After having very little luck with a digital formaldehyde meter to test indoor air quality I decided it was time to switch to a more proven way to do DIY formaldehyde testing. I did a lot of research and decided to try colorimetric gas detector tubes to measure formaldehyde levels which don’t require sending samples off to a lab. The most common way for homeowners to test formaldehyde levels is using a sampler which is placed in a room for 24 hours then sealed up and mailed off to a lab for analysis. Some of the home test kits have a test tube and an air pump which you run for about 30 minutes, these are more expensive tests and also require sending away to a lab. If you only want one data point then mailing off an air sample to the lab is more cost effective. I wanted quick results from measurements so I could make a change and see the impact without having to wait for a lab so a formaldehyde colorimetric gas detector tube was the logical choice outside of an electronic meter.
To use colorimetric gas detector tubes you need a few things; a gas detector tube for the specific gas you are testing for, an air sampling pump that can accurately meter a specific amount of air through the tube, a thermometer to make corrections to the formaldehyde detector tube’s reading. I went through a lot of trial and error trying to find a good low cost air pump, partway into this project I found the Public Lab formaldehyde testing article which is a great resource. In the Public Lab article they used a modified aquarium pump for their air pump which works well and the article helped me wrap up the details to get this test setup up and running. I wanted to expand upon the Public Lab article and go into more detail for anyone interested in doing VOC and formaldehyde indoor air quality testing using colorimetric gas detector tubes.
The air sampling pump, flow meter, and other miscellaneous parts I used can be sourced for about $70 which is a one time purchase. The Kitagawa 710 formaldehyde test tubes are about $135 shipped for a box of 20, the GASTEC 91PL tubes are $83 shipped for a box of 10. The cost per test excluding the one time equipment purchase is $6.75 for the Kitagawa 710 tubes (but requires a larger purchase price up front) and $8.30 for the GASTEC 91PL tubes. I went with the Kitagawa 710 tubes as I wanted to do more than 10 tests.
1. Colorimetric Gas Detector Tubes
Below is a list of formaldehyde colorimetric gas detector tubes designed to work with an automatic air sampling pump which I will show how to build in this article, they will not work with a manual hand pump.
Sources for formaldehyde test tubes:
2. Air Sampling Pump
2.1 Manual Piston Hand Pump
All of the formaldehyde colorimetric gas detector tubes I could find sensitive enough to measure indoor formaldehyde levels are made for the automatic sampling pumps not manual hand pumps but I figured I would mention them for background and they can be used to measure very low levels of other gases using different tubes. Also, building your own automatic pump is actually cheaper than the hand pumps on the market.
Most manual hand pumps allow for a 50ml half stroke or 100 ml full stroke. These pumps are designed to work with tubes designated for use with hand pumps, many tubes required an automatic air pump which pumps a constant air volume through the tube for a specified time period which I detail a build of DIY auto air pump later in this article. There are a lot of manual hand pumps on the market but the RAE Systems pump seems to be the lowest cost, around $200 and the cost goes up from there.
2.2 DIY Air Sampling Pump
I could not find sources for analog low volume air sampler pumps on the market, all the air sampling pumps have seemed to switch to digital which is great if you are using it daily on job sites but are way too expensive for the average DIY’er trying to do this at home on a budget. The digital automatic air pumps on the market designed for use with colorimetric gas detector tubes are $1500+ high accuracy equipment meant for industry use with NIST traceability. If you are willing to piece a setup together you can get high accuracy at a fraction of the cost. I knew I could use an air flowmeter and a small pump and get similar results.
For the formaldehyde test tubes you will need to be able to pump 200-300 ml/min with the restriction of the test tube. There are a ton of options for vacuum pumps on the market that can be picked up from surplus or on Amazon. From low voltage, low volume vacuum pumps to aquarium air pumps that can be converted to vacuum pumps. I spent a lot of time and ended up buying a lot of different pumps as the first couple I purchased did not provide a steady enough airflow to accurately meter the air volume on the flow gauge. You will also need a flow meter with an air valve to dial in the flow to set flow rate required by the formaldehyde test tube.
2.2.1 Pump Option 1: Low Voltage Vacuum Pump
I purchased a bunch of low voltage DC vacuum pumps as I was benchingmarking them and trying to find one to give me a stable flow, the first couple I bought created too much of a “pulsed” flow to get an accurate reading on the flow meter but I found a few that worked very well. Most are in the 6-12VDC range with one I tested was 3VDC. From trial and error I found the larger and more powerful pumps cause too inconsistent of an air stream when you choke them down to such a low airflow rate. The pump I settled on is about $10 and has three diaphragms internally which also helps create a smoother airflow over a single piston or diaphragm pump.
