E2648-PID User Manual
Annex 3 Properties of VOC: ionization energy, correction factor
Sensors used in E2648-PID are calibrated using iosbutylene, but the PID is a broadband VOC detector, with a sensitivity that differs for each VOC. If you know what VOC you are measuring, then the table below will allow you to calculate the
concentration for your specific VOC.
NB
These are approximate values, so for best accuracy you should calibrate with the relevant VOC.
Only the most common substances are listed in the table, if you can’t find your compound of concern, please contact us.
The table includes following columns:
1 The most common
name
for the VOC or or other substance. I
2.
CAS No
. You can find the VOC using the CAS No.
3.
Brutto-formula
.
4.
Ionization energy (IE)
, eV
5
Relative Response/ Correction Factor (CF)
Also called the
Response Factor (RF)
. Multiply the displayed concentration by the Relative Response/ CF/ RF to calculate the actual concentration of the VOC.
6
Minimum Detection Level (MDL)
Also called
Minimum Detectable Quantity (MDQ)
. Typical lowest concentration that can be detected. The sensor used in E2648-PID-40 has greater sensitivity than that in E2648-PID-200, so the MDL for the
E2648-PID-40 will be much less than the MDL for the E2648-PID-200.
The Relative Response/ CF/ RF is measured in dry air; high humidity will reduce this factor by 30% to 50%, so the CF/RF should be increased in high humidities.
Relative sensitivity is the inverse of the correction factor, specifying the percent response of the VOC, relative to isobutylene. If less than 100%, then the VOC is less responsive than isobutylene; if the relative sensitivity is greater than 100%, then the
VOC is more responsive than isobutylene. Relative sensitivity (%) is specified the same way as cross-sensitivity for toxic gas sensors.
VOC response
The PID can not measure all VOCs or gases: two types of VOCs are not measured:
NR:
No response. The lamp does not ionise the VOC and the VOC cannot be measured.
NV
: The vapour pressure of the VOC at 20°C is less than a few ppm, so this Semi-Volatile Organic Compound (SVOC) cannot be measured.
NA
: Not available
Occasionally you will be measuring a mixture of VOCs. If the total concentration is within the linear range of your PID, then it is reasonable to assume that the concentrations are additive without interference between the different VOCs.
Remember that if you are measuring a combination of VOCs, then accurate measurement of one of these VOCs will be difficult; without careful data analysis, you will get only a CF averaged measurement. Be cautious when reporting actual VOC
concentration if you know that there may be several VOCs present.
Balance gas
The relative response is measured in laboratory air, with 20.9% oxygen, balance nitrogen. Some gases absorb UV light without causing any PID response (e.g. methane, ethane). In ambient atmospheres where these gases are present, the measured
concentration of target gas will be less than is actually present. Methane absorbs UV strongly, so for accurate measurements in methane containing atmospheres, calibrate with a calibration gas containing the expected methane concentration. 50% LEL
methane reduces the reading by up to 50%. Gases such as nitrogen and helium do not absorb UV and do not affect the relative response.
The correction factor for a gas mix containing PID detectable gases A, B, C… with response factors RF(A), RF(B), RF(C), in relative proportions a: b: c… is given by:
CF(mix) = 1 / [(a/CF(A) + b/CF(B) + c/CF(C)…]
Accuracy of the Table
This table is for indication only. Table accuracy is 1 to 2 digits only, so when calculating concentration for a specific VOC, specify to 1 or 2 digits only.
Index
Chemical name
Alternative name
Formula
CAS no.
IE, eV
Response Factor (RF)
10.6 eV
E2648-PID-40
(ppb)
E2648-PID-200
(ppb)
1 Acetaldehyde
C2H4O
75-07-0
10.23
5.5
25
480
2 Acetamide
C2H5NO
60-35-5
9.69
2
3 Acetic acid
C2H4O2
64-19-7
10.66
28
180
3615
4 Acetic anhydride
C4H6O3
108-24-7
10.14
4
20
400
5 Acetone
2-propanone, dimethyl ketone
C3H6O
67-64-1
9.69
1.17
5
70
6 Acetonitrile
CH3CN
75-05-8
12.2
NR
7 Acetophenone
1-phenylethan-1-one, methyl phenyl ketone
C8H8O
98-86-2
9.29
0.6
9 Acetylene
ethyne
C2H2
74-86-2
11.4
NR
10 Acrolein
propenal
C3H4O
107-02-8
10.22
3.2
20
400
11 Acrylic Acid
propenoic acid
C3H4O2
79-10-7
10.6
21
15
275
