4-20 ma

Hi there,

This question has been answered in the instrumentation section.

Briefly - with 0 - 10V or 0 - 20mA your zero is 0. With 4 - 20mA you have an elevated current zero. So 4mA will be your reference point for zero and 20mA will be your reference point for the transmitter span.

The NAMUR standard states that your over current limit is 22mA - indicating a short, and your under current value is 3.6mA which will indicate a wire break. So by having an elevated zero we can now determine a wire break where it would not be possible with 0 - 20mA or 0 - 10V.

There are other calculations that would apply to intrinsic safe areas where your loop inductance and capacitance have to be below certain values.

For further reference visit the NAMUR web site.

Regards,

Craig

The electrical instruments systems can transmit the data in various forms (Parallel/Serial/Digital/Analog)

In the Analog form, you have broadly two methods

a) Voltage Signal- where transducer output voltage is proportional to the parameter.

b) Current Signel (Current loop instruments) where the signal, now as per standard HART protocol 4-20 mA, is transmitted, the value a function of the parameter.

The Voltage signal are much easier form but has some accuracy problem.

Due to the long distance between the transducer and the reciever (Voltmeter), even though the current is small, the voltage drop may affect the value as interpreted. Added to that other effects which may affect the resistance of the communication wire may be there.

In the current loop systems, this problem is taken care of, since we here use a current source and not a voltage source.

The current usually is converted to the voltage for digitising/ analog voltage signal at reciever end by putting a small resistor in series and measuring drop across it.

It jhas nothing to do with hazard (the voltage anyway were low) rather with the signal accuracy and also now almost universally the HART

http://www.smar.com/hart.asp

http://www.arcomcontrols.com/products/pcp/Gateways/Hart/AN19960601.pdf

Off topic

Other common protocol is PROFIBUS-DP

www.profibus.com/celummdb/doc/RPA%20UNITED%20KINGDOM/Downloads/CPIC-PROFIBUS%20Introduction.doc.doc

http://www.rtaautomation.com/profibus/

and a few others.

PS: This is a thread for instruments section, there may be more experts in that section.

Hi,

The main advantage of a current signal over a voltage signal is the immunity to disturbance. A current which enters a cable must come out. As long as the cable is intact, no leakage, the current which enters is the current which will exit. A voltage signal may be infuenced from outside the cable. This is the main reason to choose a current signal over a voltage signal. See for some examples "http://www.divize.com/techinfo/currentloop-connection.html"

Another advantage is the live zero (4 mA) which alows to detect wire breakage,

Regards, Heinrich