Flow totalisation in PLC
Flow totalisation in PLC
(OP)
I have an application where I have to totalise the flow value of coke oven gas (in a steel plant)in a PLC based automation system. For this, I have a flow transmitter in the field which gives a 4-20mA DC output corresponding to the flow range.
Can I use this 4-20mA signal directly in the PLC(through analog input card)for totalisation of this flow? Or, should I convert this 4-20mA DC signal into a pulse signal through an external pulse convertor and then feed the pulse signal to a pulse card of the PLC for totalisation?
Which philosophy will result in better totalisation accuracy?
Or is it better to use an instrument generating a direct pulse signal corresponding to the flow and then feed the pulse signal to a pulse card of the PLC for totalisation?
BMSG
Can I use this 4-20mA signal directly in the PLC(through analog input card)for totalisation of this flow? Or, should I convert this 4-20mA DC signal into a pulse signal through an external pulse convertor and then feed the pulse signal to a pulse card of the PLC for totalisation?
Which philosophy will result in better totalisation accuracy?
Or is it better to use an instrument generating a direct pulse signal corresponding to the flow and then feed the pulse signal to a pulse card of the PLC for totalisation?
BMSG





RE: Flow totalisation in PLC
The best would be a flowmeter with built in integration and a serial connection to the PLC.
If integrating 4-20 from the flow meter beware that when the flow is low accuracy is low. If the flow is zero you may still have a reading from the FT and you could be integrating it leading to a falacious reading. So you may need logic to switch integration off when you know the flow is zero.
RE: Flow totalisation in PLC
RE: Flow totalisation in PLC
To show our accuracy at the export station we typically fill an open 55 gallon drum @ 200 - 250 gpm to the top of the drum without spilling a drop.
RE: Flow totalisation in PLC
RE: Flow totalisation in PLC
BMSG
RE: Flow totalisation in PLC
I asked what the flowmeter was because it matters. If it is an orifice plate it is not going to be better that +/-1% at best, and so 4-20mA, even when integrated may be good enough.
If it is a turbine (say) then you should read the pulses from the meter directly, since that is the primary signal.
Ask:
What are the accuracy requirements?
What is the accuracy of the flowmeter?
What is the accuracy of the 4-20mA input card, in fact of all the compoments of the 4-20mA loop?
Given these you can work out whether integrating in the PLC will degrade the accuracy significantly.
If you do decide to integrate in the PLC then make sure that the integration logic is reliable - for example it should run on a time interrupt fast enough to respond to changes in flow rate without losing accuracy.
Your PLC supplier by making dogmatic statement is not impressive!
RE: Flow totalisation in PLC
I agree with all you have said.
My flowmeter is an orifice plate with a SMART differential pressure type flow transmitter, which outputs a 4-20mA DC signal proportional to the DP.
My only confusion is that if I integrate the transmitter signal (after square root extraction) in the PLC, will there be any significant error in the process of A/D conversion and integration vis-a-vis direct counting of pulses from a pulse converter?
RE: Flow totalisation in PLC
If not then you must linearise it in the PLC then do the integration.
A/D conversion errors depend in the resolution of the PLC'a Analog input. If it is 12 bit then your error will only be about 0.025%, which is far less than the error in the Flow meter, and so it is not likely to degrade overal accuracy significantly.
If it is 8 bits then you have a cheap input card. Even so that would contribute 0.4% which may still be acceptable.
(When adding errors by the way is it normal to use Root Mean Square so adding 1% from the flowmeter to 0.4% from the A-D still only makes 1.07%)
What accuracy are you aiming to achieve?
RE: Flow totalisation in PLC
RE: Flow totalisation in PLC
scozans
Philippines
RE: Flow totalisation in PLC
RE: Flow totalisation in PLC
RE: Flow totalisation in PLC