Calibration of a speed meter 4-20mA

<...why...measure 4-20mA and why not 0-20mA...>

So that a current of less than 4mA (such as 0mA for example) can be used to detect a wiring fault.

Typically, 4 mA is lowest value and often used as zero instead of something like you have, 20 M/min. If it's calibrated to run 20-120 M/min from 4-20 mA, then it should be a linear transfer function: 16mA per 100M/min or 6.25 M/min per mA.

As PW says, 0 mA means there's an open circuit, which is detectable.

Dear all,

Other than fault detection, 0-4mA can be used to supply power for instrument so that 2 wire instruments can work without power cable.

Regards

Use range versus range, not limits.

You have a measuring range of 20-4ma = 16ma. You have a range of 120-20 M/min = 100M/min. Now 100M/min divided by a 16ma range = 6.25M/min/ma. Since the zero point is 4ma, not zero, there will be a correction factor of 5.

Try it - (4x6.25)-5 = 25-5 = 20M/min and (20x6.25)-5 = 125-5=120M/min. And, we can then say that 12ma would be 70M/min. Programmers of PLC's on process sytems with loop instrumentation have to do this daily for analog in and out.

From: Tim Hawley Master Mech.

Hello Signode,

I use a 4-20mA converters all the time.

Think simply: 2 volts is equal to 4mA.

10 volts is equal to 20mA.

When the fixed resistance across the power source is equal to 500 ohms.

This means 6 volts is equal to 12mA which in the middle or center of the range.

So for every.001ma of movement or change is equal to .0005 volts or .5 millivolts.

An other example:

If you have a fixed resistance of 1000 ohms across a 1 volt power source; then for every 1 volt of increase of change you will see 1 milliamps or .001 amps of change. This gives you a linear speed scale that is very accurate and sensitive.

Example: 4ma across 1000 ohms is equal to 4 volts, and 20mA across 1000 ohms is equal to 20 volts an so on...

Use this ohms law calulator link http://www.onlineconversion.com/ohms_law.htm

Best Regards,

Dear Friends;

Thanks a lot for these useful informations. I have completely undesrtood about 4-20mA.