I have two instruments for measuring my blood pressure, arm and wrist.  The diastolic readings (lower) tend to agree.  However the systolic (higher) readings tend to be far apart.  The arm reading runs about 30 points higher.  Is this a meaningful difference or a sign of instrument error?

It’s usual to get a higher reading of both at the wrist.

All the numbers you hear regarding healthy BP refer to brachial (arm) BP so I'd stick with that if you're going to be discussing it with your health providers. Here and here confirms nevim's statement. Ideally, should also be sat for at least one minute and the arm should be supported, or at least taken in a consistent manner to allow more meaningful comparisons across time.

13 hours ago, Prometheus said:

Ideally, should also be sat for at least one minute

... with both feet on the floor. For some (), crossing the legs when sitting down is a habit, and it definitely messes with your BP readings.

23 hours ago, nevim said:

It’s usual to get a higher reading of both at the wrist.

My readings:  Systolic much higher at arm, Diastolic approximately the same for both.

1 hour ago, mathematic said:

My readings:  Systolic much higher at arm, Diastolic approximately the same for both.

I don’t know why that is, sorry, but if it helps, I was taught that the diastolic reading is the more important one. So it would seem you have an accurate result for that if arm and wrist are giving same readings.