Filter Leakage Testing Using Analog and Digital Photometers

This bulletin is intended to familiarize users of analog instruments with digital photometers by comparing the legacy TDA-2D or TDA-2E photometers with ATI’s first digital TDA-2G model.

Setting 100% Upstream Level

Analog Unit – To set the 100% baseline on an analog unit perform the following:

  1. Place the Range Selector switch in the 100% range.
  2. Turn the Selector Valve to Upstream and sample the aerosol air mixture challenge upstream of the filtration system under test.
  3. Adjust the Gain control for a 100% reading.

TDA-2G – To set the 100% baseline on a TDA-2G digital unit perform the following:

  1. Press <ENTER> key followed immediately by <100>. The red LED over the 100 key is illuminated while the unit is setting the 100%, and extinguishes when the process is completed.
  2. Turn the Selector Valve to Upstream and sample the aerosol air mixture challenge upstream of the filtration system under test.
  3. Press <ENTER> again. Since the unit is auto ranging there is no range switch to place in the 100% position.  The red LED over the 0 key illuminates and flashes prompting the user to straylight or 0% the unit by pressing the <ENTER> key.

Establishing 0% Baseline (Straylight)

Analog Unit – To set the 0% baseline on an analog unit perform the following:

  1. Turn the Selector Valve to Clear. This allows particle free air to be drawn through the detector from the internal HEPA “reference” filter.
  2. Place the Range Selector switch in the 0.1% or 0.01% range.
  3. Adjust the Straylight control adjusted for a 0% reading.

TDA-2G – To set the 0% baseline on a TDA-2G digital unit perform the following:

  1. Place the Selector Valve in the Clear position. This allows particle free air to be drawn through the detector from the internal HEPA “reference” filter.
  2. Press <ENTER>. The digital unit will automatically zero (straylight) itself, beep, and extinguish the red 0 LED to indicate straylight is completed. The display reads either 0.000% or 0.0000%, depending on the number of decimal places selected.

The 100% and 0% baselines have been established and the photometer is ready to begin testing.

Filter Leakage Testing

Analog Unit – Leakage testing using an analog photometer:

  1. Place the Selector Valve in the Downstream position. A sample, usually through the scanning probe, is drawn through the photometer detection optics.
  2. Begin scanning the downstream filter surface in over-lapping strokes. If the reading exceeds the scale range, the Range Switch is changed to a higher, less sensitive range, to allow the indicator to remain on-scale.
  3. Identify and repair any leakage readings exceeding a pre-determined leakage threshold (i.e. 0.01%).

TDA-2G – Leakage testing using a digital photometer.

  1. Place the Selector Valve in the Downstream position. A sample, usually through the scanning probe, is drawn through the photometer detection optics.
  2. Begin scanning the downstream filter surface in over-lapping strokes.
  3. Identify and repair any leakage readings exceeding a pre-determined leakage threshold (i.e. 0.01%).

Concentration Measurement

Analog Unit – Measuring concentration using an analog photometer.

  1. Turn the Selector Valve to Clear. This allows particle free air to be drawn through the detector from the internal HEPA “reference” filter.
  2. Place the Range Selector switch in the 10% range.
  3. Activate the Internal Reference light source
  4. Adjust the Gain control adjusted for a 10% reading (full scale).
  5. Deactivate the Internal Reference light source
  6. Place the Range Selector switch in the 0.1% or 0.01% range.
  7. Adjust the Straylight control adjusted for a 0% reading.
  8. Turn the Selector Valve to Upstream and sample the aerosol air mixture challenge upstream of the filtration system under test.
  9. The established gain level (or sensitivity level) will result in a 1:1 ratio between ug/l and %. A sample of 100 micrograms per liter of DOP (or PAO, if the unit was calibrated for PAO) causes a 100% response and a 65 ug/l sample causes a 65% response.

TDA-2G – Measuring concentration using a digital photometer.

