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The new CY7 Series Sensors from OMEGA representthe first truly new cryogenic sensor technologyintroduced in the last decade. The sensors incorporateuniform sensing elements that exhibit precise,repeatable, monotonic temperature response over awide range. The elements are mounted into rugged,hermetically sealed packages that have beenspecifically designed for proper behavior in a cryogenicenvironment.

The result is a family of sensors with temperatureresponses so predictable, tightly grouped, and stablethat the sensors can be routinely interchanged with oneanother.

A New Proprietary Silicon Diode Chip

The key to the sensors temperature response lies withthe basic sensing element itself. The small silicon chipin each sensor has a temperature characteristic that isso stable, so predictable, and conforms so well fromchip to chip, that the CY7s sensors are the first mass-produced, interchangeable cryogenic sensors.

As shown on the graph on page Z-93, the temperatureresponse profile of a CY7 is comprised of two distinctelements. With their inherent dual sensitivity, CY7sensors can cover a wide temperature range (up to 475Kelvin) and at the same time exhibit high sensitivity forcritical low temperature measurement.

Precise thermal response of the sensing element itselfis of little benefit if thermal errors generated in installing

and using the sensor swamp out its capability. It is inminimizing these frequently unsuspected errors that theCY7 excels.

A Sensor Package Designed for Cryogenics

Sensors for higher temperatures fall far short forcryogenic use. The complex thermal link between thesensing element and its entire environment must betaken into account, as must the effect of anymeasurement-induced self-heating of the sensor, if oneis to achieve accurate results. In addition, the packagemust also withstand repeated cycling to lowtemperatures without mechanical failure.

Cryogenic Temperature SensorsCY7 Series Silicon Diodes

USA

MADE IN

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AverageSlope-2.3mV/K

AverageSlope-26mV/K

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0 20 40 60 80 100 200 300 400

Temperature, K (kelvin)

Standard Temperature Response (Curve 10) for CY7 Series Sensors.

All Sensors Track this Curve Within Specified Tolerance Bands.

ForwardVoltageVf(volts)

0 10 20 30 40 50 60 70

The development of the CY7 Series has included thedesign of unique sensor packages to solve many of theproblems encountered in low temperature thermometry.For example, the CY7 hermetic package incorporates a

sapphire substrate for high electrical isolation yet goodthermal conductivity. The base bottom is metallized foreasy anchoring to a sample. Large strong leads form anintegral part of the package and are thermally sunk into thesubstrate. This simplifies making connections to the sensorand at the same time helps reduce measurement errorsthat could be caused by heat conduction along the leads.

10 Microampere Excitation CurrentKey to the achievement of error-free measurement is lowexcitation current. The lower the current, the less power isdissipated in the sensor and the less self-heating occurs.

One measure of the effectiveness of a cryogenic sensorsthermal design is the variation in reading obtained

between operation in a vacuum at liquid heliumtemperature and immersion directly in the liquid. In a fieldwhere discrepancies of a degree or more have beenreported, OMEGA CY7 sensors exhibit variations as lowas 5 millikelvin.

Cryogenic Temperature SensorsCY7 Series

2.0

1.5

1.0

0.5

0.00 50 100 150 200 250 300

Tolerance Bands for CY7 Series Sensors allowselection of appropriate (and economical)accuracy levels for a given application.

Tolerance,

K(kelvin)

Temperature, K (kelvin)

BAND 4

BAND 2

BAND 1

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CY7-SD The SD configuration is thesmallest package in this series, and is

designed primarily for bonding or clamping to a flatsurface. Since the sensing element is in best thermalcontact with the base (largest surface) of the package,the package should be mounted with that same surfacein good contact with the sample. Mounting materialsand methods which will not expose the sensor totemperatures above 200C are required. Low

temperature indium-lead-tin based solder or lowtemperature epoxy is recommended. The SD packagestyle is usable at temperatures up to 475 K.

CY7-ET This convenientscrew-in package is formedby soldering a basic SD

configuration into a recess in one flat of a hexagonalcylinder. The cylinder terminates in a standard 6-32SAE thread. Thus the sensor can be threaded (fingertight only) into a mounting hole in the sample. A lightcoating of vacuum grease on the threads furtherenhances the thermal contact between the sensorpackage and the sample. The solder used in mounting

the SD package to this adaptor constrains the upperuseful temperature of this configuration to 325 K.

CY7-MT The MT packageis similar to the ET versionexcept the SD package is

mounted in a slot in the center of the cylinder and thestud is a 3 mm x 0.5 metric thread.

CY7-CO A spring-loadedclamp holds a standard SD sensorin contact with the surface of thesample in this configuration. Thisallows the sensor to be easilychanged or replaced. It also

enables the sensor to be used over its full operationaltemperature range of 1.4 to 475 K. Extra clamps areavailable to accommodate applications where frequentrelocation of the sensor is desirable. The 4-40 brassscrew used with this clamp has a formed shoulder sothat, once the screw is properly seated, the springapplies correct pressure to the clamp.

CY7-BO In addition to beingsoldered to the mounting block,the SD sensor in this design hasits leads thermally anchored(without epoxy) to the block viaa beryllium oxide insert. Sinceleads can be a significant heatpath to the sensing element, and

can lead to measurement errors when incorrectlyanchored, this configuration helps maintain the leadsat the same temperature as the sensor. Mounting ofthis block is accomplished with a 4-40 screw (notsupplied). Usable temperature range of the CY7-BOsensor is 1.4 to 325 K.