Above is a picture of the pump I used, it can be found on Ebay by searching micro air sampling vacuum pump. The 6VDC and 12VDC version will both work, make sure it has two ports, the side port is the vacuum port. To power this pump you just need a simple 12VDC adapter which you can probably find around the house but if you need one they are cheap on Amazon.
2.2.2 Pump Option 2: Converting an Aquarium Air Pump
This idea came from the Public Lab article, they used a standard aquarium diaphragm air pump and reversed the flow direction of the pump. This produces a very smooth air stream and is very cost effective, if you use an aquarium pump with two diaphragms you can connect together and get a smooth 120Hz pulse rate on 60Hz mains power. I tried this aquarium air pump but I ended up using the low voltage pump in the above section as it worked just as well.
2.2.3 Measuring / Metering Air Volume, Flowmeter
I used a Dwyer series VF Flowmeter which has an accuracy of 5% of full scale. The smaller the total scale range the better resolution and the higher the accuracy, I used a 50-500 cc/min (ml/min) flowmeter.
You will need two 1/8″ NPT to 1/8″ barb fittings to connect the flow meter to some aquarium tubing which is 3/16″ O.D. 1/8″ I.D tubing.
- Dwyer VFA-21-SSV – 0.06-0.5 LPM with stainless steel valve, 5% of full scale accuracy (what I used)
- Dwyer VFA-21-BV – 0.06-0.5 LPM with brass valve, 5% of full scale accuracy
- Dwyer RMA-11-SSV – 30-240 cc/min (ml/min) with stainless steel valve, 4% of full scale accuracy (too small measuring range for Kitagawa formaldehyde test tubes)
- Dwyer RMA-12-SSV – 50-500 cc/min (ml/min) with stainless steel valve, 4% of full scale accuracy
- Dwyer RMA-12-BV – 50-500 cc/min (ml/min) with brass valve, 4% of full scale accuracy
The Dwyer RM Series is technically more accurate but is panel mounted which makes it harder to use freestanding, the VF series is rectangular block (see pic above) that can set freestanding on a flat surface. I originally purchased the Dwyer RM (RMA-12-SSV) series and beside the freestanding issue I was having problems getting a steady reading, the float ball would go randomly from a steady state reading to bouncing, it sounded like it was bouncing off the wall of the flow channel in the meter.
In-line Air Filter (optional):
This is really not necessary, I just did it to keep dirt out of the flowmeter and pump.
- HEPA Sanitary Air Filter 5/16″ barbed connections
- (2) Brass Hose Fitting, 5/16″ Barb x 1/8″ NPT Female – Used with fittings below to create an adapter from the small aquarium air tube to the larger air filter
- (2) 1/8″ NPT to 1/8″ barb
- Vinyl Tubing – 10 feet 5/16 ID – 7/16 OD
3. Performing a Formaldehyde Air Test, Collecting an Air Sample
3.1 Collecting an Air Sample
- Turn on the air pump built above and roughly dial in the flow meter to the flow rate specified by the air sampling tube you are using (will need to be readjusted once tube connected). 300ml/min for Kitagawa 710, 200ml/min for GASTEC 91PL.
- Have a countdown timer ready preset to 30 minutes, don’t start it yet.
- Break off the ends of the sampling tube with plyers, a tube breaker, or some tubes come with a tube breaking tool.
- Push the aquarium air tube over the end of the formaldehyde sampling tube making sure the the flow arrow on the sampling tube is pointing towards the air tube you are connecting the sampling tube too. You may need to use needle nose plyers before hand to expand the end of the air tubing.
- Dial in the flow meter to the flow rate specified by the colorimetric gas detector tube you are using. Kitagawa 710 is 300 mL/min and the GASTEC/Nextteq 91PL is 200mL/min.
- Start the countdown timer.
- Once timer is finished turn off pump and disconnect tube, read the tube immediately.
3.2 Reading and Calculating Formaldehyde Level
- Read the tube immediately after sampling
- The reading on the tube needs to be determined and it is not always very clear what the reading is, here is some examples to help clear this up.
