PIDs As HazMat Response Tools

Addressing PID Sensor Moisture Sensitivity 

Technical Note TN-163 

rev 5 ab.05-07 

The RAE Systems’ photoionization detectors (PIDs) 

were designed to have little or no response to 

moisture. However, when the sensor is dirty, bent 

out of shape, or corroded, RAE Systems’ monitors 

may show a strong response to humidity at or above 

about 85% RH (relative humidity). 

Dirty Sensor Causes Current Leakage 

A dirty sensor is the number-one reason for humidity 

response. The sensor has two electrodes: a bias 

electrode and a sensing electrode. One has a greater 

positive charge than the other. As the UV light 

breaks down chemicals into positive ions and 

negative electrons, they migrate to the oppositely 

charged electrodes. With clean, dry air and sensor 

components, no current can leak across the air space 

between the two electrodes. 

Clean Sensor 

Bias Electrode 

Sensing Electrode 

No dirt build-up to 

foster a decrease in 

airspace resistance 

However, even microscopic dirt accumulations 

on the 

electrodes and Teflon parts can promote leakage, 

especially when the relative humidity is greater than 

85%. This is because water adheres more readily 

onto dust particles than onto a clean Teflon or glass 

surface. A sensor may appear to be visually clean, 

but actually can be dirty enough to cause current 

(electron) leakage. This current leakage is 

interpreted by the PID as a drifting reading of 

typically more than 10, and as much as hundreds of 

ppm, with no VOC present (over 1 ppm for a 

ppbRAE). 

Dirty Sensor 

Bias Electrode 

Sensing Electrode 

Dirt build-up absorbs water and 

breaks down airspace resistance, 

leading to sensor “ leakage” or 

moisture response 

At very high levels of leakage due to a dirty sensor, 

the MiniRAE 2000 displays a “Clear Sensor, Quit?” 

message on the screen. Pressing the “Y” key tells the 

MiniRAE 2000 to continue. The “Clear Sensor, 

Quit?” message can also be displayed if the 

MiniRAE 2000 pump has been shut off with a high 

level of sample left in the instrument. 

Prevention is the Best Cure 

Preventing dirt and dust from entering the PID sensor 

is the best cure for moisture response. Make sure that 

all of the filters in the instrument are clean and 

effective. In dirty environments, it may be necessary 

to add an additional filter. However, under normal 

circumstances there should be no response to 

humidity with just the standard sintered metal and 

external filter installed. A Humidity Filtering Type II 

Tube can be used to dry the sample stream and 

measure VOCs at high humidity for about a half hour 

(see Technical Note TN-178). Many compounds can 

be measured this way, especially those commonly 

encountered in soil remediation such as gasoline and 

trichloroethylene. Caution needs to be taken because 

there are some 

very heavy compounds that can be 

lost on the tube 

Quick Humidity Response Tests 

Try exhaling gently into a MiniRAE 2000, ppbRAE, 

or ppbRAE plus for 10 to 15 seconds or cupping your 

hand over the inlet probe; moisture from your hand 

provides a fairly continuous high humidity stream. 

Be sure not to block the air flow. This test can also 

be preformed on the UltraRAE, MultiRAE and 

EntryRAE in a similar manner. 

1 

RAE Systems Inc. 

3775 N. First St., San Jose, CA 95134-1708 USA 

Phone: +1.888.723.8823 

Email: raesales@raesystems.com 

Web Site: www.raesystems.com


rev 5 ab.05-07 

inside the probe, including a green dust filter and a 

white 

“C” filter. Both of these should be removed. 

The instrument should show little or no response 

from these tests. If the MiniRAE 2000 shows more 

than 5 ppm isobutylene units or the ppbRAE shows 

more than 500 ppb increase in reading, then 

something is wrong. The design of the instruments 

makes it quick and easy to remove and clean the 

probe, lamp, and sensor to address this problem. 

Focus on the following areas: 

1. Replace filters 

2. Clean the probe, PID lamp, and sensor 

3. Inspect the sensor for damage or corrosio n. 

4. Clean the sensor housing. 

5. Clean the sensor using an ultrasonic bath. 

MiniRAE 2000, ppbRAE Plus, and UltraRAE 

When testing the UltraRAE monitor, make sure that the 

tube of interest is installed before proceeding with the 

test. If you plan to test for Benzene, install the Benzene 

tube and use the same moisture test discussed for the 

MiniRAE 2000. If the readings increase more than 2 

ppm Benzene units, then there is a problem with dirt in 

the monitor. 

