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 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

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 2 and 3 are for silicon-based thermopiles, of which “S” type silicon-based models using argon as encapsulating gas (model 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 LCC package is 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

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.