OK, a 99 Century MAF sensor is a hot-wire cooling differential type sensor.
I know how it works. What I don't know, and need to know, is the current that normally flows through the little teeny tiny platinum sensor wires in normal operation.
50ma ? 100ma ? Something else ?
For my purpose, the thing isn't being used as a MAF at all, so generic stuff won't help, unless you've put a voltmeter directly across the platinum element itself. That would be valuable information to me.

The reason I need to know, has to do with pressing the platinum elements into service in a gas analyzer, not air flow, and I'd like to not burn them up. ( again )
In this re-designed application, there won't be air flow cooling, so I need to not over-voltage the elements and burn them out.

Even knowing the total current into a normal sensor in normal operation would be more than I know now ?

 

Honestly I can't remember off the top of my head, it is listed somewhere in one of Vader's posts. Here is a peek at an old thread that touches on some odd MAF stuff that you can start with (here were two more ancillary threads but I can't find them):

https://www.thirdgen.org/forums/tech...f-session.html

 

https://www.thirdgen.org/forums/tpi/...-2-8-mpfi.html

There is one more. For some reason I'm getting far, far better results searching on Google than when trying to use the alleged serch engine on our own site.

Now you have to let us in on the design and plan so we can make our own!

 

Google is giving me quite a few hits with info I already know, but none with the info I want.
Searching several sensor manufacturing sites produces a bunch of theory, but no hard numbers, and no adequate replacement sensors, either.

It's an old, simple design. Most of the guys are using a wide band O2 sensor of one type or another, and that's not a bad way to go. THIS thing, I got off e-bay for $30. Needed a little TLC, but worked fine. I broke it, so....
An O2 sensor of any kind, responds to oxygen only, meaning it can register lean on rich misfire, due to the unburned oxygen. The hot wire responds to ALL gasses present, so can be compensated in the calibration. This also makes them unsuitable for measuring a specific gas in the presence of others, such as in an exhaust mixture, but I'm not after a specific gas, so it works for me. The very fine platinum wire is also fragile, and easily broken. ( as I've learned the hard way )

It works on the same principal as a MAF, that a platinum wire changes it's resistance ( greatly ) in its operating range with temperature changes. In a MAF, it's air flow cooling.
In an air-fuel ratio meter, it's the specific thermal absorption properties of the specific gasses present.
The sensor element housing sheilds the wire from flow, so it's not movement that causes cooling of the wire ( or heating ) but simply the differing heat absorption capability of the gas present.
There are two elements to the sensor. One is in a sealed chamber such that ambient temp is applied to both wires, keeping differential currents due to ambient temp to a minimum.
The wire NOT in the sealed chamber is exposed to the gas present. Whether that gas will absorb more heat than air, like CO2, or less heat than air, will cause the exposed wire to change its resistance due to being either hotter, or cooler than the sealed reference wire.
The difference in resistance means that the two wires have different conductivity, thus different current flow. By measuring that difference with a simple bridge circuit, the difference current can be calibrated to the gas of choice, by comparison with a known concentration sample gas. THAT's the hardest, and most expensive part, unless you happen to know someone with a reliable and accurate method to calibrate against, like a shop with a 5 gas analyzer that you can calculate actual ratio, and calibrate against.
The absolute current is irrelevant. It's the difference that's important. In my case, the absolute current can burn out the fine platinum wire if too high, or ignite combustible gasses if the temp is too high due to high current, but the wire is more sensitive to change the hotter it is.
The one I broke worked at about 4 volts at 25 milliamps. VERY fine wire filament.
The MAF wires I've got start to work around 150 milliamps, but the sensitivity isn't yet there.
Might be the type of sensor, or might be I haven't got the current high enough. ( which doesn't contribute to batery life )
Platinum is expensive, so simply increasing current to burn-out to discover the limit seems a bad idea. Kinda like detonating until the pistons hole just to see what they'll take. Bad plan.

These days, everyone wants to sell me a 5 gas infra-red analyzer, but that's no fun !
If one would really wish to persure this approach other than me, I've attached a schematic for the Heathkit CI-1080, a very similar meter, also not produced in 20 years or so.
The original manufacturer of the sensing element, these days, never heard of it.
In mine, I already have the bridge circuit, and the meter face, so a bit of effort seems appropriate before tossing the unit entirely.

 

To answer your initial question, the current flow through the sense circuit in a hot wire MAF is variable, not constant. That's how the comparator circuit determines the rate of cooling (air flow mass) in the sensor. The current in the circuit is constantly varied to maintain a 75ēC differential temperature between the inlet sensor and sensing wire. Current during a burn-ff cycle is much higher, to force the wire to become incandescent and drop its contaminants.

The wire used in the older Bosch analog sensors is AWG41 (0.0028" / 0.071mm) Platinum or Pt/Ni wire, annealed, plain, .54189 ohms/cm). Current capacity depends upon the exact alloy, but is 35.5mA for "standard" Pt/Ni wire.

The last batch I purchased about five years ago was right at about $22/ft. Obviously, I didn't get any 500' spools. I still have a few inches left, somewhere (may the joking commence).

 

Found some .002 99.9% pure platinum wire at $8.30 per foot.

Anyhow, that 35ma is the info I was looking for. Don't care that it isn't constant, I know that, but whether it's 10ma or 500ma makes a difference.

Thanks.....