CR:270 Series User Manual
Page 8
In the right position, marked >80 the instrument measures levels in the range 80 to
140dB.
Thus, the total measuring range is about 32 to 140dB(A) for an instrument fitted with
the MK:202 Microphone Capsule, and 27 to 140dB(A) for an instrument fitted with the
MK:224 Microphone Capsule. Please note that the lower figure will depend on the
microphone capsule in use. See section 10 for the instrument specifications.
Response
This switch defines the three response speeds "Slow", "Fast" and "Impulse". The IEC
651 standard now calls these 'S', 'F' and 'I' respectively, so that they are the same in
any language.
On the "I" response it should be noted that the rise time constant is 35mS but the
descent time constant is 1.5 seconds, thus it should only be used for single impulses.
When in the "Impulse Max" position there is no decay time ie when the instrument is
set to Impulse Time Weighting, and the Max Hold button is pressed.
Display
This is a three position switch. In the centre position, coded PEAK(C) the CR:27x will
measure C weighted peak Pressure, in the right hand position 'SPL' and in the left hand
position it will measure Leq.
The position of the switch does NOT affect the acquisition of data by the Leq circuitry,
the switch simply determines what is displayed on the meter. Thus the switch can be
moved between the three positions while a measurement is being made. Indeed, it is
quite normal to leave the switch in the SPL position to see how the noise is varying and
then switch to Leq at the end to see what the average level was or to PEAK for the peak
value since last reset.
Max Hold
When this switch is pushed in, the maximum SPL level as selected by the response
switch is held and automatically updated by a further incoming signal. This is
conventionally called 'Max Hold'. The hold functions has a decay time of better than 1
dB per minute.
The Max Hold function can be reset whilst viewing by pressing the RESET DISPLAY
button.
The Max Hold function is automatically reset when the Display switch is moved from
PEAK to Max Hold. Therefore if the user wishes to make simultaneous measurements
of Max Hold with Leq or Peak then the Max Hold reading should be recorded before the
display switch is moved to Peak or Leq.
Reset Display
This button, when pressed, resets the function that has been selected for display.
