You can fit a ds18b20 ti the metal plate that holds the element as close to the thermostat hole as possible.
I have a ds18b20 on the plate and a geysewise thermostat and the data is very close.
There however is a marginal difference of a few degrees that you could probably compensate for.
I must confess that commercial temperature measurement has produced some interesting technologies. I thought it would boil down to either RTD 100 or a thermocouple. Looks like neither as more and more measurement is done by themistors (but arenāt they non linear??)
Iām with Goldilocks on this one. 3 temperatures are good enough for calibration of any geyser. Too hot, too cold and just right.
The actual numbers are just unnecessary details. Turn the knob until ājust rightā and leave it alone forever.
Yes, but they are fairly linear where it mattersā¦ and of course, a lookup table is not a problem for a small computer 
Obviously you arenāt using a solar geyser: My mixer is constantly being adjusted from no cold to plenty cold waterā¦ (Maybe this is why a solar feeder geyser is used to supply a mains powered one??)
There is also the endless debate about what is the actual temperature of the water. Due to layering of the hot water it varies considerably. (Iāve now installed a temperature sensor at the water outlet on top of the geyser to see what the temperature difference is)
going to keep it short, crazy day, but nodemcu spliced into my geyser wise, 2 x NO x 2 NC contactor that switches between eskom and offgrid solar, node controls that(via Home assistant, node running esphome) plus can use the node to switch geyser wise on and off, it also has a 220v relay used as a switch to tell HA that geyserwise is on or off.
safeties on eskom is geyserwise plus standard old thermal thing set at 10 deg higher than GW(55, actual 51C).
Safety on solar is only the standard thermal disconnect(do test them they do fail, build in safety in HA aswell)
i tried to splice into the ntc of the GW, but get more accurate readings from a ds18b20 senor, my tank has 2 jackets so one is for the thermal probe other one has ds18b20 and extra ntc i was testing, then i also have a ds18 on the out of the geyser so have top and bottom temps.
I still need to do some node red things to make it more efficient but have other priorities atm.
I might redo as i need more controll(extra relay from nodemcu to control GW, few more probes like water sensor in geyser pan ect)
some pics
Interesting,
I am looking at doing a similar project.
Im curious why you used a contactor? I guess there is no chance of the NO and NO contacts powering the circuit at the same time.
I already have some smart control over my geyser, including a Geyserwise thermostat, and an SCR for power control as I run it on my inverter to try and optimize solar usage. But there have been days I wanted to move it to grid to heat up.
I was looking at this project for inspiration
But I like your design and might just use a contactor.
NO/NC contactor, I have not had any issues with this contactor(yet), but i do have another one iām probably going to use for v2, its 2 contactors with an interlock in between, the switch over time should be alot slower(still ms) so less chance of connecting the 2 sources of power. reason for contactor was to split the sources aswell as handling the current(dont trust these 10-20 amp small relays) other reason for this contactor was no engagement needed when on eskom its just always passthrough on the NC side.
those are nice links thanks going to steal some ideas from there
current contactor is: LOV BG09.T2A230 4KW Mini Contactor 4P 230V AC 2NO+2NC - R380 ex vat
Other one is: *cant find it but its the black and blue from ACDC, was 120 ea back then think now around 200 ea plus interlock was about 220 bucks.
power on geyser is monitored via shelly EM with 50 amp ct
Right, feedback on the new Geyserwise IoT display:
So far, so good.
Next level, want to control the geyser based on battery charge/discharge hence NodeRED interfacing.
Iām here now, still a long way to go: node-red-contrib-tuya-smart-device (node) - Node-RED
Have asked Geyserwise if they intend to add a GeyserWise NodeRED flow for download.
Looked at that, and the Shelly ranges, the trick one must not miss, I had a Geyserwise already installed.
Hence the display upgrade for me was much cheaper and I keep the temp sensor and thermostat (safety) already in place.
If you start from scratch, nothing in place OR the Geyserwise model is too old for the new IoT display, then it can be much more expensive when adding an authorised installer cost to go Geyserwise IoT.
My BIG thing, with DIY, I need the geyser temp without compromising the safety. Learned a few things the last few days from @Gman and @Gh3kko.
I donāt need the tempā¦ just the On/Off and monitoring! So a nice cheap (relatively) solution ā¦
Going to flash Tasmota and into Node Red she goes!
Looks like this are moving along with a Tuya based Geyserwise. Not seen them for sale online yet.
See post 31, I bought one, can get it direct from Geyserwise. Officially comes out in Oct.
Ā±R675 must have a TSE unit that can take it, older TSE units cannot.
Tried to connect it to NodeRED, donāt like the Tuya App interface, or maybe Iām just stupid.
So if anyone can make it work stable connecting to NodeRED ā¦ Iām very interested
Works perfectly otherwise.
Maybe look at some YouTube videos where people set up āLocalTuyaā. It explains how to link the SmartThings app with the Tuya IOT cloud thing, and then get the keys so you can talk to the device locally.
I did it once, just to see if I like it. I promptly decided I will not buy a device I cannot flash to Tasmota. Itās somewhat of a personal taste though. The fact that Tuya is even willing to allow local access and opening up the IOT platform like that is impressive in some ways, I just donāt particularly like the effort and depending on an external entity to make hardware run.
I like Shelly though. They focus on making good hardwareā¦
Agree.
Got access to the Tuya, got the keys, but with too many attempts to connect, it drops the Wifi ā¦ I do not like it at all.
So as it is installed, it can be the manual user override. 
And Shelly, got 3 of them ā¦ Iām seriously impressed. Should have listened to @Gman a long time ago.
