If I have multiple inclinometers, do I have to measure the spiral sensor offset each time before I make a spiral survey?

No, because that just isn’t practical. Here’s a suggestion:

1. Make a precise measurement of offset before you leave the office. Write down the measurements per the instructions in the manual.
2. Now you’re at the first inclinometer. Insert the instrument and take a measurement at the top of the hole. Note the inclinometer ID and measurement value on paper. Then do the normal spiral survey.
3. Now you’re at the next inclinometer. Repeat the steps above. You’ll probably see a slightly different value for the top of hole measurement. You shouldn’t see a huge difference. Be sure to note the inclinometer and the value. You can calculate the change later.
4. When you return to the office to crunch the data, you’ll have to enter the precise offset measurement. For the first inclinometer, you’ll enter the measurement as obtained at the office. For the other inclinometers, enter the precise measurement + the change from the top-of-hole measurement.