A question: Can changing the length of a cable used to attach a detector and instrument cause a difference in the readings obtained?
Sometimes the answer to a question is simply another question. Why would this be necessary? Well, do you want a good answer or not? (Did you see what we did there?) When a customer contacts Ludlum Measurements with a question regarding the application of one of our instruments we nearly always require more information in order to provide an accurate response.
For example, today’s question concerned the customer’s desire to know if a different cable length on a detector would require the instrument to be recalibrated to this new length. This is an excellent inquiry. Even so, it is going to generate an equally important question: What detector are you using with it?
Though we know the theory and could guess what would happen, we nonetheless performed some real-world testing to demonstrate its validity. A Model 177 Benchtop Counter served as the base instrument, with a Model 44-9 GM Pancake detector, a Model 44-2 NaI Scintillator detector, and various lengths of otherwise identical cable completed the test kit. Cable lengths varied from a mere 7.6 cm (3 in.) to 30.5 m (100 ft). Both detectors were calibrated with a standard 1 m (39 in.) cable.
As expected, increasing the cable length led to a drop in voltage for the GM Pancake detector. The reading never changed. A GM tube operates on a voltage plateau so the voltage drop was insufficient to move the instrument off the plateau. This result will follow with all GM tubes no matter what type.
The second detector, a NaI Scintillator type, does not operate on a plateau. Decreases in reading were noted even at 3.05 m (10 ft), and once the testing proceeded to 30.5 m (100 ft) values were less than 1/3 of the original value. Note that the opposite effect would be seen with an instrument calibrated with a longer cable that was then used with a shorter cable.
So can the length of a cable affect the readings from a calibrated instrument-detector pair? If the detector is a GM tube (and cable length reasonable), then it is not much of a concern. If it is another type of detector, however, a cable length change as little as 2 m (7 ft) can make a difference. To be safe it is best to recalibrate. It can also be a good idea to take readings, using a check source, with both cable lengths to verify if they have changed or not.
We appreciate the opportunity to be of further service to our customers, and those who may become customers. If you have a question you’d like to ask our training team, engineers, or sales staff, let us know and we will get you your answers – after we ask our own questions, of course.