Getting started with emonTx and emoncms

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(Dave Howorth) #1

So I have an emonTx installed and connected to my emonBase. It has an optical pulse sensor on the solar generation meter, a 100 A CT on the cable from that meter and uses a 9V AC voltage reference. The meter has conventional 1 Wh flashes.

I can see various inputs on my emoncms installation: power1, pulse & vrms. There’s also something called rssi that I don’t understand (it’s value is -33).

I understand what vrms is showing 253.1 is my mains voltage, presumably. But what is the 256 that power1 is showing? milliamps, milliwatts? Does it take account of the vrms, or do I need to do some more setup somewhere? And what is the 568 that pulse is showing?

I’m sure this must all be explained somewhere, but I haven’t managed to find it. Sorry.

(Paul) #2

RSSI is the signal strength of the received RF from the emonTx as in -33dB, nearer to 0dB is stronger, -80dB is weak.

power1 is power in Watts as in 256 Watts realpower, so yes it does include the Vac element to determine direction and power factor in in the reported power value.

the 256 is the raw pulsecount, no calibration is required with 1Wh pulses except where scaling to kWh is required, this pulsecount which continue to increase until the device is restarted/reset or it rolls over at 2,147,483,647, there are processes in emoncms designed specifically for counting energy/pulses that can “ratchet” to accomadate resets and rollovers eg the “whaccumulator” and the “total_pulse_count_to_pulse_increment” processes.

(Dave Howorth) #3

Thanks Paul, that helps some. The emonTx is only about a metre from the emonBase, in the same room, so is -33 dB as strong as it gets, or is it an indication of something wrong with the signal? I’m kind of surprised the emonBase doesn’t have an external antenna.

So for the pulse counter, I need to click on the spanner, then choose ‘Wh accumulator’. I now see there’s a description of the process underneath - hadn’t noticed that before. It says ‘See forum thread here for an example’ but there doesn’t seem to be a link. Which forum thread?

Again, is there some documentation somewhere that I’ve missed? I’ve no idea what I may break if I just play around, because I still don’t have any real understanding.

(Trystan Lea) #4

You can unwind the wire antenna of the RFM69Pi adapter board so that it points outside of the emonBase case. I have seen it improve the signal by 5-10 dB.

There’s emoncms documentation in the guide (see emoncms section of the contents):

The guide pages & emoncms screencasts are also linked from the emoncms github repository here: alongside a list of emoncms terminology and link to the input and feed API reference.

I will look into that missing link

(Trystan Lea) #5

I can see now what the missing link was referring too. The forum thread mentioned was converted into a guide page here: it covers both the power_to_kwh and wh_accumulator input process (for use with pulseCount). I will update the link.

(Dave Howorth) #6

Thanks Trystan; I’d just found that page thanks to your previous post :slight_smile:

(Dave Howorth) #7

Just to say thanks for this. I set up my second emonTx in the garage today and initially the signal was very flaky, with many dropouts (rssi around -80). So I unwound the wire antenna in the emonBase and now the signal seems reliable (rssi around -71 - -72). Good result.

(Robert Wall) #8

You could also try a groundplane.

(Dave Howorth) #9

Hmm, that looked an attractive idea and so simple to do. But if anything it seems to have made things a couple of dB worse. I suppose my signal must be very dependent on the exact position of the antenna and the various things around it. I’ll experiment more tomorrow.

(I might even try drilling a hole in the right place in the case rather than using the pre-drilled raspberry-shaped holes :slight_smile: )

edit: hmm, the antenna wire is nothing like the 165mm mentioned in the thread Robert referred to?

(Robert Wall) #10

What frequency are you working at – 433 MHz or 868 MHz?

(Dave Howorth) #11

433 MHz

(Robert Wall) #12

So your wire whip is ~ 83 mm? That is the length for 868 MHz. I’d suggest you replace it - and while you’re at it, just check that you have the correct RFM69 - check the appearance here.

(Dave Howorth) #13

Never mind. Red herring. It is 165 mm when it’s actually stretched out.

(Robert Wall) #14

A ground plane will work at either end, or both ends. If one Tx is weak, that’s the candidate for a ground plane.

That whip is one quarter wavelength, so moving something by that distance represents a long way, if there’s some cancellation going on due to reflections.

(Bill Thomson) #15

Coiling the antenna is a compromise at best. Coiled up, the antenna is far from the desired
quarter-wavelength. The placement of the ground plane isn’t overly critical, but its dimensions must be
a quarter-wavelength or more. Shape isn’t critical, circular or square perform the same. It does need to
be located quite close to the whip.

Did you view the video clip in the thread?

(Dave Howorth) #16

Using a ground plane at the transmitter is more difficult, since it’s wall-mounted.

I looked at the video and believe I understand what it’s showing. I suspect in my case the isolated ‘ground’ plane (i.e. sheet of foil) is also coupling to my nearby Wi-Fi router and increasing the noise seen by the emonBase. But perhaps it’s just the precise positioning of the antenna that matters. Difficult to tell.

I think a better solution might be to connect a remote antenna instead of a piece of wire straight on the receiver. That would give me better control over position and perhaps an antenna with more gain.

(Bill Thomson) #17

An RSSI of -33 is good. Very good, actually. That being the case, you should be good to go, as is.
The receiver sensitivity extends well past an RSSI of -90. At -33, you’re in good shape.

(Dave Howorth) #18

-33 is the value for my first emonTx, which is about a metre from the emonBase. My problem is with my second emonTx, which is 40 m or so away, and that’s what the discussion has been about.

I’m getting a lot of dropouts (unusably many) at around -80 indicated. Though it seems to be mostly working at around that level at the moment.

But I don’t want a system that depends on exactly where my emonBase happens to sit or exactly which way a piece of wire is bent. I’d like to achieve a system that is reliable and that I believe would be reliable if I took it away and then put it back in roughly the same places again.

(Robert Wall) #19

In that case, forget the ground plane at the emonBase, and look to having a ground plane, or if that’s not possible, a dipole (I think that was mentioned in the thread about ground planes) at the problematical transmitter.

(Dave Howorth) #20

I’ve got some dipoles on order; I’ll let you all know how they work after they arrive.

In the meantime I’m curious about how the optical pulse sensor data is interpreted. There’s a glitch in the graph of my solar generation; perhaps the easiest way to show it is wih the CSV data:

2018-12-07 14:22:00, 12200.0
2018-12-07 14:24:00, 12213.0
2018-12-07 14:26:00, 12028.0
2018-12-07 14:28:00, 12235.0
2018-12-07 14:30:00, 12244.0

That feed uses a whacc process. I thought they were supposed to increase monotonically?

1 Wh Accumulator whacc Node emontx3: solar generation optical pulse sensor (10464.00)

(BTW, why is the current value shown wrong?)