Re: I need a simple power meter for a phase-controlled heater.
Dr. Slack, Sat Feb 18 2017, 03:50PM

so many options, so few meet your requirements.

Any current meter must also be constant resistance, as any tempco that's different to the heater will mean a gain error with respect to the heater.

The simplest passive type would be a 'hot wire' ammeter, which naturally reads true rms. A man of your skills ought to be able rig one up and calibrate it fairly easily. Use a 'resistance wire' rather than steel or copper for low tempco.

Hot filament power substitution method is used for RF meters. A small incandescent lamp is fed with DC through an RF choke, and heated to a temperature where its resistance is 50 ohms, to match impedance. RF is fed in through a capacitor, and the DC feed is servoed to maintain the temperature by reducing its power input. The difference in DC power is the amount of RF arriving. For a low frequency measurand, you could switch to a high frequency substitution supply.

You could make a 'square and average' front end for your meter. Apart from the standard AD835 multiplier circuit, I've always had a hankering to build a sigma delta multiplier, but never got round to it. The general idea is that the output of a sigma delta converting an input is used to control a multiplexer that switches between the input and its inverse, so the mean of that output is proportional to the input squared. The first sigma delta converter I built when I invented the concept independently (a year after Phillips, Sony and Toshiba, so no patents on that) ran at 10kHz with a TL072 and HC74, so it's very easy (for appropriate meanings of the word 'very').

Then again, borrowing a true RMS meter, or even buying the one you've always wanted, might be easier.

Re: I need a simple power meter for a phase-controlled heater.
Sulaiman, Sun Feb 19 2017, 01:16AM

I was hoping for some magic trick ... no surprises.
I gained a Fluke dmm from work as it was not eonomical to repair, worth a try
last year I remembered to remove the expired batteries ......

No problem, I'll just callibrate the dial in effective heating power




DONE

Re: I need a simple power meter for a phase-controlled heater.
Hazmatt_(The Underdog), Sun Feb 19 2017, 01:36AM

I've got several RF ammeters that are thermocouple type and are true reading from DC to 10 MHz.

Surplus Sales of Nebraska has a 20A thermocouple for this purpose, and I bought one but I never had enough current that I could source to see any action out of it.

I checked and it looks like they don't have any RF shunts anymore. They are getting a bit scarce, but they do have a 40A RF ammeter.

Re: I need a simple power meter for a phase-controlled heater.
Electra, Mon Feb 20 2017, 12:12AM

I think The next most accurate method if you can't measure the rms by any means.

If you can measure the phase angle using a scope,and use that to draw a calibrated dial for the control

Calculate the rms voltage or power from the phase angle.

Found this formula on line, as I couldn't recall it


Po = ( V Squared / pi * R ) * [pi - α + (sin2α / 2) ]


the firing angle α = ωt, where ω = 2πf

V = rms volts.

I have not checked it's correct mind you.

Re: I need a simple power meter for a phase-controlled heater.
Uspring, Mon Feb 20 2017, 10:28AM

It's correct. If you don't have a scope but a meter, which can measure average current, you can determine the phase angle from there and plug it into the equation for the power.

Re: I need a simple power meter for a phase-controlled heater.
johnf, Mon Feb 20 2017, 06:40PM

The biggest problem with using a multimeter is that phase fired waveforms have too high a crest factor for the rms circuitry. In good multimeters ie Fluke there is a crest factor spec in the AC ranges.

If you simultaneously multiply your instantaneous voltage with instantaneous current you get iinstantaneous power you can use a linear multiplier then a voltage to freq convertor and count the pulses for a very accurate power reading.
I did this at work and got to better than 1 part in 36million for power applied to an electrolysis cell but this did require a very special current sense resistor from germany to get to this accuracy

Re: I need a simple power meter for a phase-controlled heater.
Hazmatt_(The Underdog), Tue Feb 21 2017, 01:39AM

I always wanted to try the analog route and see how that worked out.

The approach being taking the scaled voltage, and current measurement (CT) and applying them to a log amp, sum them, and take the anti-log to the meter. The major drawback to this is the typical BB log amp chip is in the $10 range.

The other approach would be cram it into the arduino, code whatever filter you want, and a look-up table with scaled correction factors would probably be cheaper then the log amp.
Plus, you could code peak and average, and moving average history all on a 16x2 LCD.

All depends on what you want.

Re: I need a simple power meter for a phase-controlled heater.
radiotech, Tue Feb 21 2017, 09:38AM

Connect a small wattage light bulb in parallel with your 240 volt 300 watt phase controlled heater.

Now connect a similar light bulb up to a DC power supply that you can easily measure current and
voltage to easily determine power level.

Place the two lamps in a shroud so you can judge if both are lighted equally.


If both are glowing equally, then the power applied to both lamps is equal.

The effective voltage in both lamps will be the same. This voltage squared times the resistance
of the 300 watt heater will be the true power.
Many


This topic was discussed here in 2010 in this posting , and I (radiotechnician) mentioned the power lamp.

