The % Leakage display on a digital photometer may become erratic for no apparent reason when in the Clear sample position.
Small movement in the right-most decimal place is normal. An occasional 0.0001% or 0.0002% while using the Internal Reference or when the 100-µg/l level is chosen as 100% is typical. Reducing the Internal Reference setting or 100% concentration level cause a proportional increase in the observed instability. A 0.0001% movement at 100µg/l will become 0.0010% when 10µg/l is selected. For this reason ATI recommends setting the display for three (3) decimals when an upstream Internal Reference setting or aerosol challenge of less than 50-µg/l is used.
In the absence of particulate contamination within the detection chamber, instability may also be caused by electro-magnetic interference (EMI), radio frequency interference (RFI), or a combination of both.
EMI is caused by electrical and electronic devices that are operated by AC line voltage or batteries. RFI is caused by turning electrical and electronic appliances on and off. It may also be caused by certain circuits in appliances or electrical devices. For example, EMI is common in offices where a printer is positioned in close proximity to a CRT monitor. The EMI radiating from the printer affects the video scanning electron beam that is viewed on the monitor. This interference causes the images on the monitor to behave erratically. The problem can be solved by increasing the distance between the printer and monitor.
Likewise, EMI and RFI can cause the % Leakage display of a photometer to behave erratically. Most photometers contain a photomultiplier tube that is very sensitive to EMI and RFI. One method of reducing EMI and RFI interference in a photomultiplier tube is to shield it by covering it with a metal tube containing a small hole. This technique is employed in ATI photometers.
Sometimes an anomaly caused by EMI or RFI will resolve itself. If it does not, one of the easiest solutions to reduce EMI or RFI is to move the photometer a few feet in one direction or another. Often, this will reduce the interference to a point where it is insignificant. Another possible solution may be using a different electrical outlet. This may decrease or eliminate any interference entering through the AC line voltage connection.