Why Does My Formaldehyde Reading Keep Changing? Tenmars TM-802 Measurement Tips Malaysia
Why Does My Formaldehyde Reading Keep Changing? Tenmars TM-802 Measurement Tips Malaysia
When using a formaldehyde meter, users sometimes notice that the HCHO reading changes from one location to another—or even changes while the meter remains inside the same room.
Does this mean the meter is faulty?
Not necessarily.
Indoor formaldehyde concentration can be affected by measurement location, ventilation, environmental conditions, nearby materials, activities and measurement timing.
The Tenmars TM-802 Formaldehyde Meter measures formaldehyde (HCHO), temperature and relative humidity simultaneously and is designed for professional indoor air quality and occupational monitoring applications.
Understanding how to use the instrument correctly is important for obtaining meaningful measurements.
1. Don't Expect Every Room to Have the Same Reading
Indoor air is not necessarily uniform throughout an entire building.
For example, a newly renovated meeting room may contain:
New table
New cabinets
Wall panels
Flooring
Adhesives
New chairs
An older office beside it may contain very few new materials.
It is therefore possible to obtain different HCHO readings even though the rooms are located in the same building.
This is exactly why room-by-room investigation can be useful.
2. Measurement Location Matters
Consider these three measurement positions:
Position A: Directly beside a new cabinet
Position B: Centre of the room
Position C: Beside an open window
You should not automatically expect identical readings.
A measurement close to a suspected emission source can be useful when trying to locate potential sources.
However, it should not automatically be treated as representative of the entire room.
Likewise, a reading taken directly beside fresh outdoor airflow may not represent conditions where occupants normally work.
Always document where measurements are taken.
3. Ventilation Can Change the Reading
Ventilation is another important factor.
Opening doors or windows, operating mechanical ventilation or changing air-conditioning conditions may alter the indoor environment.
Therefore, when comparing measurements, record whether:
Windows were open or closed
Doors were open or closed
Mechanical ventilation was operating
Exhaust fans were operating
Air-conditioning was operating
The room had recently been ventilated
Without this information, two readings taken at different times may be difficult to compare meaningfully.
4. Temperature and Humidity Provide Useful Context
One advantage of the Tenmars TM-802 is that it measures:
HCHO + Temperature + Relative Humidity
simultaneously.
This allows the user to document the surrounding environmental conditions together with the formaldehyde reading.
If measurements are being compared across different days, rooms or operating conditions, recording temperature and humidity provides additional context for interpreting those measurements.
5. Allow the Sensor Time to Respond
Do not move the meter rapidly from room to room and assume that every number displayed immediately represents a stable measurement.
The TM-802 has an HCHO response specification of approximately:
T90: 30 seconds
This means the sensor requires time to respond to a change in concentration.
For professional inspection, users should allow appropriate response and stabilisation time rather than recording the first number that appears after moving to a new location.
6. Avoid Rapid "Walk-Through" Testing
A common mistake is:
Room A → 5 seconds → record
Room B → 5 seconds → record
Room C → 5 seconds → record
This may produce numbers, but it does not necessarily produce a good investigation.
A better approach is to establish a consistent procedure.
For example:
Select the measurement location.
Document room conditions.
Position the instrument appropriately.
Allow the instrument to respond.
Observe whether the reading is reasonably stable.
Record HCHO, temperature and humidity.
Move to the next location.
Repeat the same procedure.
Consistency makes comparisons more meaningful.
7. Measure Under Relevant Operating Conditions
Suppose a factory wants to investigate formaldehyde during production.
Testing only before workers arrive may not answer the real question.
Similarly, if a complaint occurs only after several hours of occupancy, a quick measurement early in the morning may not represent the conditions associated with the complaint.
Depending on the investigation, consider:
Normal production conditions
Actual ventilation conditions
Normal worker activity
Materials being used
Time since renovation
Occupancy
Process operating status
The measurement procedure should reflect the purpose of the investigation.
8. Compare the Same Location Under Different Conditions
One useful troubleshooting method is controlled comparison.
For example:
Test A — Before Ventilation
Measure HCHO, temperature and humidity at a defined location.
Test B — After Ventilation
Repeat the measurement at the same location after ventilation conditions have changed.
Or compare:
Production OFF vs Production ON
Cabinet closed vs cabinet area ventilated
Meeting room unoccupied vs occupied
Before renovation vs after renovation
Keeping the location consistent makes the comparison more useful.
