What is Parallax Error on a Dial Instrument?

Paralax error is caused by the pointer of an instrument being in a different plane than the scale. The larger the distance, the greater the error.

The images below show the same dial thermometer, at the same temperature. As you can see, changing the viewing angle has the perceived effect of aligning the pointer at different positions on the scale. In this case that equates to an error of approximately +/- 10°F.

To reduce parallax-error, it's always best to read an instrument from directly in front if possible.

Tel-Tru's line of Industrial thermometers incorporates an Anti-Parallax Dial which ensures the pointer and scale are closely aligned, dramatically improving accuracy when reading at an angle as can be seen in the images below.

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