal is fed to the power amplifiers for Peltier cooler control.

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OTHER MAGNETIC SENSORS Other schemes exist for the measure-

ment of magnetic fields. There is fre- quently a tradeoff among cost, type of output signal, magnetic measurement range, bandwidth, and support equip- ment required. Table 1 provides infor- mation about some of the various mag- netic sensor technologies available. Hall effect sensors, which can be purchased for a few dollars, are useful for measur- ing fields > 10 gauss.

At the other end of the spectrum are the SQUID (superconducting quantum interference device) magnetometers. Be- cause they require liquid helium for cooling, the complete instruments are relatively bulky. SQUIDS can be ex- tremely sensitive-but they are expen- sive.

Nuclear precession magnetometers are sensitive, very stable, and fairly expen- sive. They provide total (magnitude) field measurement, and are not direc- tional. Other technologies include opti- cally pumped, fiber-optic, and magneto- optic magnetometers.

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Continued from fiage 26 ture in order to achieve lower mirror temperature. This in turn cools all parts of the sensor, including the optics, below the ambient temperature. If, however, the optics are cooled below the ambient dew point, they can fog and erroneously indicate a signal reduction at the mirror. The hermetic optical path prevents this problem.

AP P L I CAT1 0 N NOTE The chilled mirror dew point sensor is

used extensively in the thermal process- ing of metals. One subset of this applica- tion is the sintering of parts formed from powdered metal. Parts are heated in a moving belt furnace to - 2000°F to facil- itate microscopic joining of one metallic particle to another. Dew point control is crucial to this process because dissoci- ated water vapor can damage the metal surfaces in a variety of ways. Steel and iron may become oxidized and subject to rusting, or oxygen ions may combine with carbon in steel, causing surface decarburization and the formation of carbon monoxide.

CONCLUSIONS We have shown through geometric

arguments that a well-focused optical sys- tem will outperform an unfocused system in terms of both sensitivity and crash immunity in the chilled mirror dew point sensor. We have stated that a large opti- cal S/N ratio is always desirable, and have reviewed several ways of achieving this. Optical layouts have been discussed, along with descriptions of some of the components used in these layouts. Finally, the built-in microscope, ambient light rejection, and hermetic optical path were presented as examples of recent advances in the state of the art.

Stephen M. Tobin is a Project Engineer, Gen- eral Eastern Instruments, 20 Commerce Way, Woburn, MA 01801; 61 7-938-7070, X-252, fax 61 7-938-1 071.

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SENSORS October 1994 65

Product Sheets cover the features, bene- fits, and applications of all-silica fibers, plastic- clad silica fibers, standard and specialty fiber- optic bundles and specialty fiber coatings. Fiberguide Industries, Inc., Stirling, NJ. SEN Circle 269

MEDICA Business Report is a 1993/94 compendium of international businesses that focus on the biomedical field: “Contacts for Contracts” lists offers and requests for busi- ness relationships. Market Intelligence Part- ners, Cupertino, CA. SEN Circle 270

Industrial Vibration Sensors covers pre- dictive maintenance and machine condition monitoring programs, including accelerome- ters and other hardware. Dytran Instruments, Inc., Chatsworth, CA. SEN Circle 271

I994 MVA/SME Machine Vision Industry Directory (a) lists suppliers of vision systems, components, and services. Cross referenced alphabetically, geographically, and categori- cally. (b) 1982-1 992 Bibliography of Ma- chine Vision Technical Resources contains abstracts, papers, books, and videotapes, or- ganized by category. SME, Dearborn, MI. (a) SEN Circle 272, (b) SEN Circle 268

Analytical Instrumentation catalog fea- tures elemental analyzers for nitrogen, sulfur, carbon, and fluoride for procesdon-line appli- cations. Antek Instruments, Inc., Houston, TX. SEN Circle 273

Advanced U V Technology covers methods and product specifications for disinfection and contaminat ion control. Aquionics, Erlanger, KY. SEN Circle 274

Siemens Optoelectronics I 9 9 4 Short Form optocouplers, LED lamps, IR and fiber- optic emitters and custom products. Siemens Components, Optoelectronics Div., Cuper- tino, CA. SEN Circle 275

Metal Repair, Techni- 1, describes putties and cast iron, carbon, and

SS components in burners, furnaces, and other such structures to 2000°F. Aremco Products, Inc., Ossining, NY. SEN Circle 279

Hermetical ly and Environmentally Sealed Products and Capabilities, Techni- cal Product Bulletin No. 301, describes probes, viewports, optical windows, feed- throughs, connectors, and terminations for fiber-optic DC, and RF applications. Three E Laboratories, Lansdale, PA. SEN Circle 280

Elegtironic Interface Panel brochure de- tai ’fhe capabilities of the CE-1555, an elec- 2 onic interface panel which can be incor- porated into other products. Cherry Electrical Products, Waukegan, IL. SEN Circle 281

Temperature Measurement Designer’s Guide is a 500-pp. manual for designers and plant operators who are responsible for tem- perature measurement and control. Techni- cal data includes applications and specs for RTDs, thermocouples, wire and cable, con- nectors, panels, and instrumentation. Thermo Electric, Saddle Brook, NJ. SEN Circle 282

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