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.

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.

Table 3. Output voltage, Responsivity, SNR, and time constant Multipliers for “ST” type Silicon Based Thermopile detectors relative to Nitrogen.

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

LCC package, SLA32: Available with Nitrogen gas only.

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.