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Question:
Will all pens keep accurate time on the chart?
What is the pen to chart time accuracy?

Answer: No, only the red pen (position 1)* will properly track the chart from 0-100% of the scale. The reason is simple, since all four pens (maximum pens allowed on a C1900) are charted at the same time with separate ink pens, they must be positioned apart roughly 3/8" to avoid collision with each other. This initial error is approximately 8 minutes apart on a 24 Hr. chart @ 100%.
* The True Time line event pen is the exception to the rule. This pen option has a custom pen arm that lines the event pen (purple or black - position 4) with the red pen (position 1). See photos below and at bottom of page.

Standard Pen Position Only the red pen is set to the time line	True Time Line Event Pen Position The red and black pen share the same time line

 

Since all four pens have different lengths, the accuracy will not be linear with the chart from 0-100%. This is due to the arc of each pen line being slightly different from each other. The physics at work here make it impossible to solve this problem on any chart recorder that uses separate pens on a single chart. The photos below illustrate the different arcs of each pen.

Red Pen Arc - Pen Position 1	Green Pen Arc - Pen Position 2
Blue Pen Arc - Pen Position 3	Black/Purple Pen Arc - Pen Position 4

 

The following table is an example of the pen error using a 24 Hr. chart set at 5:00
Scale Position	0%	25%	50%	75%	100%
Red	5:00	5:00	5:00	5:00	5:00
Green	4:15	4:41	4:46	4:49	4:52
Blue	5:45	5:22	5:15	5:10	5:08
Black/Purple	3:29	4:18	4:32	4:39	4:43

 



As mentioned earlier, the True Time Line option is the exception to the rule. This photo illustrates that the red pen and the black pen are indeed on the same time line.

Please note: The True Time Line  event pen position is for illustation purposes only. In reality, the event pen would only mark at the outer edge of the chart.

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