Replace Filter 

The MiniRAE 2000, ppbRAE, ppbRAE plus, and 

UltraRAE all use two filters, an external filter and a 

sintered metal filter. The external filter (PN 002- 

3022-005 for a pack of 5, 002-3022-010 for a pack of 

10, and PN 002-3022-100 for a pack of 100) is 

attached to the end of the probe and is white. If it 

appears to have any discoloration, then it is time to 

replace the filter. If the instrument goes into pump 

alarm with the filter attached and is able to run 

without any problems when not attached, it is time to 

replace the filter. When the instrument is responding 

to moisture, it is time to replace the filter. The metal 

sintered filter (PN 490-0047-005 for a pack of 5) 

should be replaced when it is dirty and when the 

instrument responds to moisture. The MiniRAE 

2000 and ppbRAE instruments have other filters 

Clean the Probe 

Unscrew the lamp housing cap; inspect the sintered 

metal filter at the bottom of the probe. If it appears to 

be dirty, it needs to be replaced. A dirty internal filter 

indicates that the probe has dirt inside and requires 

cleaning. Cleaning the probe depends on the kind of 

exposure or dirt that has contaminated the probe. If it is 

a chemical that might affect the reading of the PID, use 

methanol to clean the probe. If it is just dirt or dust that 

has been sucked into the instrument, blow clean, dry air 

through the probe to clear the internal tubing. 

Clean the PID Lamp 

Access to both the lamp and the sensor of a MiniRAE 

2000 and ppbRAE plus requires no tools. To remove 

the PID sensor, grasp the top lip of the sensor and pull 

straight out, to avoid bending the electrical pins on the 

sensor. If the sensor does not pull out immediately, a 

gentle rocking motion should help loosen the sensor. 

Also try rotating your grip as you pull the sensor. Once 

the PID sensor is removed, set it aside to clean or to 

inspect for damage or corrosion. 

The PID lamp has a white O-ring that keeps it from 

sitting too low in the lamp housing. Before reaching for 

the lamp, get out the lamp cleaning kit (PN 081-0002- 

000), which includes methanol, finger cots, cotton 

swabs, and lens tissue. Use finger cots on the fingers 

that will handle the lamp. 

CAUTION: Never touch the window surface with 

anything that might scratch, including your fingers, or 

any liquid that might leave a film. 

Carefully remove the lamp by grabbing onto the white 

O-ring, and set the instrument aside. Dip a cotton swab 

into the methanol and wipe the window surface of the 

lamp. The sides of the lamp can be cleaned if it is 

obviously dirty, but the flat-surface window of the lamp 

is the only part of the PID lamp that requires cleaning. 

Once the lamp is cleaned, use a piece of the lens tissue 

to dry and polish the lamp before dropping it back into 

the instrument. If there is visible dirt, clean the lamp 

housing prior to placing the lamp back into the 

instrument. Do not force the lamp all the way down to 

the bottom of the lamp housing. Allow the PID sensor 

to push the lamp the rest of the way into place. 

2 



Phone: +1.888.723.8823 




rev 5 ab.05-07 

To access the UltraRAE lamp for cleaning, you will 

need to first remove any RAE-SEP tube from the probe. 

Then unscrew the four screws holding the bar code 

reader cap to the instrument base. Lift the Bar Code 

Reader cap straight off of the instrument base until the 

PCB (printed circuit board) clears. Set it aside and 

make sure that the screws do not fall out and disappear. 

Take the silver sensor adapter off of the PID senso r. 

Underneath is a rubber tube connector and a silver 

sintered filter. Remove the rubber tube connector with 

filter. If it appears to be dirty, it needs to be replaced. A 

dirty internal filter indicates that the probe has dirt 

inside and requires cleaning. Cleaning the probe 

depends on the kind of exposure or dirt that has 

contaminated the probe. If it is a chemical that might 

affect the reading of the PID, use methanol to clean the 

probe. If it is just dirt or dust that has been sucked into 

the instrument, blow clean, dry air through the probe to 

clear the internal tubing. 

bent or the Teflon mask is warped, the sensor needs 

replacement. Note: A warped Teflon mask can be a 

sign that the sensor has been exposed to too much heat. 