Image Source: Ray Systems Gas Detection tubes and Sampling Handbook
The reading is taken as the furthest distance along the tube that the color change just becomes visible. If the leading edge is diagonal instead of perpendicular to the axis of the tube, use the average of the minimum and maximum values. The three tubes shown above are all read as 0.9
- Next this reading must be corrected for environmental conditions of the test which is different for each formaldehyde test tube brand and model
3.2.1 Kitagawa 710 Formaldehyde Detector Tube Measurement Corrections
The Kitagawa 710 Formaldehyde detection tube only requires temperature correction. A temperature coefficient is provided in the table that comes with the tube. Below is a scanned copy of the correction table.
To to adjust for ambient temperature first find the current ambient temperature in the above table, so say the ambient temperature at the time of the test was 24°C (75.2°F) you would start in the third row of the table labeled “20” and the six row labeled “4” and you would then trace back where those two intersect which would give you a value of “0.88” this is your coefficient for temperature correction which you need to multiply times the value read on your detection tube.
I scanned instruction manual for reference:
– Kitagawa formaldehyde detector tube 710 instruction manual scan page-1
– Kitagawa formaldehyde detector tube 710 instruction manual scan page-2
3.2.2 GASTEC/Nextteq – 91PL Formaldehyde Gas Sampling Tube Measurement Corrections
The Kitagawa 710 Formaldehyde detection tube only requires temperature correction. A temperature coefficient is provided in the table that comes with the tube (see below). A correction is needed if the humidity is outside of 20-90% relative humidity but no correction coefficient is given in the manual so make sure you take a reading when the humidity is in the given range. The final correction needed is for atmospheric pressure, most modern cell phones have a barometer sensor in them that can be read by a free barometer app.
To to adjust for ambient temperature first find the current ambient temperature in the above table, so say the ambient temperature at the time of the test was 24°C (75.2°F) you would start in the third row of the table labeled “20” and the six row labeled “4” and you would then trace back where those two intersect which would give you a value of “0.83” this is your coefficient for temperature correction which you need to multiply times the value read on your detection tube.
4. Formaldehyde Reference Levels
|0.0002–0.006 ppm||Rural and suburban outdoor air [source]|
|0.0015–0.047 ppm||Urban outdoor air [source]|
|0.04 ppm||Health Canada long-term exposure 8- hour average exposure limit [source]|
|0.10 ppm||Recommend upper limit for residences by ASHRAE, ANSI, EPA, NIOSH for short term exposure limit (STEL)
Level at which sensitive individuals have reported symptoms [source 1, source 2, source 3]
Health Canada short-term exposure 1-hour average limit [source]
|0.30 ppm||Threshold Limit Value (TLV) American Conference of Governmental Industrial Hygienists (ACGIH), 2005|
|0.40 ppm||Department of Housing and Urban Development, Office of Manufactured Housing Targeted maximum for ambient levels of formaldehyde in manufactured housing. Proposed, never enacted|
|0.50 ppm||OSHA 8-hour time-weighted average (TWA) action level workplace limit|
|0.75 ppm||OSHA 8-hour time-weighted average (TWA) permissible exposure limit (PEL)|
|0.80 ppm||Level at which most people first detect odor|
|0.90 ppm||AEGL-1, Acute Exposure Guideline Levels|
|1.0 ppm||ERPG-1, Emergency Response Planning Guidelines|
|2.0 ppm||OSHA 15-minute short term exposure limit (STEL)|
|10 ppm||ERPG-2, Emergency Response Planning Guidelines|
|14 ppm||AEGL-2, Acute Exposure Guideline Levels|
|40 ppm||ERPG-3, Emergency Response Planning Guidelines|
|56 ppm||PAC-3 (Protective Action Criteria), AEGL-3, 60 minute Acute Exposure Guideline Levels|
|~333 ppm||2-hour LC50 (Lethal Concentration) in mice
LC: A calculated concentration of a chemical in air to which exposure for a specific length of time is expected to cause death in 50% of a defined experimental animal population.
|~815 ppm||30-minute LC50 (Lethal Concentration) in rats|
|70,000 ppm||Lower Explosive Limit|
5. Lessons Learned
- Use a large amount of hose before the flowmeter, the air pump creates pressure pulse waves and the long hose section helps dampen these pulses from the pump which will give a much more stable reading at the flowmeter. I used about 20 feet in a loop.
- Buy the Dwyer VF series flowmeter over the RM series and save yourself the headache I had with it with bouncing readings and needing to mount it on a panel to keep it upright.
6. Helpful Resources
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