Is there 24V voltage on the signal wires of the four-wire mass flow meter?
Thread Content
Have you ever encountered this situation: in a four-wire mass flow meter, there is 24V on the signal line leading to the safety barrier? This is what happened to me today with the Coriolis mass flow meter.
Are you referring to the open-circuit voltage at the signal output terminal? Of course, there is a higher voltage; otherwise, it wouldn’t be possible to drive the load resistor and transmit the signal over a certain distance. The current output terminals of four-wire instruments have a certain voltage, and the level of this voltage determines the distance over which the signal can be transmitted, or in other words, its ability to drive loads. The signal terminals of active outputs all have a certain voltage, which is quite normal.
Why does a four-wire instrument still need to be connected to a safety barrier? ?
The mass flow meter from Emerson that I used didn’t show a 24V voltage
I’m not quite sure about this; everyone here has used safety barriers
There are indeed devices designed in this way; what I’ve seen are electromagnetic flowmeters, whose output goes through a safety barrier before reaching the card reader. It doesn’t constitute a safety system on its own, but it is likely used for isolation to prevent the components from being damaged due to high voltage in the circuits.
The analog output of the instrument can be designed to be passive; this output circuit functions like a two-wire transmitter, and there is no problem with that.
4-wire system: one power supply line and one signal line. The power supply provides a voltage of 24 volts. The signal line transmits signals of 4-20 milliamps; this current only exists as a result of the voltage applied. From the perspective of the safety barrier, it is not powered, but from the perspective of the flow meter, it is powered. The 24V for the signal line is provided by the flow meter and connected to the unpowered terminal of the safety barrier.
Can connecting a safety barrier still cause vibrations in the mass flow meter?
This is correct; some instruments require four-wire power supply, and when the signal terminal carries an active signal, there will be 24V
I’ve encountered this situation before; I forgot whether it was a mass flow meter or something like Emerson’s concentration meter. It requires a separate power supply, and the signal line also needs an active 24V supply.