Straight Electrodes 

with mask removed 

for clarity 

Good Sensor 

(partly disassembled) 

Warped 

Teflon 

Mask 

Damaged Sensor 

Bent 

Electrode 

If the sensing electrode is exposed to UV light from the 

lamp, it gives an undesired positive response to the 

light. If the fingers look aligned but are not in the same 

plane (some fingers are bent towards or away from the 

lamp), then the sensor should be replaced. The fingers 

should be close to, but not touching, the Teflon mask. 

In some cases it is possible to carefully bend the 

electrodes back into the correct position if the damage is 

very minor. 

Clean the Sensor Housing 

If the PID sensor is not damaged, thoroughly wipe clean 

the sensor housing area using a cotton swab dipped in 

lamp-cleaning methanol. Let the sensor housing area 

dry completely. Reassemble the PID, and perform a 

humidity response test. If it shows no response, 

calibrate the instrument and return it to service. 

Follow the directions for the MiniRAE 2000 and 

ppbRAE plus for PID sensor and lamp removal. The 

UltraRAE PID sensor and lamp are the same as those in 

the MiniRAE 2000. 

Sensor 

Housing 

Inspect the Sensor for Damage or Corrosion 

If the sensor appears corroded, it should be replaced. If 

the sensor is damaged or even slightly bent, it can show 

increased response to moisture. The bias and sensing 

electrodes are two sets of “fingers” in the MiniRAE 

2000 sensor. These fingers should be straight and 

parallel. The bias electrode is easy to see when the 

bottom of the sensor is inspected. However, the sensing 

electrode should be blocked from sight (and the UV 

light from the PID lamp) by a Teflon mask. If the 

sensing electrode can be seen because the fingers are 

Cleaning the PID Sensor 

Cleaning the lamp and sensor surfaces using the lamp 

cleaning kit with cotton swabs is not good enough to 

remove humidity response. In fact, cotton swabs can 

leave behind fibers that continue to cause moisture 

issues after cleaning. An ultrasonic cleaning bath 

MUST be used. Ultrasonic cleaners are available 

from laboratory suppliers. Alternatively, a jewelry 

3 



Phone: +1.888.723.8823 




rev 5 ab.05-07 

cleaner from a department store can be used. To 

clean the sensor, remove it from the instrument 

and 

immerse it in lamp-cleaning solution (anhydrous 

methanol) in a small beaker or baby food jar. Fill the 

ultrasonic cleaning bath with water. Place the beaker 

containing the sensor into the water bath of the 

ultrasonic cleaner. The ultrasonic waves pass through 

the water and container and assist in cleaning the 

sensor. The lamp-cleaning solution in the jar can be 

reused a few times until it becomes dirty. Remove the 

through the tube easily. Some heavy vapors such as 

diesel fuel or polar compounds such as acetone and 

alcohols require extended sampling times. See 

Technical Note TN-178 for more details and cautions 

on use. (Note that Type I Humidity Filtering tubes 

cannot be used during measurements.) 

Flex-I-Probe 

Tube Tip Breaker 

(p/n 021-2400-100) 

Tube Adapter 

(p/n 025-3002-000) 

Humidity Filtering II Tube 

(10-pack, p/n 025-2002-010) 

Water Bath 

Methanol 

in Jar 

Ultrasonic Cleaner 

sensor and allow it to dry for at least a few hours, and 

preferably overnight (warm but not hot air will speed 

the drying process). 

Caution! Do not overheat the sensor, and never 

disassemble the sensor! 

It is very important to blow or shake out any residual 

lamp-cleaning solution from the sensor before letting it 

air dry. Otherwise, some of the dirt that was just 

extracted into the cleaning solvent could be deposited 

back onto the sensor components as they dry. 

Reinstall the clean, dry sensor into the MiniRAE 2000. 

Exhale into the instrument and check the response. If it 

shows no response to the moisture in your breath, 

calibrate the instrument and return it to service. 

Using Humidity Filtering II tubes 

Humidity Filtering Tubes (Type II) can be used to dry 

the sample gas and measure many VOCs for about 15 

minutes to 1 hour. Their use prevents moisture effects, 

even on a dirty sensor. The tubes (p/n 025-2002-010) 

are inserted into an adapter (PN 025-3002-000) attached 

to a Flex-I-Probe (PN 021-2400-000). Many 

compounds such as trichloroethylene and gasoline pass 

MultiRAE Plus and EntryRAE 

While the MultiRAE and EntryRAE do not normally 

have as many issues with dirt in PID sensor, 

contamination is possible. The moisture test is done 

by cupping your hand over the end or breathing 

gently into the end, being careful not to block the air 

flow. If the VOC reading increases over 5 ppm 

isobutylene, there is a problem with dirt. 

