LMI instruments with counts per minute (CPM) meter faces are usually calibrated with a pulse generator (pulser). The pulser should have a negative pulse, a 0.5 µs lead, a 1.5 µs width and a 5 µs tail. The pulse size is similar to the size of the pulse the instrument is capable of recognizing. In our calibration department at LMI, we use the Ludlum Model 500 Series pulsers to calibrate instruments which require CPM readouts. The Model 500 Series supply the correct pulse size and a digital readout in CPM with continuous adjustment from 1 to 9,000,000 CPM, along with a high voltage readout.

To begin calibration, connect the instrument to the pulser and determine the sensitivity of the instrument. For example, the Ludlum Model 3 and 14C survey meters with fixed thresholds have sensitivities of 30 ± 10 mV and 40 ± 10 mV respectively. Simply multiply this sensitivity reading by 2 and set the pulser to that reading. Next, check the HV (high voltage) of your instrument. This can be achieved on the Model 500 Series by looking at the high voltage readout. The HV on our portable survey meters may vary depending on the probe used with that particular instrument. LMI instruments used with GM detectors should be operated at 900 V. The HV on instruments using scintillation detectors is determined by running a plateau.

To continue to calibrate the instrument, follow the ANSI 32-1978 guidelines, which states the instrument should be calibrated to 80% and 20% of the meter scale. Determine the 80% scale of your instrument and set pulser to same reading. If your instrument does not read the same as the reading on the pulser, adjust the calibration potentiometer so your instrument reads the same as the pulser. Below is an example of the necessary steps to calibrate an instrument.

For example, we will use a LMI Model 3 Survey Meter with a 0-5k CPM meter face. 80% scale = 4k, 20% scale = 1k. The ranges on the Model 3 include x100, x10, x1, x0.1.

To calibrate this instrument, set the pulser to 400k CPM and calibrate the x100 range to equal this amount. To check the 20% scale, dial the pulser down to 100k CPM and make sure the instrument is within ±10%. If your instrument does not comply within these figures, consult your instrument manual for meter linearity. Continue to decrease both pulser and instrument down to the next decade range and follow the same procedure as above until you have calibrated each range.

Training Course

For a more in-depth overview of instrument calibration, LMI offers a two day training course that involves calibration, repair, and maintenance on Ludlum manufactured instruments. Click here to learn more.