Basically when you are doing the troubleshooting of
any instrumentation system you assume that every instrument has at least
one input and at least one output and that the output(s) should accurately
correspond to the input (s). In normal circumstances, if the instrument’s
output is not corresponding to its input according to the instrument’s design
function, then there could be something wrong with the instrument. Lets
consider the inputs of the following examples of instruments that are
commonly used in process control systems:
- Differential Pressure transmitter
- Temperature Transmitter
- Controller
Each of the above instruments takes in (input) data
and generates the (output) data. In an instrumentation loop, the output
of one instrument feeds into the input of the next. Such information is passed
from one instrument to another.
You can also read:
By intercepting the data communicated between components
of an instrument system, we are able to locate and isolate faults. For us to
able to properly understand the intercepted data, we must understand the inputs
and outputs of the respective instruments and the basic functions of those
instruments. From the above diagrams, we are able to highlight the kind
of inputs and outputs for each of the instruments indicated.
To be able to check the right correspondence
between the instrument inputs and outputs, we must therefore use appropriate
test equipment to intercept the signals into and out of these instruments e. g.
in case of analogue instruments using 4-20 mA signals we can use the electrical
meters capable of measuring the current and voltage.
So what are some of the key considerations when using milliameters to
measure loop current?
For you to measure the loop current, you have to
break the circuit to connect the milliameter, in series with the current, and
which means the current will fall to 0 mA until the meter is connected.
Interrupting the current means interrupting the flow of information that is
conveyed by that current, be it a process measurement or a command signal to a
final control element. This can have adverse effects on the control system
unless certain preparations are made before hand. The preparations can be
in form of:
- Informing the personal in charge that signal will be interrupted - state the number of times you intend to do the interruption.
- For case, where the signal is coming from a process transmitter to a controller, the controller should be placed in manual mode, so that it will not cause an upset in the process.
- If the current drives process shutdown alarms, these should be disabled on temporarily basis, so that nothing shuts down upon the interruption of the signal.
- All process alarms should be temporarily disables so that they do not cause panic.
- If the current signal to be interrupted is a command signal from a controller to a final control element, the final control element either needs to be manually overridden so as to hold a fixed setting while the signal varies or it needs to be bypassed completely by some other devices (s)
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