PIDs As HazMat Response Tools

rae.be

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


Technical Note TN-163

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

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


Technical Note TN-163

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

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


Technical Note TN-163

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

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


Technical Note TN-163

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

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

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