Why Doesn’t the Pen Match the Display?

Causes and Troubleshooting:

The chart recorder and digital display are actually two independent measuring instruments, with different specifications. There is certain amount of discrepancy allowed between chart and display. 

1. The amount of the discrepancy depends on the range of the chart recorder. An SL4120, for example, has a scale of -20 to +120 degrees, that's a range of 140 degrees. That means a 2.8 degree tolerance for the chart. 
2. The digital display has tolerance of 3.6 degrees, which means there is a maximum allowed discrepancy of 6.4 degrees between the chart and display. The chart may be adjusted using the pen adjust screw on the face of the unit. If the display is out of specification, the display can be replaced but not calibrated.

Causes and Troubleshooting:

1. Dip switches set for a specific range. Using a chart for another range could cause mismatching display and chart readings. Make sure the proper chart is installed and that the dip switches are set for that specific chart.
2. The “Pen Home” may not be set properly. To check and adjust the “Pen Home” setting, press and hold the Chart Rotation and Alarm button, then press the On/Off button. Hold down all three buttons until the unit comes on, the light should be flashing amber. When the amber light becomes steady, press the Alarm button twice, the light should alternate between Red and Green and the pen will move toward the top of the chart. The pen should stop on the Maximum Temperature Line of the chart that is being used. If the pen is not exactly on the Maximum Temperature Line, use the Alarm button to move the pen down and the Chart Rotation button to move the pen up.
3. Ensure that the gears are not binding or slipping by manually advancing the chart one full rotation with the chart rotation button.
4. Another possibility is that the pen(s) are not completely inserted on the pen arm(s).