Multi-meter sensitivity

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bdickens
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Multi-meter sensitivity

Post by bdickens » Mon Sep 24, 2007 2:39 pm

I'm using a Seapoint turbidity sensor on a project and I need to be able to measure down to 10mV accuracy for a 5V output line (.001 to 5). Now my MM has that many decimal places, but I got to wondering about sensitivity. Each FTU (Formazin Turbitity Unit?) is .01.

How does one find out how accurate , or what the resolution of the meter is ? I'm betting that since i have a 200mV setting, that I have the room, but I'm not all that certain and I don't have enough experience with this.

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haklesup
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Post by haklesup » Mon Sep 24, 2007 3:14 pm

The specs would be in the DMM owners manual. Look at similar specs online if you lost yours to get a feel for the ballpark.

The 200mV range will be fine up to 200mV but you won't get full scale to 5V if you needed it. Should be 1mV resolution or better in that scale, even the next range up may work.

Some examples:
http://us.fluke.com/usen/support/manuals/default.htm

http://assets.fluke.com/manuals/11x_____umeng0200.pdf

Some meters have hi resolution modes. My Tek you power up while holding a button and it is much more accurate (at the expense of a little speed)

Dean Huster
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Post by Dean Huster » Mon Sep 24, 2007 6:21 pm

It depends upon the number of digits available. A 3-1/2 digit meter used on the 20 volt range to handle the full 5 volts will provide a maximum reading of 19.99 volts with 10mv resolution. A 4-1/2 digit meter will max out at 19.999 volts with 1mv resolution. Understand that almost any digital readout (other than some freaky frequency counters) will have a ±1 digit bobble in addition to the readout accuracy specification, e.g., ±0.05%. This means that an actual 5.00000 volts measured on that 4-1/2 digit readout with those specs could indicate anywhere from 5.003 down to 4.997 volts. An actual voltage of 20.000mv on that same meter would read anywhere from 19mv to 21mv as the ±1 digit bobble swamps the 0.05% accuracy (±0.01mv). With a 20mv reading, the ±1 digit bobble translates into an error of ±5% which really makes that fine meter accuracy pointless.

This concept holds true of a digital meter as well an an analog meter: for the most precise and accurate reading, use the lowest range available that will not go overrange with the measured quantity.

At least the digital meter's accuracy specification doesn't swamp the reading as it would with an analog meter. An analog meter with the same accuracy (and impossible to find) would have an accuracy of ±0.05% FSD (full scale deflection) which translates into ±10mv on the 20v range, an amount that would make that 20.00000mv actual voltage read anywhere from 10mv to 30mv on the analog instrument.

Note: Although differential voltmeters with accuracies of ±0.05% existed, that accuracy was not dependent upon its analog meter which was used solely to indicate a balance or null.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

bdickens
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Post by bdickens » Sat Sep 29, 2007 9:34 am

Good points. I'm hunting for a better meter. I've tried it with a friends and you all were correct. Too few digits for the accuracy. Thanks

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