Encapsulation Gas in Thermopile Detectors

Time constant, signal-to-noise ratio (SNR), responsivity and output voltage are the four key performance parameters affected based on the selection of an encapsulating gas in a thermopile detector package.

The effect of the molecular thermal conductivity of gases on the thermal resistance of the detector and package affects the time constant, responsivity and output voltage.

Thermopile model, type of package (resistance weld versus cold weld) and the amount of black absorber are the other factors affecting these performance parameters.

The selection of the encapsulating gas has less impact on these three parameters in the case of silicon-based thermopiles when compared to thin film-based thermopile detectors.

Encapsulation Gas Effect on Silicon- and Thin Film-Based Thermopiles

The specifications presented in the Dexter Research Center (DRC) data sheets are for nitrogen or argon encapsulation gas based on the detector model. The specifications of all “ST” detectors as well as the SLA32 are with nitrogen.

The specifications of all other models are with argon. These parameters vary by the same percentage, approximated by the multipliers presented in Tables 1, 2, and 3, for thin film-based, “S” type silicon-based, “ST” type silicon-based (thick rim) thermopiles, respectively.

As shown in Table 1, the use of encapsulating gas xenon in place argon in a detector package will increase the time constant, responsivity and output voltage by 2.4 times in the case of thin film-based thermopiles. Similarly, the increase in these parameters for “S” type silicon-based thermopiles will be by 1.6 times as shown in Table 2.

Table 1. Output voltage, responsivity, SNR, and time constant multipliers for thin film-based thermopile detectors relative to argon

Thin Film Based Thermopile in Argon (Ar)
Gas Multiplier
Nitrogen (N2) .75
Xenon (Xe) 2.4
Neon (Ne) .4

 

Table 2. Output voltage, responsivity, SNR, and time constant multipliers for “S” type silicon-based thermopile detectors relative to argon

“S” type Silicon Based Thermopile in Argon (Ar)
Gas Multiplier
N2 .87
Xe ~1.6
Ne 0.6

 

Table 3. Output voltage, responsivity, SNR, and time constant multipliers for “ST” type silicon-based thermopile detectors relative to nitrogen

“ST” type Silicon Based Thermopile in Nitrogen (N2)
Gas Multiplier
Ar 1.1
Xe 1.55
Ne 0.9

 

Table 2 and 3 are for silicon-based thermopiles, of which “S” type silicon-based models using argon as encapsulating gas (model S25, and S60M) are shown in Table 2. The “ST” type silicon-based models with nitrogen (all multi-channel models) as encapsulating gas are presented in Table 3. At present, the SLA32 and the LCC packages are only offered with nitrogen.

The multipliers shown in the aforementioned tables can differ by more than 25%. This difference is restricted by the fact that if a multiplier is more than 1.0, then it cannot have a value lower than 1.0. Similarly, if a multiplier is below 1.0, then it cannot have a value above 1.0. Argon, neon, xenon and nitrogen are the four standard encapsulating gas options offered by DRC. For each gas, the effect varies based on the type of the detector.

The encapsulation gas calculations for Dexter thermopile detector models are summarized in Table 4.