9. Don't Confuse a Peak Reading With an 8-Hour Average
The TM-802 provides several ways of looking at measurement data, including:
Current HCHO reading
Maximum
Minimum
Average
TWA
STEL
These values do not represent the same thing.
A short peak should not automatically be interpreted as an 8-hour exposure condition.
Likewise, an 8-hour TWA value does not describe every short-term concentration change during the day.
Understanding the measurement mode is essential.
10. Understand TWA and STEL
The TM-802 provides:
TWA — 8 Hours
for longer-duration exposure monitoring, and:
STEL — 15 Minutes
for shorter-duration exposure monitoring.
These functions are particularly relevant to occupational safety applications.
For example, a Safety Officer investigating a manufacturing process may want to understand both:
What happens during a particular 15-minute activity?
and
What happens across the overall working shift?
This is very different from taking a single instantaneous measurement.
11. Use the Memory Function for Multiple Locations
The TM-802 can store up to 200 manual readings.
This is useful when conducting systematic inspections.
A building investigation could include:
Location 01 — Reception
Location 02 — Open office
Location 03 — Meeting room
Location 04 — New cabinet area
Location 05 — Manager's office
Location 06 — Storage room
Location 07 — Pantry
Location 08 — Outdoor/reference area where appropriate
Recording measurements systematically reduces the chance of forgetting which reading came from which location.
It is still good practice to maintain inspection notes describing the location and conditions.
12. Why Does the Reading Increase Near New Furniture?
If HCHO readings change near new furniture, cabinetry or engineered wood materials, the observation may justify further investigation.
Potential sources in indoor environments can include materials such as:
MDF
Particle board
Plywood
Engineered wood
Adhesives
Resins
Laminates
Interior furnishing materials
However, one elevated reading near an object should not automatically be treated as proof that a particular item is the only source.
A systematic investigation is preferable.
13. Why Does the Reading Drop After Opening Windows?
If ventilation introduces more outdoor air and removes indoor contaminants, indoor concentrations may change.
This can be useful information during an investigation.
However, remember to document the ventilation condition.
A measurement taken with all windows open should not be directly compared with another measurement taken in a closed room without noting that difference.
14. Don't Diagnose Indoor Air Quality From HCHO Alone
Formaldehyde is only one component of indoor air quality.
A formaldehyde meter should therefore not be treated as an instrument that completely determines whether overall indoor air quality is "good" or "bad."
Depending on the application, a broader IAQ investigation may consider additional parameters or contaminants.
The TM-802 should be used for what it is designed to do:
professional formaldehyde monitoring together with temperature and relative humidity measurement.
15. When Should You Investigate Further?
Further professional assessment may be appropriate when:
Unexpected readings persist
Workers repeatedly report concerns
Results vary significantly between areas
A suspected process appears associated with higher readings
Ventilation changes produce significant differences
Workplace exposure needs formal assessment
Regulatory or occupational exposure compliance needs to be determined
Portable measurement equipment provides valuable data, but regulatory conclusions should be based on the appropriate standards, procedures and professional assessment.
A Better Formaldehyde Measurement Checklist
Before accepting a reading, ask:
Where am I measuring?
Why did I select this location?
Is ventilation operating?
Is production operating?
Are doors or windows open?
What are the temperature and humidity?
Has the sensor had enough time to respond?
Am I looking at current, Max, Average, TWA or STEL data?
Have I recorded the measurement conditions?
Can I reproduce the measurement later?
Good measurement practice is just as important as choosing a good instrument.
Tenmars TM-802 Formaldehyde Meter Malaysia
The Tenmars TM-802 provides a professional combination of:
Electrochemical Formaldehyde Sensor
0.00–5.00 ppm HCHO Range
0.01 ppm Resolution
Temperature Measurement
Relative Humidity Measurement
8-Hour TWA
15-Minute STEL
Max / Min / Average
200 Manual Records
High / Low Alarm
For IAQ consultants, EHS departments, Safety Officers, facility managers and industrial users, understanding how to conduct consistent measurements can make the TM-802 much more valuable than simply switching it on and looking at a number.
Contact MTM Precision
MTM Precision Sdn. Bhd.
Showroom & Service Centre
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
🌐 Website: www.mtmpre.com.my
📧 Email: mtmpre@yahoo.com
📱 WhatsApp: +6016-660 7346
14 Aug 2026
