Aerosol Photometer Response Time Constant

The Photometer RTC (Response Time Constant) is the time necessary to achieve a reading 63% of the final reading when sampling a stable aerosol source.

The values provided below are intended to help the photometer user calculate the response time constant applicable to their usage.

These calculations assume a 28.3 lpm (1 cfm) sample flow rate.

The sample train volume is assumed to be equivalent to a cylinder 13 feet (long) by 1/4 inch (ID) and is based on the characteristics of current industry photometer using a sample probe or 12 foot length of sample tubing.

Cylinder volume = πr²h

Where:
π=3.142
r=0.125 in
r²=0.015625 in²
h=156 in (13 feet total sample tube length)
7.659 in³ = 0.126 liters (total sample tube volume)
28.3 lpm sample rate (0.472 lps)
0.267 seconds per sample transit time to area of observation (0.126 liters ÷ 0.472 liter/sec)

% Leakage displays refresh rates:

ATI Photometer models:

• TDA-2E = 0.267 seconds + 3 seconds (assumed jeweled meter full-scale response time)
• TDA-2G = 0.60 second @ maximum display refresh rate
• ATI 2H = 0.60 seconds @ maximum display refresh rate
• ATI 2i = 0.267 seconds @ maximum display refresh rate (0.20 seconds)