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Know your Errors - TRMS pro's and con's
Phyllomedusa:
Hey fellow,
after I am currently crawling under a car again and would like to give it a check up (after all the rust is gone) I was thinking about the Lambda-Sensor and a check-up.
So what is it....just an oxygen sensor that tells the car's ECU how much oxygen is left in the exhaust gas and therefore helps the ECU to control the amount of injected fuel for the best fuel/air mixture.
The signal is an alternating signal that ranges somewhere between 0,2 and 0,8V (warm and healthy engine). So you could see it as an ±0,3V AC Signal with a 0,5V DC offset. If there would be a problem with the engine: let's say the fuel filter is aged and there is not enough fuel delivered, it means that there would be more oxygen left over compared to a good burn (as there will be less oxygen needed to burn the fuel). This mixture is called lean and has some problems: less engine power and even worse... higher burning temperatures that aren't healthy for your motor....
Best is to check this with a scope. In fact it is fairly interesting to watch when the engine is warming up and then the control circuit with the lambda sensor kicks in to watch over the mixture.
So somebody suggested to watch with your multimeter and talked about looking for 0.2 and 0.8V as Min/Max or that the signal somehow reaches these levels from time to time (if they don't it could be that the sensor is aged....).
As you may have seen the topic's name you may already guess my thought: well, if is is really an AC+DC Signal, our modern TRMS multimeters will consider it the same and don't tell you the Min/Max, but the effective voltage which is much lower. So in this case you would actually consider your oxygen sensor to be broken, but he isn't. Your meter is just telling you a lower voltage.
As there is actually a lot of Sensors in car and a lot of them are working in a way like that you need to consider this. My U1253B has a Peak-Hold feature that brings you a lot nearer to that if I compare the signal on the scope.
Well, it is described as 1ms Peak hold...so I suppose the max. frequency to accurately detect it is 1kHz.
....so why not build id a feature to measure and show you the actual maximum of the amplitude....?!?!.... TRMS is nice to have but if that's all you can get into problems.
Kiriakos GR:
I did read your story three times and I am still confused about accepting those as facts, which will lead in to a logical conclusion about the TRMS.
I am speculating, that what you care mostly is to verify that the sensor works correctly (it is close at their calibration standards).
a) The expected output in mV, is just an indication that the sensor is alive.
b) The pulse frequency is a key factor, and must be known so to tell if the multimeter is capable to measure volts in the specific frequency range.
c) The Min/Max Peak it is the faster mode to detect Peak voltages ( and so no one lies here of what this function does).
I am speculating again that you do not have any guidance from the manufacturer of this car, and you are walking blindly. :)
If I am suspecting that I am dealing with pulses, the first thing that I would measure is the frequency.
One good oscilloscope which haves one significant amount of memory in it, it can record one number of samples, which they are a larger window of data compared to multimeters.
But If you do not aware of what you are looking for (from the start), I do not think that even this it would be a great help.
Phyllomedusa:
As I said: A scope is the best way, but not everybody has one. Also high memory isn't necessary as the control of the injection isn't that slow.
To the sensor.....it's no magic, most of the oxygen sensors work on the same principle. They compare the ambient oxygen with the exhaust oxygen and a difference there produces a voltage. The oscillation just comes from the ECU and injection systems that tries to mix it the best way.
Here is the problem: The sensor will be getting lower in amplitude over the time and therefore the ECU thinks that there is less fuel in the mixture (it's lean) and shove in some extra-fuel. That increases the fuel consumption (you'll notice that) and actually isn't healthy for the exhaust system and the catalysator.
As I said the Sensor is just measuring what the ECU control and it is an AC Signal. I doubt that the multimeter is smart enough to know that you want to measure the amplitude/Positive peak.
As far as I remember the TRMS measures the effective voltage (230V on the mains doesn't mean the Max. Voltage is at 230V, but 230*SQRT(2)).
So the sensor won't be bad, but there is a high possibility that you think it's dead when you use your TRMS meter as it shows you the effective voltage.
For example:
If I use a sine of 600mVpp the meter gives me a reading of roughly 211mVrms. If you don't remember/or know that this is the effective Voltage and not the actual Maximum of the Signal.
And regarding the Sensor output....you don't need any advice from the manufacturer to read the voltage ;). If it just is cycling for about 200mV or stays high/low with a warm engine...... put in a new one ;)
Maybe check this one for the principle of the sensor. Perhaps you'll understand then what I suppose.
Phyllomedusa:
--- Quote from: Phyllomedusa on October 26, 2012, 07:21:04 PM ---As I said: A scope is the best way, but not everybody has one. Also high memory isn't necessary as the control of the injection isn't that slow.
To the sensor.....it's no magic, most of the oxygen sensors work on the same principle. They compare the ambient oxygen with the exhaust oxygen and a difference there produces a voltage. The oscillation just comes from the ECU and injection systems that tries to mix it the best way.
Here is the problem: The sensor will be getting lower in amplitude over the time and therefore the ECU thinks that there is less fuel in the mixture (it's lean) and shove in some extra-fuel. That increases the fuel consumption (you'll notice that) and actually isn't healthy for the exhaust system and the catalysator.
As I said the Sensor is just measuring what the ECU control and it is an AC Signal. I doubt that the multimeter is smart enough to know that you want to measure the amplitude/Positive peak.
As far as I remember the TRMS measures the effective voltage (230V on the mains doesn't mean the Max. Voltage is at 230V, but 230*SQRT(2)).
So the sensor won't be bad, but there is a high possibility that you think it's dead when you use your TRMS meter as it shows you the effective voltage.
For example:
If I use a sine of 600mVpp the meter gives me a reading of roughly 211mVrms. If you don't remember/or know that this is the effective Voltage and not the actual Maximum of the Signal.
And regarding the Sensor output....you don't need any advice from the manufacturer to read the voltage ;). If it just is cycling for about 200mV or stays high/low with a warm engine...... put in a new one ;)
Maybe check this one for the principle of the sensor. Perhaps you'll understand then what I suppose.
--- End quote ---
So in this case you may be able to see it as a kind of pulsing DC and check it that way. But what if the frequency would be about 40Hz or so. Then I am pretty sure you'll get the DC component only. An example for a similar signal but with higher frequency could be wheel speed sensor. It can get dirty or the gap between the sensor and the wheel could be too wide (really happens). Then the voltage of the sensor itself.
I agree with you that the peak feature is meant for voltage spikes and so on.....but well, it seems to work in this case too. And I could really appreciate the Max Peak of a signal. Could be interesting for educational purposes as well, as I think a lot of folks out there think that there is only a Max of 230V on the mains ;)....
Kiriakos GR:
The above video speaks clearly that what they are interest at, is the Duty Cycle.
And this is something that the DMM can do.
The scope that this dude has looks interesting, and I made a quick search about it and found the below video.
About True-RMS in automotive repairs, what I know from experience is that they use average measuring multimeter, but is not my field of expertise, and I do not wish to get more to it.
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