I did some very simple tests on a stainless steel BrewBuilt fermenter.
I closed the fermenter lid and all the ports and butterfly valve. The BLE signal was -71dB
I then used this product on a single 1.5"TC port on the lid:
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With this attached the BLE signal was averaging -68dB. A small difference but still noticeable. The 1.5" port is quite small.
I then got a round piece of plastic the same size as the 4" TC lid and covered the whole lid hole and the BLE signal improved to -61dB.
These tests were done with my mobile phone and averaging the readings over about 1min and trying to to moving anything else when doing the comparison.
Based on this I would say it's fair to say switching over to plastic does seem to make a substantial difference on this basic testing. I think we might need to make a 4" TC lid that looks like this one:
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But is compatible with 4" TC fittings so it suits the wide range of fermenters with 4" TC at the top. What do you guys think?