Standard signal in fields Current and Pneumatic

[haideralibutt]

It would be highly appreciated if some one could let me know these questions regarding the signals.
Why we use 3-15 psi why not other limits i.e 0-15 psi or any other values other then this?
Why we use 4-20 mA why not other values i.e 0-20 mA any other values other then this?

Regards,

[amshah]

I think you are not from instrument field,

1) 3 requires for differentiate live zero and dead zero (instrument fail), 15 requires to maintain linearity with flapper nozzle design. Also you can find other signals as 3 to 30 psi, 3 to 50 psi in rare cases for specially designed instruments.

2) same way for 4 ma requires for live and dead zero differentiate, 20 ma and 4 ma combination requires to convert to voltage through 250 ohm std. rasister linearly to 1 to 5 v dc. Above 25 ma as per biomedical laws a human can get shock( internal to body cell) with 24 vDC. there are some instrument which have option of 0 to 20 ma span of measurement.

[haideralibutt]

well sir i am from instrument field

well i know that we need to keep the circuit thts y we use 3-15 psi and 4-20 ma but the question is y this y not 1-15 psi or 2-15 psi psi or otherwise and same case with 4-20 mA y we do not use 1-20 mA y do we have to use 250 Ohms resistance we can have a resistance of higher range and is it coz of the 1-5 v requirement of the cards? what if we use lower limits of 1-4 V?

[bega]

Perhaps the link below will help you

[link Point to another website Only the registered members can access]

[amshah]

Based on my erlier reply and begas link, i would like to summurise here, if not correct- please correct

1) pneumatic : 3-15 psi- signal starts other then zero is for live and dead zero, 3 to 15 psi is meant for linear response of the flapper nozzle design in this region.

2) 4-20ma : above zero requires for live and dead zero, in europe still it is 0 to 20 ma.

4 to 20 is requires to convert the same to 1 to 5 volt which normally can be measured at receiver side very easily with standard highily (.0005%) accurate 250 ohm resistor. If it is 4 to 21 or other values, non standard resistor is requied to manufacture which is difficult to get from open market and there (may be) a std. for resister values.
and above 25 mA shock hazards remains to human cell.

Hope it meets your requirement.

Nice point for discussion : to refresh basics