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Old 20th Sep 2021, 11:43 am   #1
Superscope
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Default Crompton Parkinson Wattmeter

Can anybody tell me anything about how you actually use this
Meter?

Looks a bit involved.
Anybody actually used one in anger?

I have a number of these I need to re-home for a Family who
have recently lost a loved one who was a Calibration Engineer.

I will post them up shortly, but I am interested to know how they
were/are used?

I assume these were mostly used in Colleges and Universities.


Ian





Ian
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Old 20th Sep 2021, 12:17 pm   #2
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Default Re: Crompton Parkinson Wattmeter

Watts = volts times amps.

So to measure watts, you need to sense the current and voltage a thing is using. and then you need something to do the multiplication.

THe deflection of the needle of a moving coil d'Arsonval meter is proportional to the product of the current in the coil, the number of turns in the coil and the strength of the magnetic field, divided by the strength of the return spring.

So if you don't use a permanent magnet, but use an electromagnet, the deflection becomes proportional to the product of the currents in the moving and field coils.

Set one coil up to measure the voltage to the device under test, with a burden resistor to scale it appropriately. Set the other coil up to measure the current the device under test is taking, with a shunt resistor to scale it appropriately.

You now have a direct reading wattmeter.

If the magnetic poles are laminated and good for the frequency range you're interested in, it will read AC watts without needing any rectifiers. It will also read DC.

THe unit inthe photos is a nice quality one with a selection of built-in shunt and burden resistors to suit a variety of uses.

David
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Old 20th Sep 2021, 12:41 pm   #3
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Default Re: Crompton Parkinson Wattmeter

Ian,

The information you need is on the terminal markings and the scale plate. There are two current ranges, for the low current range the current coils are connected in series and for the high current range they are in parallel. (There may be a range table on a data plate on the right hand side of the casing).

You are right about these being used in colleges. I found three of these wattmeters in a store in a college in virtually unused condition. I fitted them with an acrylic terminal cover and a 13A plug on an input cable and a 13A trailing socket on the output and preseneted them to the elctrical section, somewhat reluctantly as I would have liked to keep one for myself, but honestly prevaled

Within a few days one 'student' managed to wreck all three in a few minutes by connecting them wrongly and finding there was no deflection.

I have been meaning to rewind the self supporting moving coil on the damaged one I was given for some time, so if I might be considered for one of those looking for a home I would be very interested.

PMM
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Old 20th Sep 2021, 2:02 pm   #4
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Default Re: Crompton Parkinson Wattmeter

If you apply only current you will get no deflection if there is not a suitable voltage. Students forget this and increase the current until the coils burn out. Then they check the wiring, get the voltage right but by then it is to late.
Most of these instruments in college labs have some form of fuse inserted in the current circuit by the lab technician.
Ian
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Old 20th Sep 2021, 3:03 pm   #5
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Default Re: Crompton Parkinson Wattmeter

Quote:
Originally Posted by g8jzyian View Post
Most of these instruments in college labs have some form of fuse inserted in the current circuit by the lab technician.
Ian
That is a really good idea. I wish I'd thought of that!

I expect, however, that most college students regard anything with an analogue scale as being ante-deluvian and "not as accurate as a digital one".

PMM

Last edited by Cobaltblue; 20th Sep 2021 at 3:18 pm. Reason: Fixed quote
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Old 20th Sep 2021, 4:34 pm   #6
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Default Re: Crompton Parkinson Wattmeter

what a lovely thing, I will try to be in "the hat" when offered. The exposed mechanism is just right for educational use.
 
Old 20th Sep 2021, 5:45 pm   #7
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Default Re: Crompton Parkinson Wattmeter

I have a similar meter in a wooden case, current coils in series with the load and voltage across the supply. I wired mine up and fitted a 13 amp socket outlet so that I can test any load up to 10 amps. some reading you have to multiply by 2, the details are on the side. I fitted a single pole switch across the current coil when testing an unknow load ( a motor) and open the switch when the initial surge has passed. This prevent the needle hitting the end stop and braking it. Very expensive when new. CP type £400 in 1986. Ted
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Old 20th Sep 2021, 6:11 pm   #8
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Default Re: Crompton Parkinson Wattmeter

Wattmeters are now listed on the For Sale Section of the Forum.

https://www.vintage-radio.net/forum/...29#post1408329


Ian
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Old 20th Sep 2021, 6:42 pm   #9
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Default Re: Crompton Parkinson Wattmeter

Thanks to everybody who enlightened me on how you use these things.
Especially the very in depth description by Radio Wrangler.




