hi all,

why do we have a separate UPS power for mass flow meter?

i want to know , as we come across many transmitter(loop powered), why do we have a seperate power source for mass flow meter??

1-what is the reason behind??
2- how will identify, the transmitter/application shall be field powered or loop powered??
3- can you name any other transmittwer, beside mass flow meter?
4-and what is the threshold of condition to be loop powered & field powered...??
5-and what is the max allowable/tolerable distance that is sufficient for 4-20ma signal to be not attenuated?? i heard about approx 1 km, (hardwired)..and after this, is fiber optic is only left solution?

 

Coriolis Mass flow meters require too much power to be loop powered. Micro
Motion's loop powered coriolis meter is a study in compromises, and doesn't fully conform to the 4-20 mADC standard. Expect to use another power supply for your Coriolis meters.

Any transmitter that draws more operating current than 4 mA continuous must be a four-wire instrument.

Either or both the P&ID or the Instrument Data Sheet will identify loop powered instruments or four wire instruments.

Generally speaking, the furthest I've run 4-20 mADC is 1000 feet. If I have to go further I use a field mounted current isolator/retransmitter. Fibre optics works too.

 

I don't know what kind of mass flow meter you have, but many field instruments require more power than can be delivered on the 4-20ma line. Do the math: instruments powered from the line must operate with 4ma current. With a 24 volt power supply, that is only 24 x .004 = 0.096 or 96 milliwatts of power. Some instruments just require more power - such as your mass flow meter. For example, all magnetic flow meters require significant AC power that cannot be delivered on the 4-20ma DC line. Coriolis mass flow meters are also in this category.

 

> I don't know what kind of mass flow meter you have, but many field
> instruments require more power than can be delivered on the 4-20ma line.

I just want to know the basic meaning of loop powered transmission and simple transmission.

 

A 2 wire loop powered instrument uses about 3.5mA from the signal line to power itself and do whatever it needs to do. So loop powered instruments can never be 0-20mA because at least 3.5mA (or thereabouts) are needed to run the instrument.

A running 'loop' that is not in fault mode always has aminimum 4.0mA of power available for the instrument to use for its own power. A transmitter in Fault mode can drive the signal down to somewhere around 3.7 or 3.8ma to indicate a fault condition.

3.5mA at a nominal 24Vdc is only 84 mW. Some technologies work fine with 84mW available, others do not. Any technology that requires a heater probably is not a candidate for 2 wire loop power (thermal dispersion, FID). Mr. Boyes mentioned the tradeoffs for Coriolis. Same with magmeters - marginal. Those technologies that are not prospects for 2 wire loop powered become 4 wire instruments. The higher power requirements are met by supplying power on one pair of wires; the signal output is on another pair of wires. Add the number of wires together, you get 4 wires: two for power, two for output signal.

I don't know the context of 'simple transmission'.

 

Actually, in small (up to 4 inch or 100 mm nominal) sizes, several companies now make two-wire magnetic flowmeters that are actually very good and do not compromise accuracy and reliability for power consumption. Azbil (formerly Yamatake), Endress+Hauser, and others do this. They are usually slower in speed of response than a four wire unit, but for most applications work quite well.

Walt

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]

 

There are several loop powered magnetic flow meters on the market which work very well, there are even battery powered mag meters on the market which work very well!