There's previously been some discussion about E61 grouphead thermometers but I haven't found specific mention of TheBat device version.
Nothing ground-breaking about the design, just a different take on mounting to the grouphead compared to "Eric's device" which uses a modified Swagelock compression fitting to seal against the thermometer probe. With his, the tip of the thermometer is directly exposed to water in the grouphead when it is flowing. The downside however is the aesthetics.

Thebat device uses a modified hex head cap screw, (similar to what you remove to install this one), which has been blind drilled for a tolerance fit of the thermometer probe. The last few mm have been turned down externally creating a thinner tip for quicker heat transfer. This modified screw "thermowell" has thermal paste applied internally before installing the thermometer probe to ensure the best possible heat transfer. In effect it's the same as Eric's device just with an additional thin layer of metal over the thermometer tip. Sensitivity would be near identical. The big benefit is that the overall installation is a lot neater and less obtrusive. It looks like the original screw-head with a short length of probe and smaller thermometer head on it.


(photo from web)
Just a mention regarding cost. Both Eric's device and Thebat device seem quite expensive for what they are. As they aren't mass produced, the price reflects the effort involved. There is a fair bit of manual labor involved in modifying the length of the thermometer probe and also the mounting components to suit. The thermometers in themselves aren't expensive its just the labor component that determines the price.
Anyway, first impressions are mixed. I always start machine warm-up by running water through the grouphead and then lowering the lever to the middle position (not fully lowered to drain). This leaves water rather than air gaps in the grouphead which I believe helps with quicker and more even heating. Its early days but I've come to the conclusion that temperature readings are only indicitive and not accurate when there is no water flowing in the grouphead. Part of the reason for this may be explained by the image below. What are referred to as "Infusion channels" in the image are shown to rise towards the "plug" (which is replaced by Thebat device mounting screw) and then fall towards the "Dispersion screw" (diffuser). I believe that when there is no water flow, the water drains out of the falling section of the "Infusion channels" leaving the tip of the thermometer or thermowell just short of the water in the rising section. There will still be radiated heat from the surrounding metal but you won't get a true water temperature at rest.

However, when the E61 grouphead has been given time to fully heat-up, the temperature displayed is closer to the brew-water temperature. You see much less variation between rest temp & when brew-water is flowing. It takes the initial couple of seconds of flowing water before actual water temp is displayed but you learn to allow for this. The closer to optimal grouphead temp, the less time it takes. On my machine (Vibiemme E61 Single Boiler), once past this, I have about 0.5 deg C variation over the 25-30 sec brew cycle. I still flush 3-5 secs before brewing depending on how close to the previous heating cycle it is. If insufficient (slightly) heat-up time has been allowed, the temperature will fall 2-4 deg C during brewing which isn't desirable. At the moment I'm experimenting with controlling temperature in the 2nd shot as I sometimes see a 2 deg c rise if brewed too soon after re-heating occurrs following the 1st shot.
I found Thebat device has given a better insight into the timing of grouphead heat-up and also timing requirements between shots. Originally I set the machine thermostat to 93 deg C using a multimeter and bead thermocouple as water exited the shower screen. Yet to compare the 2 readings but on the to do list.
Nothing ground-breaking about the design, just a different take on mounting to the grouphead compared to "Eric's device" which uses a modified Swagelock compression fitting to seal against the thermometer probe. With his, the tip of the thermometer is directly exposed to water in the grouphead when it is flowing. The downside however is the aesthetics.
Thebat device uses a modified hex head cap screw, (similar to what you remove to install this one), which has been blind drilled for a tolerance fit of the thermometer probe. The last few mm have been turned down externally creating a thinner tip for quicker heat transfer. This modified screw "thermowell" has thermal paste applied internally before installing the thermometer probe to ensure the best possible heat transfer. In effect it's the same as Eric's device just with an additional thin layer of metal over the thermometer tip. Sensitivity would be near identical. The big benefit is that the overall installation is a lot neater and less obtrusive. It looks like the original screw-head with a short length of probe and smaller thermometer head on it.
(photo from web)
Just a mention regarding cost. Both Eric's device and Thebat device seem quite expensive for what they are. As they aren't mass produced, the price reflects the effort involved. There is a fair bit of manual labor involved in modifying the length of the thermometer probe and also the mounting components to suit. The thermometers in themselves aren't expensive its just the labor component that determines the price.
Anyway, first impressions are mixed. I always start machine warm-up by running water through the grouphead and then lowering the lever to the middle position (not fully lowered to drain). This leaves water rather than air gaps in the grouphead which I believe helps with quicker and more even heating. Its early days but I've come to the conclusion that temperature readings are only indicitive and not accurate when there is no water flowing in the grouphead. Part of the reason for this may be explained by the image below. What are referred to as "Infusion channels" in the image are shown to rise towards the "plug" (which is replaced by Thebat device mounting screw) and then fall towards the "Dispersion screw" (diffuser). I believe that when there is no water flow, the water drains out of the falling section of the "Infusion channels" leaving the tip of the thermometer or thermowell just short of the water in the rising section. There will still be radiated heat from the surrounding metal but you won't get a true water temperature at rest.
However, when the E61 grouphead has been given time to fully heat-up, the temperature displayed is closer to the brew-water temperature. You see much less variation between rest temp & when brew-water is flowing. It takes the initial couple of seconds of flowing water before actual water temp is displayed but you learn to allow for this. The closer to optimal grouphead temp, the less time it takes. On my machine (Vibiemme E61 Single Boiler), once past this, I have about 0.5 deg C variation over the 25-30 sec brew cycle. I still flush 3-5 secs before brewing depending on how close to the previous heating cycle it is. If insufficient (slightly) heat-up time has been allowed, the temperature will fall 2-4 deg C during brewing which isn't desirable. At the moment I'm experimenting with controlling temperature in the 2nd shot as I sometimes see a 2 deg c rise if brewed too soon after re-heating occurrs following the 1st shot.
I found Thebat device has given a better insight into the timing of grouphead heat-up and also timing requirements between shots. Originally I set the machine thermostat to 93 deg C using a multimeter and bead thermocouple as water exited the shower screen. Yet to compare the 2 readings but on the to do list.

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