Re: I need a simple power meter for a phase-controlled heater.
Sulaiman, Tue Feb 21 2017, 10:20AM

Thanks for all of the feedback,
I've decided to go with a simple linear scale and 'callibrate' it later when I have access to a true rms meter.

radiotech ... I had considered a front panel filament lamp in parallel with the heater as a visual power indicator.
my pathetic excuse for not doing it is
a) I find low wattage 240 V lamps have a short life, a false 'nopower' indication is worse than not having an indicator
b) I don't have one in stock
Somewhere I've got one of those small circular transformers intended for running elv filament panel lamps on 240 Vac ....

Re: I need a simple power meter for a phase-controlled heater.
radiotech, Tue Feb 21 2017, 10:34PM

The elevator lights transformer may mess with the chopped AC.

You also could hook two clear 120 volt Christmas lights in series to make a 240 volt lamp.

Re: I need a simple power meter for a phase-controlled heater.
klugesmith, Wed Feb 22 2017, 02:42AM

>> Place the two lamps in a shroud so you can judge if both are lighted equally.
>> If both are glowing equally, then the power applied to both lamps is equal.

That's my favorite approach, for reasonable accuracy at low cost. Lamps respond to extreme crest factors a lot like heaters do.

For objective visual comparison of brightness, you can arrange the lamps to cast partly-overlapping shadows or spots of light on a white surface.
One alternative, which I read about decades ago, puts the lamps inside an opaque box at opposite ends. At the midplane is a thin sheet of white paper prepared with a small spot of oil or melted wax in the middle. Observing the paper through a viewport from either side, the spot appears brighter or darker than the surrounding field, according to the relative illuminance on far and near sides.

The DC voltage on lamp #2 can be servo'd to automatically keep the brightnesses matched.
Visual or electronic matching accuracy can be improved by using the same sensor for both lamps, with a mechanical optical chopper. Or switching a single lamp between unknown AC and reference DC voltages -- the human eye could be a pretty good null meter if the blink frequency is well chosen. The brightness of a single lamp could be measured with an analog lightmeter and non-linear scale markings, calibrated with DC.

- - -

If low wattage 240 volt lamps are hard to come by, you could use a low voltage lamp(s) with power resistor instead of transformer. Then no worries about transformer frequency response. Voltage division ratio would no longer be constant (not even close!), but that's easy to handle with DC calibration or identical DC reference lamp circuit.

- - -

As for analog op-amp methods, it's not easy to do better than the RMS-to-DC converters in mainstream RMS-reading multimeters.
Unknown AC voltage (typically AC-coupled!) is precision-rectified, then squared and low-pass filtered. A DC voltage source maintains a matching output from an identical analog voltage squarer. Readout settling time depends on the low-pass filter.

Of course this is about measuring RMS voltage or current, not measuring true power to non-resistive loads.

-Rich

Re: I need a simple power meter for a phase-controlled heater.
Dr. Slack, Wed Feb 22 2017, 05:44AM

Posted: Tue Feb 21 2017, 10:20AM
Thanks for all of the feedback,
I've decided to go with a simple linear scale and 'callibrate' it later when I have access to a true rms meter

Not sure what you mean by 'simple linear scale'. The thing we've been discussing is either how to square effectively, or to measure heating power in a some sort of resistor, the point about a phase-controlled switching waveform is the mapping between mean current and power is non-linear with phase angle. But at least it is consistently non-linear.

Re: I need a simple power meter for a phase-controlled heater.
Sulaiman, Wed Feb 22 2017, 09:39AM

I meant;
I will use a ready made linear dial (e.g. 0 to 10) for the potentiometer
and have a look-up table/graph for actual power vs. dial setting.

This way I immediately get a scale that allows me to use a previously tested power setting.
(this is just to control a heating mantle for a flask to do fractional distillations at a constant boil-up rate,
as I am learning the techniques of distillation for my chemistry hobby)

Re: I need a simple power meter for a phase-controlled heater.
Physikfan, Sun Feb 26 2017, 05:22PM

Hi Sulaiman

Your main goal is:
"I will be measuring the effective heating power by the rate of boiling and condensing of water,
and I want to compare it to the actual electrical power input."

I would replace your triac controlled AC source by a dc source.
In my opinion it would be the easy way to solve your problem, measurement of the rate of boiling and condensing of water
as a function of the actual electrical power input.

Re: I need a simple power meter for a phase-controlled heater.
Sulaiman, Sun Feb 26 2017, 06:35PM

dc would be easier to measure, but the £1.24 controller works well and I do not want huge electrolytic capacitors etc.
(cost, volume, reliability etc.)

I only wondered if anyone had a simple circuit that could measure the electrical power input using phase-control,
because I could not remember or conceive of a simple circuit,
just so that I can determine heating efficiency, which in fact, is not that important.

I will just use a callibration curve/graph to convert between dial setting and boil-up rate, which is the important parameter.

I can use and easily measure mains voltage direct for full power, and half-wave rectified for 1/2 power
then use those two callibration points only ... good enough for this application.

I THINK THAT THIS THREAD CAN NOW BE CONSIDERED CLOSED.