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the voltage of air - sorry rookie here

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irishire

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so ive two different multimeters which are displaying small ac milli-volt readings simply been exposed to the air. one of the meters is a cheapy and the other is decent. its possibly a coincidental fault but unlikely with both of them

is this a normal occurrence for multimeters? (when i say normal im mean do you or others that you know generate these types of readings with any sort of frequency)
what readings have you detected?
is there such a thing as a constant-variable air voltage?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
so ive two different multimeters which are displaying small ac milli-volt readings simply been exposed to the air. one of the meters is a cheapy and the other is decent. its possibly a coincidental fault but unlikely with both of them

is this a normal occurrence for multimeters? (when i say normal im mean do you or others that you know generate these types of readings with any sort of frequency)
what readings have you detected?
is there such a thing as a constant-variable air voltage?
No, but meters (and scopes) have high input impedances (they need to) so they pick up stray RF, mains hum, and general noise - in a modern environment mains hum is probably the biggest culprit.

So it's perfectly normal.
 

spec

Well-Known Member
Most Helpful Member
Hi irishire,

Greetings from ETO.

Which part of Ireland are you from. If you put it next to 'Location' on your user page it will show in the box at the left of your posts. Knowing your location helps us with answers.

is there such a thing as a constant-variable air voltage?
Briefly, no there is not, but air can acquire a voltage, it can create a voltage, and it can conduct a voltage.

The first thing to know is that the air is full of electromagnetic waves, including light, both natural and man-made. That is why your radio and TV work.
Air can also squire a static charge, more so if it is dry. Air can also cause a charge by passing over an object and stripping off electrons.

The upshot of this is that your multimeter can pick up these signals and give you a random reading, especially if you have long open circuit leads on the input of your multimeter (acting as an antenna). This is quite normal. If you short the leads together the reading should be greatly reduced.:)

spec

(crossed posts Nigel)
 

irishire

New Member
thanks for the reply, could you tell me what the upper readings could be. im detecting close to half a volt ac at times, which means that i cant test the output from a small electret - (have an experiment to do and the readings are expected to be 10-20mV)
 

spec

Well-Known Member
Most Helpful Member
thanks for the reply, could you tell me what the upper readings could be. im detecting close to half a volt ac at times, which means that i cant test the output from a small electret - (have an experiment to do and the readings are expected to be 10-20mV)
There is no upper reading as such, but 0.5V seems like a lot.

Is the 20mV a direct voltage or is it an alternating voltage as in the audio signal from the microphone (there are problems measuring the output from electret microphones because of their high output impedance).

There are many techniques for making low-level measurements: screened & short leads, screened room/container, good earthing.

If your meter is set to the voltage range that you need and you short the leads together, what reading do you get. Also what range is the meter set to?

spec
 

irishire

New Member
spec - yeah the readings for the electret experiment are for alternating volts. also the half volt reading was the uppermost reading ive seen but its generally reading approx 10mV constantly.
when the leads are shorted the readings drops to 0V

followup - the half volt reading could be a meter calibration issue or something like that. the upper most reading on the lowest ac setting (2) is .05v but with the next setting (20) the reading remains the same but the decimal place has shifted giving the half volt reading
 

spec

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Most Helpful Member
spec - yeah the readings for the electret experiment are for alternating volts. also the half volt reading was the uppermost reading ive seen but its generally reading approx 10mV constantly.
when the leads are shorted the readings drops to 0V

followup - the half volt reading could be a meter calibration issue or something like that. the upper most reading on the lowest ac setting (2) is .05v but with the next setting (20) the reading remains the same but the decimal place has shifted giving the half volt reading
Hi irishire,

You have a hard, if not impossible, task on your hands there, afraid to say. The easiest way to check an electret mike is to amplify the signal before you make a measurement. The other issue is frequency response. Audio is 20Hz to 20KHz and multimeters can have a frequency response of only 1 KHz so you need to ensure that the signal from the microphone is not above this frequency.

By the way, making an amplifier for an electret mike is fairly simple.

spec
 

Nigel Goodwin

Super Moderator
Most Helpful Member
thanks for the reply, could you tell me what the upper readings could be. im detecting close to half a volt ac at times, which means that i cant test the output from a small electret - (have an experiment to do and the readings are expected to be 10-20mV)
A multimeter isn't the correct tool to try and measure something like that, you need an oscilloscope or AC Millivoltmeter - both of these would use screened leads to reduce stray pickup.

Even making an amplifier for the mike wouldn't be that good, as multimeters are mainly designed for use at specific low frequencies (mains frequencies), and not audio.
 

spec

Well-Known Member
Most Helpful Member
This is what you wrote in a previous post.
It's because the meter only works at low frequency AC, specifically it's designed for mains frequencies, 50Hz or 60Hz, but generally work a good bit higher. The actual spec will be given in the meters instruction leaflet, I just checked a Velleman meter I have here, and it's spec says 40Hz to 400Hz...
If you built an amplifier and exposed the mike to an audio source of around 400Hz, most multimeters (digital that is) will work fine.

As a matter of interest, most digital multimeters will measure 50Hz to 400Hz. If you go slightly upmarket you can get 20Hz to 5Khz (my multimeter does this) and higher quality multimeters can measure up to 100Khz, typically.

Normally though, the accuracy is lower at AC and is less again at higher frequencies.

It is odd that most digital multimeters do not measure up to around 10Khz because the technology is low cost and available- I suppose there is little demand.

UPDATE: I just remembered what the issue is with a high frequency response for multimeters: it is the frequency response of the high resistance input attenuators.

spec
 
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