Thermopile Detectors Affected by Gas Encapsulation

The choice of encapsulating gas in a thermopile detector package influences four significant performance parameters: the responsivity, output voltage, time constant, and signal-to-noise ratio (SNR).The molecular thermal conductivity differs for different gases. The thermal resistance of the detector and package are affected by the molecular thermal conductivity, which in turn affects the time constant, responsivity, and output voltage. It must be noted that there are other factors that affect these parameters including the thermopile model, type of package (cold weld vs. resistance weld), and the amount of black absorber. The influence of the encapsulating gas have on these three parameters is more with Thin Film-Based Thermopile Detectors than Silicon-Based Thermopiles.

The specifications given on the Dexter Research Center (DRC) data sheets are for Nitrogen or Argon encapsulation gas, based on the detector model. While all “ST” detector specifications as well as the SLA32 are with Nitrogen, the remaining are with Argon (see individual data sheets for encapsulation gas specified). These parameters vary by the same percentage, approximated by the Multipliers depicted in Tables 1, 2, and 3 for Thin Film Based, “S” type Silicon Based, and “ST” type Silicon Based (thick rim) thermopiles, respectively.

For instance, when a detector package is encapsulated with Xenon in place of Argon, the output voltage, time constant, and responsivity would increase by 2.4 times for Thin Film Based Thermopiles (see Table 1 below). On the other hand, these parameters would increase 1.6 times for “S” type Silicon Based Thermopiles (see Table 2 below). The backfill gas calculations for all DRC detector models are shown in Table 4.

DRC provides four standard encapsulating gas options: Argon, Xenon, Nitrogen, and Neon. The effect differs for every gas based on the type of thermopile detector - whether it is a Thin Film, “S” type Silicon Based, or “ST” type Silicon Based. The tables below show an approximation of the encapsulating gas factor (Multiplier) for these three groups of thermopile detectors. The Multipliers below can differ by over 25%. This variation is restricted by the fact that the multiplier cannot go below 1.0 if a Multiplier is greater than 1.0, and if a Multiplier is less than 1.0, then the multiplier can not go over 1.0.

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) 0.75
Xenon (Xe) 2.4
Neon (Ne) 0.4

 

Two tables for Silicon Based Thermopiles are shown below: Table 2 is for “S” type Silicon Based models with data sheets using Argon (model S25, and S60M).

Table 3 is for “ST” type Silicon Based models with data sheets using Nitrogen (this includes all multi-channel models). Presently, the LCC and the SLA32 packages are only available with N2 encapsulating gas.

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 0.87
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 4 below, shows the encapsulation gas calculations for DRC’s thermopile detector models.

2M Time Constant Example

To demonstrate how the above encapsulating gas Multipliers work, the DRC model 2M is considered. From the DRC data sheet for the 2M, the time constant is 85 ms when encapsulated with Argon gas.

In order to measure the approximate time constant in Xenon, multiply the Argon time constant of 85 ms by the Ar to Xe Multiplier of 2.4 (see Table 1) which gives 85 ms x 2.4 = 204 ms.

Thus, by encapsulating the model 2M with Xe, the time constant is about 204 ms instead of 85 ms for Ar.

2M Output Voltage Example

The same holds true for the output voltage as well. From the DRC data sheet for the model 2M, the output voltage is 250 µV when exposed to 330 μW/cm2 radiation and encapsulated with Argon gas.

To find out the approximate test stand output voltage for the 2M encapsulated with Xenon, multiply the voltage of 250 µV by the Ar to Xe Multiplier of 2.4 (see Table 1) which gives 250 µV x 2.4 = 600 µV.

Consequently, by encapsulating the model 2M with Xe, the test stand output voltage is about 600 µV instead of 250 µ for Argon.

