voltmeter with more accuracy

[sharath_bn]

hi

i am very interested in doing the digital voltmeter project.but the problem is that i need it to measure in steps of 0.1 V (lesser the better)..i am ready to sacrifice the max. the meter can measure for it's accuracy...is there any simple modification that can do this..
hurry i need fast answers

ur new friend
sharath :)

[audioguru]

Hi Sharath,
Welcome to our forum.
Our projects section has two digital voltmeter projects that use the same ICL7107 IC. Which one are you talking about?

The "Digital Voltmeter" project has its reference voltage set to 0.1V, just like the datasheet for the IC shows, but uses an extremely low-value voltage divider on the IC's input for its voltage ranges. For its 0.2V range, it uses a 10 ohms resistor across the IC's input on its voltage divider.

The "LED Display Digital Voltmeter" project has its reference voltage set to 1.0V and also uses a voltage divider for higher ranges. For its 2.0V range, it uses a 0.0 ohms resistor across the IC's input (must be a mistake) on its voltage divider.

The accuracy of both projects are determined by its voltage divider and reference setup resistors' drift and your calibration.

In addition to wanting better accuracy, you also mention wanting "steps of 0.1V or better". Since these are four-digit meters, the steps are exactly 0.1V for their 200V range, 0.01V for their 2V range and the 1st project has steps of 0.001V for its 0.2V range. When set for a 0.2V or 2V range, the IC doesn't need a voltage divider, and can be extremely accurate depending on only its reference setup resistors' drift and your calibration.
Which voltage range will you be using?

[sharath_bn]

thanks for the speedy reply,

i am most likely to use the 2V range with 0.01V accuracy....
actually i am planning to use this as a part of hot wire anemometer
to measure velocities accurately....
basically it works like this...
a thin strand of wire is part of a balanced wheatstone bridge...the wire is heated by passing electric current..so when kept in flow the wire starts to cool and reaches steady state...so resistance of wire changes and the extra voltage developed is measured and hence the flow velocity is measured
the voltage measurement requires voltmeter...

if anyone has better ideas or suggestions .please do so...

[audioguru]

Hi Sharath,
Instead of using a fragile wire that might break or burn-out, you could use a dual transistor package. One of the transistors is the heater and the other is the temp sensor. You can get them in a sturdy metal package.

[abad]

i wanna mkore info

[ante]

Hi Sharath,

The technique you described is commonly used to measure the airflow in the intake manifold of EFI engines. For this principle to work with high precision it must have one part of the bridge in a protected and controlled environment. The sensing wire must also be placed in clean (filtered) air to prevent any particles to build up on it that would cause faulty measuring. I don

[MP]

Sharath,
The hot wire method is a long proven design. I am surprised you are building one from scratch. You will get much better sensitivity with this setup than you will using transistors. The hot wire method is also a standard in the meteorological industry and air quality measurement equipment. This is the method used for the flow sensors in most all environmental quality equipment that has a mass flow controller. In these devices, precision is most important so that the accumulation of mass particles can be accurately measured in a cubic meter of air.
What are the values of your high and low measurements? This will determine what type of voltmeter you will need. You will want at least 2 decimal places for resolution. 3 would be much better. You will also want some type of conversion from voltage to a flow measurement. Had you planned on using a microprocessor to do this?

MP