Sunday, August 19, 2012

MT87 Clamp Meter

I always enjoy a new toy. And quantifying something is great, too. And I'm cheap. So I had to buy this clamp meter when I found it on eBay last week. It was a grand total of $8.45. That's much less than the ones you'd elsewhere online and much much less than something quality like a Fluke. Is it as good as a Fluke? I doubt it. (I'd love to compare if I had access to a Fluke. Don't count on it.)

It's generically called Digital Clamp Meter on the box and has MT87 on the device itself. There's no brand or website or anything. A manual I found online says 3% accuracy ± 5 digits. It's got 3 AC current modes, depending on the current level, AC voltage, DC voltage, beeping continuity, and a single resistance mode.

I already own two multimeters so what's so special about this one that I had to buy it? It's got that clamp! Squeeze the trigger to open the jaws to clamp on to a live electrical wire. Doing that allows you to measure how much AC current is going through the wire.

The clamp part is acting as a transformer, where the live wire is like a single winding around the metal core of the clamp. Current through the live wire induces a current on the other side of the transformer, which is inside the meter. Circuitry inside the meter measures that induced current to tell me just how much power is going through that live wire. Awesome.

How much power is that window air conditioner using?

Here I've put a 1 ft. extension cord (Monoprice 5296) in between the socket and the AC plug. I've stripped the cable of its outer shielding so I can have access to the hot, neutral, and ground wires which are colored black, white, and green, respectively. If I clamp around all three at once, I get a reading of zero, since the currents from the hot and neutral are out of phase. You can imagine water flowing in the black is like electrical current. It's going the opposite direction of water flowing in the white. The sum of the two currents is zero since they are the same in opposite directions. So we place the clamp on only one of the wires. This picture shows a reading of 4.47 amps. At the time I was reading about 122 volts, which gives over 540 watts (power = volts*amps).

Here the fan is blowing but the cooling part is off. At 0.37 amps that's less than 50 watts of power being used by just the fan.

That was fun, but what about my whole house? How much power am I using everywhere?

You probably have a circuit breaker like this in or on the outside of your home. This is the one in my apartment. For safety, the power in your home is divided across multiple breakers. If too much power goes through one of these switches, it will automatically shut off (or "trip"). This prevents fires and other hazards. I'm not sure a lot of forethought went into my own apartment. The 7 switches in my box are assigned as:

  1. Lights
  2. Living room and bedroom plugs
  3. Kitchen plugs and fridge
  4. DSWN (?) Plug
  5. Washer
  6. Door Bell
  7. Range (oven)
I have no idea what 4 is and I'm surprised the door bell gets its own 20 amp breaker (did you notice the doorbell transformer on top of the breaker box?). Let's take a look inside.

I do not recommend doing this. These voltages are live and dangerous. Any mistake and you could end up in the hospital or dead. PLEASE don't do this.

Notice three thick cables at the top, two black and one white. This is a common setup in most homes in the USA. It's known as a split-phase system. The potential difference between the two black is 240 V and the neutral is a center tap on the transformer where the two black came from. So the potential difference between neutral and either black is 120 V. That's how you get 240 V and 120 V in your home. Going from top to bottom, the breakers alternate which black line feeds into the breaker. The grey breaker gets both so the oven can have 240 V.

Just how much power am I using right now?

The left line has 2.29 amps through it right now. But that's just part of the picture.

The right one has a load of 9.16 amps. So the two aren't as balanced as one would like but that can vary with time and with what's turned on. Remember that the living room and the bedroom are on the same breaker. We had two AC units running on that one.

So we are using almost 1.4 kW of energy at that moment. For Utah residential rates with Rocky Mountain Power, as of the time of this writing, the rates are:

   8.7035 cents/kWh (kilo watt hour)
   8.4004 cents/kWh (1st 400 kWh)
   10.3481 cents/kWh (next 600 kWh)
   12.8709 cents/kWh (all additional kWh)

So using 1.4 kW constantly for 31 days in August is about 1040 kWh (31 days * 24 hours = 744 hrs, 744 hrs * 1.4 kW  = 1041.6 kWh) costing just over $100 (left as a problem for the reader (I hate when books say that)). Ouch. Keep those AC units off when you can.

So the meter gives me some insight into how much power I'm using at any given time. With my threaded extension cable I can see where individual appliances stand in terms of power usage. It wasn't rigorously tested but it puts you in the ball park. I'll compare it's voltage readings to my other two multimeters in another post. And best of all, we had fun! Plus, that's one more thing...