This is where digital units are more versatile than analog.  TDA-2G digital units provide three Internal Reference settings: P1 for DOP, P2 for PAO, and P3 a factor for alternative liquids.

If mineral oil is to be used and the systems under test are normally tested with DOP, the factor for mineral oil (0.90) would be used for P3.

To set the sensitivity to 100 micrograms per liter on the digital TDA-2G:

  1. Press <ENTER> followed by Ref, then <ENTER> again.
  2. P1 flashes on the display if the unit was set for DOP, P2 if set for PAO, and P3 if set for an alternate liquid. A number is displayed (user selectable). Set to 100 (default).
  3. Press <ENTER>. The unit sets the detector sensitivity level for 100 micrograms per liter of PAO, DOP, or the entered P3 factor.  The red 0 LED begins blinking when complete
  4. Press <ENTER>. The digital unit will automatically zero (straylight) itself, beep, and the red 0 LED will stop blinking will to indicate straylight is completed. The display reads either 0.000% or 0.0000%, depending on the number of decimal places selected.
  5. Place the Selector Valve in the Downstream position. A sample, usually through the scanning probe, is drawn through the photometer detection optics.
  6. Begin scanning the downstream filter surface in over-lapping strokes.
  7. Identify and repair any leakage readings exceeding a pre-determined leakage threshold (i.e. 0.01%).

100% Adjustment Using Internal Reference

Analog Unit – Setting 100% using calculated upstream aerosol concentration.

  1. Calculated upstream challenge concentration.
    1. Total air flow present in system under test. e.  540 cfm
    2. Laskin nozzles in use operating at 20 psi (DOP) or 23 psi (PAO-4).  e. 1 nozzle
    3. 13,500 x 1 nozzle ÷ 540 cfm = 25 ug/l
  2. Calculate the required Internal Reference setting using the concentration calculated in the preceding step.
    1. 100 (desired 100% response) ÷ 25 ug/l (calculated concentration) x 10% (Int Ref) = 40%.
  3. Place the Range Switch in the 100% position.
  4. Activate the Internal Reference light source.
  5. Adjust the Gain control adjusted for a 40% reading.
  6. Deactivate the Internal Reference light source
  7. Place the Range Selector switch in the 0.1% or 0.01% range.
  8. Adjust the Straylight control adjusted for a 0% reading.
  9. Place the Selector Valve in the Downstream position. A sample, usually through the scanning probe, is drawn through the photometer detection optics.
  10. Begin scanning the downstream filter surface in over-lapping strokes.
  11. Identify and repair any leakage readings exceeding a pre-determined leakage threshold (i.e. 0.01%).

This was one of the most complicated operations using the analog photometer and it is sometimes difficult to learn. Essentially the total flow of the cabinet would be measured; for example, assume a total flow of 540 CFM. One Laskin nozzle at 20 psig using DOP provides 100 micrograms per liter into 135 CFM, that 135 can be divided by the new CFM (540), assuming one nozzle, resulting in a calculation of 25 micrograms per liter. Since the concentration is now 1/4 of 100 micrograms per liter the Internal Reference setting would be increased from the normal 10% to 40% (4 times the gain) to have the correct sensitivity for a 100% response using 25 micrograms per liter.

TDA-2G – Setting 100% using calculated upstream aerosol concentration.

  1. Press the <ENTER> key
  2. Press the REF key.
  3. Using the arrow keys, scroll up or down until the calculated concentration is displayed (25 micrograms per liter).
  4. Press <ENTER>. The unit adjusts the sensitivity level of the photometer to display 100% when 25 micrograms per liter of concentration is sampled.
  5. Press <ENTER> to 0
  6. Place the Selector Valve in the Downstream position. A sample, usually through the scanning probe, is drawn through the photometer detection optics.
  7. Begin scanning the downstream filter surface in over-lapping strokes.
  8. Identify and repair any leakage readings exceeding a pre-determined leakage threshold (i.e. 0.01%).