PPM v. PPM-M
Ppm-m is not a typo! “Parts per Million” (ppm) and “Parts Per Million Per Meter” (ppm-m) are not the same. This document will help you understand the difference and help you decide which one is right for you.
Open path systems use Path Integration. In an open path system, a light wave is projected from a transceiver through open air over a known distance and returned to the transceiver for analysis. Path Integration indicates a total mass of the molecule being monitored, which is a summation of the molecules measured through each meter of the transmitted path. This summation of molecules is dictated as Parts Per Million Per Meter, ppm-m for short. Path Averaging indicates the average concentration of the molecules for the path. This is dictated as Parts Per Million Meter Per Meter and written as (ppm*m)/m, and when simplified: ppm.
In Open Path Detection, ppm-m is the value collected by the transceiver and reported to the CCU. The CCU sends a scaled signal to the 4-20mA module, which forwards a 4-20mA signal to the customer. Below are several scenarios that will demonstrate the pros and cons of monitoring your location in ppm or ppm-m. It comes down to a difference between math and reality.
First, we consider a cloud that is 50m x 10m x 10m. This cloud contains 7 ppm HF and is the full length of the 50m path. This system will read 350 ppm-m because it sees 7 ppm for 50m, which is a total of gas being read. Dividing 350 ppm-m by the length of the path returns a value of 7 ppm. This method is commonly acceptable indoors where the atmosphere is usually well mixed.
Secondly, let us consider a 10m3 cloud containing 10 ppm HF being monitored on the same 50m path. The System will read 100 ppm-m because the laser passes thru 10 meters of 10 ppm HF. However, if we try to convert this into ppm by dividing the path length out of the units, we show the system to read an average of 2 ppm along each meter of the path. Here you can see a huge error in the ppm reading due to the assumption that the release has generated a cloud completely engulfing the entire path. The system reports 2 ppm, even though the cloud is 10 ppm in reality. This error will create IDLH atmospheres that are undetected in practice due to alarming parameters that have been averaged.
Finally, let’s look at two separate clouds that occur simultaneously. The first 10m3 cloud containing 5 ppm HF and the second 10m3 cloud containing 20 ppm HF. Again, we will use a 50m path to monitor them. We see one 50 ppm-m cloud and one 200 ppm-m cloud, but the system cannot differentiate between one, two, or any multiple of clouds. In this case, the unit reports a 250 ppm-m value. As before, if we divide the path distance from ppm-m, the unit reports 5 ppm.
As you have just seen, ppm doesn’t appear to be an issue because 5 ppm is below the OSHA IDLH level for HF of 30 ppm, but there is truly a 20 ppm cloud floating through the area, and nobody has been alerted. Alternatively, if there had been a 1m3 cloud of 250 ppm HF, the system would have also read 250 ppm-m, as shown below.
Alarming on PPM calculations can provide a false sense of security in certain environments because it will show the presence of the gas, but it will not accurately portray the magnitude of the danger 100% of the time.
On the other hand, ppm-m will provide you with a “worst case scenario” measurement and will ensure that you can react in such a manner to mitigate any IDLH scenarios earlier and more accurately. It is up to the customer to determine which alarming method is best for your site.