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Question
petergebruers 1,259
I write this topic to help you decide if you want the "external sensor" (+ 15 EUR, +10 EUR bought as a bundle)... To avoid confusion: the FGT-001 does only displays temperature if you add the external sensor. Without the sensor, it displays "--". But that is not the main reason to buy one... You might have heard the TRVs cannot measure temperature accurately, because their sensors are "not in the right spot". But the question is... how bad is it? I encourage you to have a look at the setup, the graphs and, if possible, repeat the experiment. For me it is clear: I wanted the external sensor for the FGT even before I did the experiment, based on experience with older Danfoss valves. But this is not a general advise, it probably depends on a lot of factors.
Part 1 is this topic. I'll show you graphs of the temperature "in the room" and "near a valve". So you can decide for yourself. In "Part 1" I do not use my FGT yet! I compare the result of different temperature sensors in the room. I only want to know if they match.
Part 2 is not available yet, because the FGT has a bug that causes it to behave erratically. I'll publish results after running tests with the firmware release that will fix this. In "Part 2" I'll establish if the FGT + external sensor (a) does measure accurately (b) can control room temperature.
DISCLAIMER. "My room" is not "your room" and certainly insulation and heating system are different. If you want to make sure, please do your own measurements (and post them so we can all learn from your experiments).
Setup and measurements.
I have a DS18B20 sensor connected to a Z-Uno. It reports temperature every few minutes (after running it through a filter for smoothing). It is attached to my mechanical TRV, like this:
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/monthly_2017_12/ValveSensor.jpg.9dcac03e760143164475f0a902763e0d.jpg" />As you can see, I really did "bugger all" to make this a professional solution, sorry, no time to waste... It is a temporary setup anyway. However, the measurements will be fine.
Now I need a reference device. I did slightly more than "bugger all" because this device has been my temperature reference since 2012. It is a DS18B20, probably from the same batch, but connected to a Fibaro UBS. I mounted it near "the sofa". Hence its name... "Sofa"...
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First, let us see if the sensors need calibration. So I mounted the valve sensor in the same spot as the sofa sensor.
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I would say they are identical. I am not a mathematician, I am an engineer, we have a different definitions of the concept: "the same".
So now let's put the valve sensor back as shown in the first picture.
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Let's see if I can explain what we see. The valve sensor = red almost always registers a temperature that is too high.
The left part of the graph is "the evening". If we ignore the dips on the graph, I'd say the sofa measures 1.5 - 2.0 °C lower. Is this plausible? Yes, when I used an LC13 on that radiator I had to set it about 1.5 higher.
The sudden drop and the right part of the graph are caused by two things: turning down the TRV and opening a door. The room next to this one is cooler (TRV is an LC12, controlled by the heating panel).
As you can see... For about one hour the TRV measures a temperature below the sofa sensor. I live in a could house, the TRV is close to a window. Temperature at the TRV drops quickly and the sensor does not have a lot of thermal mass (or digital filtering). The sofa sensor is probably not experiencing the same level of air flow (it is near a wooden panel).
At around 6:00 temperature increases in the other room, then I also increased the setting on the valve a bit (it is a mechanical valve). this explains the last part of the blue line.
Based on this graph I'd say the valve sensor is inaccurate, and I do not see a simple solution. Neither an offset, nor a gain correction can make the red an blue graph "fit".
Let's have a look at another time segment.
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Again, overall, the valve sensor reports higher temperatures.
Can you see I have trouble sleeping? Yes, at 4:00 I increased the temperature setting of the valve.
I cannot explain the complete graph, because I did not write down time and set-point of the valve. For comparison of the two sensors, this is no big deal.
Please note all graphs use the same temperature scale, because that is what we are trying to compare. But they do not use the same time scale.
Conclusion. In my house, in this room, I agree with the statement:TRVs cannot measure temperature accurately, because their sensors are "not in the right spot". So I understand why an FGT without external probe does not report temperature. It could do that, but then you would have to understand its limitations.
An FGT-001 with a sensor near the sofa should be able to control temperature more accurately. Whether this is important is up to you. I have learned to live with mechanical TRVs and LC12, LC13 without external sensor. I expect the FGT-001 plus external sensor to increase the comfort level. But right now, I really do not know. How people experience temperature is really complicated... Temperature control is only part of the experience...
Edit:
@Lode mentioned other sensors in the same room. Here is a graph of three sensors. The red and blue are the same (valve and sofa) and the third one is an FGMS-001 mounted roughly 0.5 m below the ceiling.
I think it is hard to say wether it better matches the sofa sensor. When you look at 1/3 of the graph and compare blue-black at 2/3 of the graph I'd say a simple offset will not do the trick. But as a rule of thumb, it measures about 1,5 °C higher because of the difference in height.
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@robmac made it clear to me... my topic title might be a bit misleading at the moment, because "Part 2" does not exist yet. So here's some more information.
They should be concerning... But maybe there is a possibility of misinterpretation of my post and my graphs. I probably caused this myself, because the topic title is misleading. In hindsight. It says "FGT-001 Fibaro Heat Controller, external sensor evaluation, part 1". In fact, in part one, there is no FGT-001. And because of that, there is no external sensor either!
I compare three sensors in my room, while there is only one plain old TRV. And my point is exactly what you say: (a) Old valves do not perform well in my setup (b) it has a lot to do with the fact that an old valve has no other reference than the sensor inside its head. And indeed, may stuck-on "valve sensor" does not match either my FMGS nor my SOFA sensor. It is not an absolute temperature test. It is only a comparison of references. Because this is a mechanical valve, I could have noted each time I adjusted its setting. But I did not, because I was not really interested in the performance of this old, mechanical valve.
When Fibaro releases a firmware update, I'll redo the measurements, with external sensor. I hope to see the room temperature (aka SOFA) will be under control and will be stable and also match the set-point of the heating panel.
I think as an absolute temperature sensor, the FGT-001 is quite accurate, the spec says +/- 0.5 °C and I have every reason to believe that. I could produce some graphs like the ones you posted. I'll think about that... I did have a quick look at the accuracy of the external sensor, but I did not save the data because I thought, it meets its specification, not much to tell about it. In part 2 I'll put the FGT external sensor next to my "SOFA" sensor and they should match. They also should match the programmed temperature.
Absolutely... I have thought about explaining this. The short version: old house, not well insulated, two rooms 4x4 with a door that is mostly open, some chilly weather in Belgium (around 0 °C) and some western wind. Doors sometimes open, number of people in the room vary, ... So the absolute performance depends on so many factors! So Part 2 will be exiting, because then the FGT should try to keep the temperature near the sofa at the expected level! So when I write part two, I'll make notes of the exact circumstances.
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