At some point you may wish to use a different detector with your survey meter than the detector the instrument was originally calibrated with. Unfortunately, this can cause issues because not all detectors operate at the same high voltage or use equivalent meter scales. Using a survey meter and a detector that have not been configured to be used together can lead to inaccurate radiation measurements.
High Voltage
Operating a survey meter and detector at the incorrect high voltage setting is a common problem many of our customers endure. This problem usually occurs when a different detector is substituted for the detector that the instrument was originally calibrated with. Because some detectors have different operating voltages, most instrument high voltage settings must be determined for that particular detector and survey meter.
The Ludlum Models 44-6, 44-7, and 44-9 GM (Geiger-Mueller) detectors operate between 900 - 1100 volts with 900 volts being the standard high voltage (HV) setting. Ludlum scintillation detector high voltages may vary operating below or above this standard setting. The LMI Calibration/Repair Department will normally set the HV at 900 volts on a survey meter that is sent in with a GM detector attached. If you try to use a scintillation detector with the instrument set at 900 volts, the scintillation detector will most likely not be set at this operating point of the plateau. That is why it is important for us to know what kind of detectors you will use with that instrument. We have many requests regarding the operation of both the GM and scintillation detectors with the same survey meter. Typically, the scintillation detector is modified to accommodate the 900 volts, which can be performed at the time of calibration. (Learn more about operating a scintillation detector at 900 volts.)
Meter Faces
Another problem our customers sometime experience is obtaining the proper readout when a different detector is used instead of the original detector and survey instrument. For example, you are using a detector and a survey meter that were calibrated together and you wanted to use another (second) detector. The original detector was calibrated on the mR/hr scale, but the other (second) detector's cpm/mR/hr will not have the same relationship as the original detector. Therefore, the meter scale and the other detector will not be compatible.
When an Ludlum Model 3 survey meter is ordered with a Model 44-9 pancake GM detector and the customer requests a dual scale mR/hr and cpm meter face, we would send a 202-608 meter face. However, if the customer ordered at Model 44-6, sidewall GM detector, we would send a 202-241. A list of meter scales and compatible detectors are listed on the individual instrument pages under the "Meter Faces" tab. If you have questions regarding a specific meter scale that is not listed on the website, please contact us.