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Test fitting schematic

Test fitting schematic

Test fitting schematic

I have in front of me a vendor drawing of a test fitting.  It has 4 ports.  In, Out, Test and Exhaust.

The interconnection of the ports is determined by and controlled by the insertion of different test probes into the test port.

When not testing (i.e. normal operation) a dummy plug is inserted into the test port.

The dummy plug, or the test fitting therefore also serve as the valve actuator.

Functionally, the possible connections are:

Normal (dummy plug inserted):  In connected to out.  Test blocked.  Exhaust blocked.

No plug or probed inserted:  Out connected to exhaust.  In blocked.  Test blocked.

Type A test probe inserted:  In, test and out connected.  Exhaust blocked.

Type B test probe inserted:  Test connected to out.  In blocked.  Exhaust blocked.

The vendor is proposing this schematic symbol

Clearly it's not exactly right.  However I'm not sure what exactly right would be.  Any suggestions on how to properly depict this schematically are welcome.

RE: Test fitting schematic

I would move the 'dummy plug' block to the place where the ports are labeled, to show that's the 'normal' position, and put the 'no plug' block immediately below that (maybe with a 'NO PROBE' label next to it), with the spring at the bottom, to suggest that the 'no probe' condition governs when the plug is removed.  I might relabel the 'dummy plug' to 'NORMAL PROBE'.

I'd leave the little 'T' handle at the top, and 'chain' it with curved mechanical linkage symbols to two loose blocks labeled 'A PROBE' and 'B PROBE'.

... but that takes up a lot of drawing space.

If you must use they symbol all over the place in a system diagram, I could live with something similar to what your vendor suggested, reordered, with a little white space between the blocks to suggest that they're physically separate.


Mike Halloran
Pembroke Pines, FL, USA

RE: Test fitting schematic

I would also add a truth table/logic table near the symbol to be absolutely clear. Granted, schematics are supposed to be clear without language, but more info is better, and faster.

What I have seen most often:
Columns across the top for three conditions (no probe, type A, type B)
Rows along the side for the ports. (out in Ex Test)
In each column, a vertical line top to bottom.
Heavy black dot in any box that line connects to in that condition.
Multiple vertical lines and dots if there are multiple connections to different ports in that condition.

Tried to insert a sample, but snipping tool not working for me.

RE: Test fitting schematic

I also came to the conclusion that the state envelopes and labels were not in the right place.

(a)    With the dummy fitting inserted "IN" is connected to "OUT" and all other ports are blocked.  This connection state is shown in the second from the top envelope.  As this is the "normal" condition of the device the port labels should be on this envelope, not the top envelope.
(b)    The envelope with "OUT" and "EX" connected should be the bottom envelope, as this is the state of the device when there is no actuation (no probe or dummy plug inserted).

However this still does not really capture that the insertion of the dummy plug or test probes is the actuation method.  

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