Ian
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Old 20th Sep 2021, 7:50 pm   #10
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Default Re: Crompton Parkinson Wattmeter

Hi Ian, I've used these type of wattmeters many times over the years.
Although they were used at mains frequencies in college labs you will find they are equally good throughout the audio range to see what is actually going into your speakers.
They give true RMS values on complex waveforms as well as sine waves

Ed
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Old 21st Sep 2021, 12:28 am   #11
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Default Re: Crompton Parkinson Wattmeter

Because of the immediacy of the multiplication, they are sensitive to the phase shift between voltage and current, so they read 'Real' power and aren't bothered showing imaginary or reactive power.

These are damned useful things when the situation arises, and the digital world isn't quite so well equipped with equivalents as you might think. A sampling digitising scope with a maths function could almost do the job, but doesn't give the earth isolation these old 'dynamometer' instruments give. Still useful, still a hard act to follow.

A power factor meter to go with them would be a great companion.

Very much a case of Tolkien's "All that is gold...."

David
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Old 21st Sep 2021, 9:53 am   #12
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Default Re: Crompton Parkinson Wattmeter

Ed, David,

Very useful extra infomation, thanks.

I'm thinking to have one myself now!
That certainly wasn't the Plan!


These are certainly nicely made Instruments, I'm surprised
they haven't been snapped up, although there are only Three
left now, out of the Five I started with.

I have just one example with a Centre Zero 40 - 0 - 40 Scale.
What would be the purpose of that ?



Ian
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Old 21st Sep 2021, 11:30 am   #13
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Default Re: Crompton Parkinson Wattmeter

It shows the direction of power flow.

Easily enough understood at DC. Consider the direction of current, but you also have to account for the polarity of the voltage. The ordinary wattmeters will try to read backwards if the power flow is the reverse of what you connected it for. This one just has the pointer sprung to the mid scale position zero marking.

They also show direction with AC. The direction business has the phase of the current reversed or not with respect to the phase of the voltage.

Some AC motors can act as generators on the overrun. Some generators can act as motors if the engine driving them is throttled back. So in a power grid network with multiple generators and motors, real power meters show whether something is contributing or consuming. It can also go between a house with a solar photovoltaic system and the mains and show whether you're contributing or consuming, and how much. Fancy electricity meters that allow for your array contributing to the grid have to handle power measurement this way. Needless to say you get a different price for contributions to consumption, but that's downstream of measuring the power in the first place.

Not relevant to these meters because of frequency range, but the same thing goes on at RF. Directional power meters are used between transmitters and antennae. They read the power from the transmitter to the antenna, and they also read the amount reflected back from the antenna. This is what's going on inside most "VSWR meters" Back in the eighties, I designed a little directional power meter for the G-QRP club. George was saddened that a commercial model with handy power scaling wasn't being marketed in the UK. You'll find it in Spat magazine, issue 61. A bit of transformer trickery to pull off a sample of the forwards power and of the reverse power, two diode detectors andtwo meters. Shazzam! one glance shows what's going on.

David
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Old 21st Sep 2021, 1:04 pm   #14
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Default Re: Crompton Parkinson Wattmeter

Thanks David,

Yes, in DC it wasn't a difficult concept, but AC had me
stumped.
Thanks for another in depth explaination.



Ian
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Old 21st Sep 2021, 9:09 pm   #15
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Default Re: Crompton Parkinson Wattmeter

Hi Ian, as David says, all you need now is a power factor meter. The normal ones are not very accurate, but if you have a decent voltmeter and ammeter in the collection it is easy to work out PF = W/ V*A and it should always come out less than unity; but ideally pretty close if you have an efficient electrical installation

Ed
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