Table 4. Encapsulation gas calculations for Dexter thermopile detector models

Single-Channel Argon
Output Voltage (µV) Signal-to-Noise Ratio (Vs/Vn) Time Constant (ms)
S25 TO-18 23.0 1,186 16.0
S25 TO-5 40.0 2,062 18.0
M5 35.0 5,000 28.0
S60M TO-18 89.0 2,320 18.0
S60M TO-5 120.0 3,125 27.0
M14 20.0 2,857 14.0
ST60 Micro 59.4 1,896 19.8
ST60 TO-18 66.0 2,108 16.5
ST60 TO-5 68.2 2,179 19.8
ST60 with Lens 324.5 10,368 19.8
1M 60.0 8,571 32.0
1SC Compensated 48.0 3,582 48.0
M34 115.0 10,088 38.0
DR34 Compensated 115.0 7,099 38.0
ST120 TO-5 198.0 5,161 27.5
ST150 253.0 7,228 41.8
ST150 with Lens 357.5 10,215 41.8
DR46 Compensated 210.0 11,602 40.0
2M 250.0 19,531 85.0
3M 440.0 25,581 100.0
6M 370.0 18,317 221.0
Multi-Channel
ST60 Dual 68.2 2,179 19.8
DR26 54.0 5,684 38.0
DR34 115.0 10,088 38.0
ST120 Dual 181.5 4,731 27.5
ST150 Dual 253.0 7,228 41.8
DR46 210.0 16,406 40.0
T34 Compensated 115.0 7,099 38.0
ST60 Quad 68.2 2,179 19.8
ST120 Quad 154.0 4,014 27.5
ST150 Quad 253.0 7,228 41.8
2M Quad 250.0 19,531 85.0
10 Channel 115.0 10,088 38.0
Single-Channel Nitrogen
Output Voltage (µV) Signal-to-Noise Ratio (Vs/Vn) Time Constant (ms)
S25 TO-18 20.0 1,032 13.9
S25 TO-5 34.8 1,794 15.7
M5 26.3 3,750 21.0
S60M TO-18 77.4 2,018 15.7
S60M TO-5 104.4 2,719 23.5
M14 15.0 2,143 10.5
ST60 Micro 54.0 1,724 18.0
ST60 TO-18 60.0 1,916 15.0
ST60 TO-5 62.0 1,981 18.0
ST60 with Lens 295.0 9,425 18.0
1M 45.0 6,428 24.0
1SC Compensated 36.0 2,687 36.0
M34 86.3 7,566 28.5
DR34 Compensated 86.3 5,324 28.5
ST120 TO-5 180.0 4,692 25.0
ST150 230.0 6,571 38.0
ST150 with Lens 325.0 9,286 38.0
DR46 Compensated 157.5 8,702 30.0
2M 187.5 14,648 63.8
3M 330.0 19,186 75.0
6M 277.5 13,738 165.8
Multi-Channel
ST60 Dual 62.0 1,981 18.0
DR26 40.5 4,263 28.5
DR34 86.3 7,566 28.5
ST120 Dual 165.0 4,301 25.0
ST150 Dual 230.0 6,571 38.0
DR46 157.5 12,305 30.0
T34 Compensated 86.3 5,324 28.5
ST60 Quad 62.0 1,981 18.0
ST120 Quad 140.0 3,649 25.0
ST150 Quad 230.0 6,571 38.0
2M Quad 187.5 14,648 63.8
10 Channel 86.3 7,566 28.5
Single-Channel Xenon
Output Voltage (µV) Signal-to-Noise Ratio (Vs/Vn) Time Constant (ms)
S25 TO-18 36.8 1,898 25.6
S25 TO-5 64.0 3,299 28.8
M5 84.0 12,000 67.2
S60M TO-18 142.4 3,712 28.8
S60M TO-5 192.0 5,000 43.2
M14 48.0 6,857 33.6
ST60 Micro 83.7 2,672 27.9
ST60 TO-18 93 2,970 23.25
ST60 TO-5 96.1 3,071 27.9
ST60 with Lens 457.2 14,609 27.9
1M 144 .0 20,570 76.8
1SC Compensated 115.2 8,597 115.2
M34 276.0 24,211 91.2
DR34 Compensated 276.0 17,038 91.2
ST120 TO-5 279 7,273 38.75
ST150 356.5 10,185 58.9
ST150 with Lens 503.7 14,393 58.9
DR46 Compensated 504.0 27,845 96.0
2M 600.0 46,874 204.0
3M 1056. 61,394 240.0
6M 888 .0 43,961 530.4
Multi-Channel
ST60 Dual 96.1 3,071 27.9
DR26 129.6 13,642 91.2
DR34 276.0 24,211 91.2
ST120 Dual 255.7 6,667 38.75
ST150 Dual 356 .5 10,185 58.9
DR46 504.0 39,374 96.0
T34 Compensated 276.0 17,038 91.2
ST60 Quad 96.1 3,071 27.9
ST120 Quad 217 5,656 38.75
ST150 Quad 356.5 10,185 58.9
2M Quad 600.0 46,874 204.0
10 Channel 276.0 24,211 91.2
Single-Channel Neon
Output Voltage (µV) Signal-to-Noise Ratio (Vs/Vn) Time Constant (ms)
S25 TO-18 13.8 712 9.6
S25 TO-5 24.0 1,237 10.8
M5 14.0 2,000 11.2
S60M TO-18 53.4 1,392 10.8
S60M TO-5 72.0 1,875 16.2
M14 8.0 1,143 5.6
ST60 Micro 48.6 1,552 16.2
ST60 TO-18 54 1,724 13.5
ST60 TO-5 55.8 1,783 16.2
ST60 with Lens 265.5 8,483 16.2
1M 24.0 3,428 12.8
1SC Compensated 19.2 1,433 19.2
M34 46.0 4,035 15.2
DR34 Compensated 46.0 2,840 15.2
ST120 TO-5 162 4,223 22.5
ST150 207 5,914 34.2
ST150 with Lens 292.5 8,357 34.2
DR46 Compensated 84.0 4,641 16.0
2M 100.0 7,812 34.0
3M 176.0 10,232 40.0
6M 148.0 7,327 88.4
Multi-Channel
ST60 Dual 55.8 1,783 16.2
DR26 21.6 2,274 15.2
DR34 46.0 4,035 15.2
ST120 Dual 148.5 3,871 22.5
ST150 Dual 207 5,914 34.2
DR46 84.0 6,562 16.0
T34 Compensated 46.0 2,840 15.2
ST60 Quad 55.8 1,783 16.2
ST120 Quad 126 3,284 22.5
ST150 Quad 207 5,914 34.2
2M Quad 100.0 7,812 34.0
10 Channel 46.0 4,035 15.2

Time Constant and Output Voltage Calculations for DRC model 2M

As shown in Table 4, the time constant for the DRC model 2M with argon encapsulating gas is 85ms. The approximate time constant for the model 2M using xenon encapsulating gas can be calculated by multiplying the time constant value of argon by 2.4 (xenon multiplier in Table 1), which gives 204ms.

Similarly, the output voltage of the model 2M with argon encapsulating gas under exposure to 330µW/cm2 radiation is 250µV (Table 4). By multiplying this value with xenon multiplier of 2.4 given in Table 1, the approximate test stand output voltage can be calculated for the model 2M using xenon as encapsulating gas. The resulting output voltage for the 2M encapsulated with xenon is 600µV.

This information has been sourced, reviewed and adapted from materials provided by Dexter Research.

For more information on this source, please visit Dexter Research.

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