Kill-A-Watt for DC?

Anybody know of a kill-a-watt like product, but for DC....ideally one that happens to be the inline plug for CPAPs?

I'd really like to know exactly what my machine is drawing....

The Dreamer

That's easy, a standard digital Volt Ohm Amp meter, is made to do this job. Most better ones handle 10 AMP D/C. I'm not talking $10 meters here, mine costs $100 but cheaper can be had in the $25 to $50 dollar range, RS in where I got my last 3. Jim

The wattage will vary as the machine works due to changing pressures and effort. Volts times AMPS = Watts

How would you hook that up then? I have a Fluke tester that was handed down to me, but I'm not sure how I would hook it up while it is running since the prongs I would think need to get connected to the unit someplace. But where?

The ammeter needs to be in series with the power conductor. You either jury-rig it (alligator clips are your friend), or build an adapter to route the power correctly.

It would take a male and female connector to match your machine, and connectors to match your meter (unless you have alligator meter leads). A little bit of zip wire and maybe some tape and you're in business. Or some light soldering if you're so inclined.

Most of the current power supplies for cpaps are about 95% efficient so if you run them through a standard Kill-o-Watt you can get a pretty good idea of the power needed to run your cpap. What GoofProof says can be done easily it is not for the average user. First you have to cut one of the power supply leads between the power supply and the cpap to insert the ammeter into the circuit. Then use the voltmeter to measure across the output of the power supply to measure the voltage. As a side note the current crop of Digital Multi Meters on the market are not designed to handle long term measurements of current. We have had quite a few of them get hot and start melting holes in their cases here at work with as little as three or four amps in a hour or so time frame.

There is a simpler way to check the amp draw. Get a "clamp meter" that can handle DC amps. I have one that can go to 100 amps. They have come down in price and a good one can be had for under $100.00.

Bruce

Clamp meters do not handle DC Current, a steady state DC field will not induce a voltage in to a clamp coil. Some of the clamp meters will do DC current but you still have to break the wire and insert the meter, the clamp part is only for AC.

DC clamp meters work great. They use the Hall effect to determine the current present in the DC line.

http://www.electrotechnik.net/2009/09/h ... -work.html

It is actually amazing how accurate they are.

No you don't have to damage, cut, or strip the wires to measure the DC current if you use these clips. I have heard these referred to as both alligator clips or crocodile clips.

You would connect the ammeter between one of the clips and the battery terminal. But it is much easier if you know what you are doing.

Here are a couple videos that may help some - others it may not. If these videos don't help you - ask someone for help.

THE BEST Multimeter tutorial (HD)
https://www.youtube.com/watch?v=bF3OyQ3 ... ure=fvwrel

Multi-meter to measure amps
https://www.youtube.com/watch?v=9Zt3P9b ... re=related

How to Use a Multimeter
https://www.youtube.com/watch?v=UsDdQaI ... re=related

Some of these meters are sort of the DC equivalent of a Kill A Watt. They don't come with "CPAP" connectors, you'll have to adapt somehow. If you don't know how to hook them up, you can damage the meter, the power supply, and the CPAP machine.

You can use a multimeter to measure the instantaneous current flow. Then, just multiply by 12V or whatever voltage the machine uses. The voltage will probably be constant enough to not worry about variations.

The problem with this is that the current draw of the CPAP machine varies greatly. When I looked at mine with a Kill A Watt on the AC line, the power goes up and down by a factor of 2 or more whenever I inhale or exhale.

I suspect you're interested in average power draw over a period of time. In order to measure this, you'd need a multimeter that averages or "integrates" current usage over time, the way a Kill A Watt does for power. There may be such meters, but I haven't seen any.

I've found that most people who try to use an ammeter end up shorting out the power supply and blowing a fuse or damaging something. Remember, the ammeter goes in series with the device under load. If you don't know what that means,you're probably going to do it wrong.

To use an ammeter, run the negative lead of the battery/power supply directly to the CPAP machine, run the plus lead of the battery to the ammeter, and run the other lead of the ammeter to the plus side of the CPAP. If you screw this up, you can burn up the ammeter, the wiring, or even the CPAP machine.

Once again, most people I've explained how to use an ammeter end up blowing something up. Even some people who I've shown how to use it and watched them do it right eventually ended up doing it wrong later when I wasn't watching.

Depending on what you're doing, it may be easiest to simply use a Kill A Watt and assume the DC load is the same number of watts the Kill A Watt shows. You will overestimate the DC load a little. To find the average watts, unplug the Kill A Watt, plug in the CPAP and run for long enough to get a kWh reading and divide by the number of hours.

Blowing 9-12cm, my Legacy ("tank") Respironics BiPap AutoSV draws about 1.7 amps (12v) through the Respironics DC-DC converter (12v to 24v).

Don't worry too much about accuracy when it comes to measuring the current draw of your Cpap. The real variable will probably be the battery. Battery AH ratings can be exagerated, and even deep cycle batteries cannot be discharged all the way down.

But, the engineerd that I am...wants to track accurate nightly Ah usage of my autoCPAP over the long term.

Where's it list what kind of connector the machine is using....I can't find my calipers. I suppose I could whip up some cables with powerpoles and....except I think my powerpole crimper grew feet the last time I had it out (at a SET).

Hmmm, PWRcheck has USB support for real-time monitoring....would be better if it had direct SNMP network/wifi connectivity (trying to keep computers out the bedroom)...then I could track in cacti

Been wanting to find a temperature/humidity probe of such, so I can also track what those conditions are around my bed at night....

The Dreamer.

TheDreamer wrote:...wants to track accurate nightly Ah usage of my autoCPAP over the long term.

(invective)

TheDreamer wrote:But, the engineerd that I am.

So I spent an hour searching Youtube because? Should we ass-u-me that you knew "how to use a multi-meter" already?

TheDreamer wrote:...wants to track accurate nightly Ah usage of my autoCPAP over the long term.

Should I ask "WHY"? Yes, I spent more than an f'n hour searching so - Why do you want to do this?
Well hopefully you know enough to disregard aa's helpful post.

TheDreamer wrote:Been wanting to find a temperature/humidity probe of such, so I can also track what those conditions are around my bed at night....

That my friend is an easy one, wish I knew this before too.

GumbyCT wrote:Well hopefully you know enough to disregard aa's helpful post.

Aww, how cute, I was worried you weren't still infatuated with me.

Yes, ignore my post, including where I gave him the link to the device that is the equivalent to a "Kill A Watt for DC."

TheDreamer wrote:Where's it list what kind of connector the machine is using....I can't find my calipers. I suppose I could whip up some cables with powerpoles and....except I think my powerpole crimper grew feet the last time I had it out (at a SET).

If you've got 12V PowerPole connectors on other devices, that would be an excellent way to do it. Don't you need a new crimper for the next SET anyway? Powerwerx sells the meters with powerpoles on them already. I'm tempted to buy one myself.

This page on cpap.com implies that the Intellipap uses the same cord as Respironics.

If this is correct, look at this thread for the description and part numbers for the connector.

They are 2.5 mm ID, 5.5 mm OD barrel connectors. The tip is positive. This is also referred to as a “size N coaxial DC power connector.”

Of course, check the fit, voltage and polarity before blowing up your CPAP machine.