CY7-LR With a CY7-SD sensormounted on a slightly more than half-round cylinder, this package is

designed to be inserted into a 1/8 inch (3.2 mm) diameterhole. Low temperature epoxy can also be used to installthe sensor, although the mounting is much morepermanent in that case. As with other soldered downsensors, the temperature range of the CY7-LR extendsto 325 K.

CY7-CU In this configuration, the SD sensor isepoxied into a flat cylindrical disk and the sensor leadsare thermally anchored to that same disk. The unit canbe mounted to any flat surface with a 4-40 brass screw(not supplied). The CU style sensor is wired in a four-lead configuration with the leads comprised of a 36-inchlength of OMEGAs color coded cryogenic wire.Temperature range is 1.4 to 325 K.

CY7-D1 This is a two-lead version of the the CY7-CU.

CY7-CY Some applications are best served by arelatively large, robust sensor, and the CY7-CY fills thatbill. It is very similar to the CU style except that the diskhas a larger center diameter with the mounting holedirectly in the center. The CY sensor has 36-inch heavyduty (30 AWG, PTFE coated) leads. Special attentionmust be paid to thermally anchoring the leads toprevent heat leak induced measurement error.

Probes The flexibility of the CY7 series sensorsmakes them ideal candidates for incorporation intovarious probes and thermowells. However, theindividualized nature of these applications usuallydemands customized designs.

Z-92

Select the Sensor Configuration Best Suited to Your Application

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TABLE 1. Chebychev fit coefficients

2.0 to 12.0 KA(0) = 7.556358 VL = 1.32412A(1) = -5.917261 VU = 1.69812A(2) = 0.237238A(3) = 0.334636A(4) = -0.058642

A(5) = -0.019929A(6) = -0.020715A(7) = -0.014814A(8) = -0.008789A(9) = -0.008554

12.0 to 24.5 KA(0) = 17.304227 VL = 1. 11732A(1) = -7.894688 VU = 1.42013A(2) = 0.453442A(3) = 0.002243A(4) = 0.158036A(5) = -0.193093A(6) = 0.155717A(7) = -0.085185

A(8) = 0.078550A(9) = -0.018312A(10) = 0.039255

24.5 to 100.0 KA(0) = 71.818025 VL = 0.923174A(1) = -53.799888 VU = 1.13935A(2) = 1.669931A(3) = 2.314228A(4) = 1.566635A(5) = 0.723026A(6) = -0.149503A(7) = 0.046876A(8) = -0.388555A(9) = 0.056889

A(10) = -0.116823A(11) = 0.058580

100 to 475 KA(0) = 287.756797 VL = 0.079767A(1) = -194.144823 VU = 0.999614A(2) = -3.837903A(3) = -1.318325A(4) = -0.109120A(5) = -0.393265A(6) = 0.146911A(7) = -0.111192A(8) = 0.028877A(9) = -0.029286A(10) = 0.015619

PROGRAM 2. BASIC subroutine for evaluating thetemperature T from the Chebychev seriesequations (1) and (4). ACS is used to represent thearccosine function.

100 REM Evaluation of Chebychev series110 X = ((V-VL)-(VU-V))/(VU-VL)120 T = 0130 FOR I = 0 to Ndegree140 T = T + A(I)*COS(I*ACS(X))150 NEXT I160 RETURN

Cryogenic Temperature SensorsCY7 SeriesPolynomial Representation

Curve #10 can be represented by a polynomial equation basedon the Chebychev polynomials which are described below.Four separate ranges are required to accurately describe thecurve, with the parameters for these ranges given in Table 1.The polynomials represent Curve #10 on the preceding pagewith RMS deviations on the order of 10 mK.

The Chebychev equation is of the form

T(x) = an tn (x) (1)n=0

where T(x) represents the temperature in kelvin, t n(x) is aChebychev polynomial, and an represents the Chebychevcoefficients. The parameter x is a normalized variable given by

x = (V-VL)-(VU-V) (2)(VU-VL)

where V is the voltage and VL and VU designate the lower andupper limits of the voltage over the fit range.

The Chebychev polynomials can be generated from therecursion relation

tn+1 (x)=2xtn(x)-tn-1(x), to(x)=1, t1(x)=x. (3)

Alternately, these polynomials are given by

tn(x)=cos[n*arccos(x)]. (4)

The use of Chebychev polynomials is no more complicatedthan the use of the regular power series, and they offersignificant advantages in the actual fitting process. The firststep is to transform the measured voltage into the normalizedvariable using equation (2). Equation (1) is then used incombination with equation (3) or (4) to calculate thetemperature. Programs 1 and 2 give sample BASICsubroutines which wil l take the voltage and return thetemperature T calculated from Chebychev fits.The subroutinesassume that the values VL and VU have been input along withthe degree of the fit, Ndegree. The Chebychev coefficients arealso assumed to be in an array A(0), A(1), ...,A(Ndegree).

An interesting property of the Chebychev fits is evident in theform of the Chebychev polynomial given in equation (4). Noterm in equation (1) will be greater than the absolute value ofthe coefficient. This property makes it easy to determine thecontribution of each term to the temperature calculation andwhere to truncate the series if the full accuracy is not required.

PROGRAM 1. BASIC subroutine for evaluating thetemperature T from the Chebychev series usingequations (1) and (3). An array Tc(Ndegree) shouldbe dimensioned.

100 REM Evaluation of Chebychev series110 X= ((V-VL)-(VU-V))/(VU-VL)120 Tc(0) = 1130 Tc(1) = X140 T = A(0) + A(1) *X150 FOR I = 2 to Ndegree160 Tc(l) = 2*X*Tc(l-1)-Tc(l-2)170 T = T + A(l) *Tc(l)180 NEXT 1190 RETURN

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