Filters 

The MultiRAE uses a smaller external filter (PN 008- 

3022-001 for a pack of 3, PN 008-3022-010 for a 

pack of 10, PN 008-3022-100 for a pack of 100, and 

PN 008-3014-000 for a single external filter with 

adapter for the silver MultiRAE). This white filter 

should be replaced when it is dirty and when the 

instrument responds to moisture. If the instrument 

goes into pump alarm with the filter attached and is 

able to run without any problems when it is not 

attached, it is time to replace the filter. 

The EntryRAE uses a small external filter (PN 490- 

0122-000), which should be replaced when it appears 

dirty or when the instrument goes into pump alarm 

with the filter attached and clears the pump alarm 

when the filter is removed. 

MultiRAE PLUS 

There are two versions of the MultiRAE Plus, silver 

housing and black housing. There are small differences 

when cleaning them. If there is a response to moisture, 

dirt has gotten to the lamp. Due to the gas flow within 

the instrument, if dirt has gotten to the lamp, it has 

probably gotten into the filter, pump, tubing (silver 

MultiRAE only) and the gas plate. 

4 



Phone: +1.888.723.8823 




rev 5 ab.05-07 

Pump 

Start by replacing the filter, and then work on cleaning 

the internal parts of the instrument. Loosen the two 

screws on the back of the instrument and remove the 

top housing cover. Unplug the battery. If you have a 

black-box MultiRAE, you have access to the gas plate, 

pump and sensors now. If you have a silver-box 

MultiRAE, you still need to unscrew the four screws 

holding the gas plate to the instrument. After loosening 

the four screws, pull the gas plate off of the silver stand- 

plugged into the 

offs. Unplug the tubing from the pump and the wiring 

PCB. 

To clean the tubing, remove both pieces of tubing 

from 

the instrument and the gas plate and immerse it in 

methanol. Agitate it by hand or in an ultrasonic bath. 

Remove it and let it dry overnight. The black 

MultiRAE does not have tubing, but there are O-rings 

in the adapter attached to the tubing. Make sure there is 

no dirt in the adapter or O-rings by removing the pump 

from the top housing, and unplug the pump from the 

adapter. Remove all eight O-rings and inspect the 

adapter and O-rings. Clean the adapter and O-rings 

with methanol. Dry the O-rings with a piece of the lens 

tissue that comes with lamp-cleaning kit. Clean the 

pump by removing the four screws that hold the head to 

the diaphragm. Use a cotton swab dipped in methanol 

to clean all surfaces, including the diaphragm. 

Reassemble the pump and set it aside to dry for 5 

minutes. Now that everything is removed from the gas 

plate, clean the gas plate by pushing air or methanol 

through the airflow path. Allow it to dry overnight. 

The PID sensor 

The PID sensor is covered by a shielding cover that 

needs to be removed. This silver cover can be dipped in 

methanol to be cleaned. To clean the sensor, remove it 

from the instrument and immerse it in lamp-cleaning 

solution (anhydrous methanol) in a small beaker or 

baby food jar. 

Agitate it by hand or in an ultrasonic cleaner. Fill the 

ultrasonic cleaning bath with water. Place the beaker 

containing the sensor into the water bath of the 

ultrasonic cleaner. The ultrasonic waves pass through 

the water and container and assist in cleaning the 

sensor. The lamp-cleaning solution in the jar can be 

reused a few times until it becomes dirty. Remove 

the PID sensor and let it dry for at least a few hours, 

and preferably overnight (warm but not hot air will 

speed the drying process). 

Caution! Do not overheat the sensor! 

It is very important to blow or shake out any residual 

lamp-cleaning solution from the sensor before letting it 

air dry. Otherwise, some of the dirt that was just 

extracted into the cleaning solvent could be deposited 

back onto the sensor components as they dry. Reinstall 

the clean, dry sensor into the MultiRAE. Exhale into 

the instrument and check the response. If it shows no 

response to the moisture in your breath, calibrate the 

instrument and return it to service. 

EntryRAE 

If moisture problems still persist, contact the RAE 

Systems Service Department at 888-723-4800. 

5 



Phone: +1.888.723.8823

PIDs As HazMat Response Tools

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?