Application Brief 7: Effects of Encapsulation Gas on Thermopile Detectors

Single-Channel Argon Nitrogen Xenon Neon
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
S25 TO-18 23.0 1,186 16.0 20.0 1,032 13.9 36.8 1,898 25.6 13.8 712 9.6
S25 TO-5 40.0 2,062 18.0 34.8 1,794 15.7 64.0 3,299 28.8 24.0 1,237 10.8
M5 35.0 5,000 28.0 26.3 3,750 21.0 84.0 12,000 67.2 14.0 2,000 11.2
S60M TO-18 89.0 2,320 18.0 77.4 2,018 15.7 142.4 3,712 28.8 53.4 1,392 10.8
S60M TO-5 120.0 3,125 27.0 104.4 2,719 23.5 192.0 5,000 43.2 72.0 1,875 16.2
M14 20.0 2,857 14.0 15.0 2,143 10.5 48.0 6,857 33.6 8.0 1,143 5.6
ST60 Micro 59.4 1,896 19.8 54.0 1,724 18.0 83.7 2,672 27.9 48.6 1,552 16.2
ST60 TO-18 66.0 2,108 16.5 60.0 1,916 15.0 93 2,970 23.25 54 1,724 13.5
ST60 TO-5 68.2 2,179 19.8 62.0 1,981 18.0 96.1 3,071 27.9 55.8 1,783 16.2
ST60 with Lens 324.5 10,368 19.8 295.0 9,425 18.0 457.2 14,609 27.9 265.5 8,483 16.2
1M 60.0 8,571 32.0 45.0 6,428 24.0 144.0 20,570 76.8 24.0 3,428 12.8
1SC Compensated 48.0 3,582 48.0 36.0 2,687 36.0 115.2 8,597 115.2 19.2 1,433 19.2
M34 115.0 10,088 38.0 86.3 7,566 28.5 276.0 24,211 91.2 46.0 4,035 15.2
DR34 Compensated 115.0 7,099 38.0 86.3 5,324 28.5 276.0 17,038 91.2 46.0 2,840 15.2
ST120 TO-5 198.0 5,161 27.5 180.0 4,692 25.0 279 7,273 38.75 162 4,223 22.5
ST150 253.0 7,228 41.8 230.0 6,571 38.0 356.5 10,185 58.9 207 5,914 34.2
ST150 with Lens 357.5 10,215 41.8 325.0 9,286 38.0 503.7 14,393 58.9 292.5 8,357 34.2
DR46 Compensated 210.0 11,602 40.0 157.5 8,702 30.0 504.0 27,845 96.0 84.0 4,641 16.0
2M 250.0 19,531 85.0 187.5 14,648 63.8 600.0 46,874 204.0 100.0 7,812 34.0
3M 440.0 25,581 100.0 330.0 19,186 75.0 1056.0 61,394 240.0 176.0 10,232 40.0
6M 370.0 18,317 221.0 277.5 13,738 165.8 888.0 43,961 530.4 148.0 7,327 88.4

 

Multi-Channel Argon Nitrogen Xenon Neon
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
Output Voltage
(μV)
Signal-to-Noise Ratio
(Vs/Vn)
Time Constant
(ms)
ST60 Dual 68.2 2,179 19.8 62.0 1,981 18.0 96.1 3,071 27.9 55.8 1,783 16.2
DR26 54.0 5,684 38.0 40.5 4,263 28.5 129.6 13,642 91.2 21.6 2,274 15.2
DR34 115.0 10,088 38.0 86.3 7,566 28.5 276.0 24,211 91.2 46.0 4,035 15.2
ST120 Dual 181.5 4,731 27.5 165.0 4,301 25.0 255.7 6,667 38.75 148.5 3,871 22.5
ST150 Dual 253.0 7,228 41.8 230.0 6,571 38.0 356.5 10,185 58.9 207 5,914 34.2
DR46 210.0 16,406 40.0 157.5 12,305 30.0 504.0 39,374 96.0 84.0 6,562 16.0
T34 Compensated 115.0 7,099 38.0 86.3 5,324 28.5 276.0 17,038 91.2 46.0 2,840 15.2
ST60 Quad 68.2 2,179 19.8 62.0 1,981 18.0 96.1 3,071 27.9 55.8 1,783 16.2
ST120 Quad 154.0 4,014 27.5 140.0 3,649 25.0 217 5,656 38.75 126 3,284 22.5
ST150 Quad 253.0 7,228 41.8 230.0 6,571 38.0 356.5 10,185 58.9 207 5,914 34.2
2M Quad 250.0 19,531 85.0 187.5 14,648 63.8 600.0 46,874 204.0 100.0 7,812 34.0
10 Channel 115.0 10,088 38.0 86.3 7,566 28.5 276.0 24,211 91.2 46.0 4,035 15.2

 

LCC package, SLA32: Available with Nitrogen gas only.

Dexter Research Center, Inc

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

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

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