# Meter Calibration

Discussion in 'The Projects Forum' started by TacMot, Dec 26, 2013.

1. ### TacMot Thread Starter New Member

Dec 26, 2013
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I wish to use a bridge circuit to calibrate a meter with ten major divisions. (I understand how a Wheatstone null meter works) What I'm really interested in doing is calibrating my meter so that I can measure the voltage in an unbalanced condition. i.e., if the meter in the balanced condition shows 25 VDC at division "0", then how can I make the meter read 0.5 VDC at each division above "0" and -0.5 VDC
at each point below "0"?

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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What changes one or more of the resistors in the bridge? Switches? A Pot? Sensor?

3. ### TacMot Thread Starter New Member

Dec 26, 2013
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The only thing that changes is the voltage.
TacMot

4. ### TacMot Thread Starter New Member

Dec 26, 2013
5
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The only thing that changes is the voltage applied to the bridge.
TacMot

5. ### TacMot Thread Starter New Member

Dec 26, 2013
5
0
Re: Meter Calibration
I've been naive in my query. I probably shouldn't be using a bridge at all.
If a meter shows 25 VDC at a given major division, then how can I make this meter read 0.5 VDC at each division above this major division and -0.5 VDC at each point below this major division? The only thing that changes is the input voltage.
TacMot

6. ### MikeML AAC Fanatic!

Oct 2, 2009
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bance and Metalmann like this.
7. ### BillB3857 Senior Member

Feb 28, 2009
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Are you wanting to make what is known as an expanded scale analog meter? That is one that with the needle at rest, it will read the low end of the voltage range, at mid position, the target voltage and at the full scale, voltages above the target with the same scale factor as those below the target.

It would help a lot to know what kind of meter movement you have. (ohms/volt or full scale current rateing, internal resistance, etc.)

8. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Please respond to the forum rather than private. Others may learn also...
Usually, the expanded scale meter setup is to use series resistance to provide the scale factor. To do that, determine what the full scale difference will be. Apply that voltage from an external source and adjust the resistance for full scale. As an example, you want the meter to be centered at 25 volts, lower end to be 20 volts and full scale to be 30 volts. The SPAN will be 10 volts. Use an external source of 10 volts and adjust series resistance for full scale.
Next, you need to set the scale. In the example, if you put a 20V zener (proper direction), and apply 25 volts, 20 of those will be "used up" by the zener leaving 5 for the meter. 5 volts at the meter in the example will give 1/2 of full scale.

Hope this is clear enough.

inwo likes this.