Why Do Two PM2.5 Monitors Show Different Readings During Johor Haze Malaysia
Placing two PM2.5 monitors in the same room does not always produce identical readings.
A small difference may be normal because the instruments can use different sensors, airflow designs, response times and calculation methods. The air inside a room is also constantly moving, so each monitor may sample a slightly different concentration of particles.
Before deciding that one instrument is faulty, users in Johor should perform a controlled side-by-side comparison.
## Common Reasons for Different PM2.5 Readings
### Different Measurement Positions
One monitor may be closer to:
- An open window
- A door
- An air purifier
- An air-conditioning outlet
- A kitchen
- A person walking past
- A dusty surface
Even a short distance can affect the reading when air is moving unevenly around the room.
### Different Response Times
One instrument may react quickly when particle levels change, while another may update more gradually.
During rapidly changing haze conditions, one display may rise or fall before the other catches up.
### Different Sensor and Calculation Methods
Many portable PM2.5 monitors use optical particle-sensing methods, but manufacturers may use different:
- Sensor designs
- Airflow arrangements
- Sampling intervals
- Internal algorithms
- Display averaging periods
- Particle conversion methods
Two models can therefore show different numbers while following a similar upward or downward trend.
### Humidity and Environmental Conditions
High humidity can influence the way some optical particle sensors respond.
Avoid comparing monitors immediately after moving them between different temperature or humidity conditions. Allow both instruments to adjust to the same room environment first.
### Dirty Air Inlets or Sensors
Dust accumulation around the air inlet can affect airflow and sensor response.
Inspect the instrument according to the manufacturer’s instructions. Do not insert tools, compressed air or cleaning liquids into the sensor unless the procedure is specifically approved for that model.
### Different Instrument Condition
Readings may differ if:
- One monitor is new and another has been used for a long period
- One instrument was stored in a dusty location
- The air inlet is partially blocked
- Battery or power conditions are unstable
- One monitor has been exposed to smoke or high particle levels repeatedly
- The instruments use different factory settings
## How to Perform a Side-by-Side Comparison
### Step 1: Use the Same Room
Place both monitors in one indoor room away from strong particle sources.
### Step 2: Position Them Close Together
Place the instruments at the same height with enough space for their air inlets to operate freely.
Do not stack one monitor on top of the other or block any ventilation openings.
### Step 3: Avoid Unstable Locations
Keep both monitors away from:
- Open windows
- Air purifiers
- Air-conditioning vents
- Kitchens
- Doors with frequent movement
- Direct sunlight
- Fans blowing directly at the instruments
### Step 4: Allow the Readings to Stabilise
Do not compare only the first number displayed after switching on the instruments or moving them into the room.
Allow both units to operate under the same conditions and observe the readings over a reasonable period.
### Step 5: Compare the Trend
Ask:
- Do both readings rise when a window opens?
- Do both decrease when the air purifier operates?
- Do they follow the same general direction?
- Is the difference stable or constantly increasing?
- Does one monitor remain unchanged when conditions clearly change?
The trend is often more useful than expecting both displays to match exactly at every moment.
## When Is the Difference More Concerning?
Further checking may be appropriate when:
- One monitor shows almost no response to changing conditions
- One reading remains extremely high in clean-room conditions
- One instrument consistently displays zero
- The difference becomes much larger over time
- The fan or air inlet appears blocked
- The monitor was dropped or exposed to liquid
- An error message appears
- The instrument behaves differently after storage
Repeat the comparison in another room before concluding that the monitor has a fault.
## PM2.5 Monitor vs Official Outdoor API
An indoor PM2.5 monitor should not be expected to display the same number as Malaysia’s outdoor API.
The API is a regional outdoor index based on official monitoring data. A portable monitor measures particle concentration at its immediate indoor location.
Differences may be caused by:
- Distance from the official station
- Indoor filtration
- Closed windows and doors
- Local traffic or construction
- Different units and reporting methods
- Indoor activities
- Timing of data updates
Use the outdoor API for regional awareness and the indoor monitor for checking the actual occupied space.
## Avoid Adjusting Readings Without a Reference
Users should not apply an arbitrary correction simply because two monitors show different numbers.
Before making any adjustment:
- Confirm correct placement
- Repeat the side-by-side test
- Check the manufacturer’s instructions
- Inspect the air inlets
- Compare trends over time
- Use an appropriate reference method when accuracy verification is required
A second consumer monitor is useful for comparison, but it is not automatically a calibration reference.
## Choosing a Monitor for Consistent Indoor Checks
For repeatable comparisons:
- Use the same monitor when tracking one room over time
- Keep the measurement position consistent
- Record room and operating conditions
- Avoid comparing different parameters or units
- Follow the manufacturer’s maintenance guidance
- Keep the monitor away from contamination when stored
MTM Precision supplies **Temtop air-quality monitors** for homes, offices, schools and workplaces. Customers experiencing unusual readings may WhatsApp us a clear photo of the monitor, model label, display and measurement location for an initial assessment.
**MTM Precision Sdn Bhd**
**Showroom & Service Centre:** No. 29-1 & 29-2, Jalan Bandar 18, Pusat Bandar Puchong, 47160 Puchong, Selangor, Malaysia
**Tel:** 03-8080 7172
**WhatsApp:** +6016-660 7346
**Email:** mtmpre@yahoo.com
**Website:** www.mtmpre.com.my
A small difference may be normal because the instruments can use different sensors, airflow designs, response times and calculation methods. The air inside a room is also constantly moving, so each monitor may sample a slightly different concentration of particles.
Before deciding that one instrument is faulty, users in Johor should perform a controlled side-by-side comparison.
## Common Reasons for Different PM2.5 Readings
### Different Measurement Positions
One monitor may be closer to:
- An open window
- A door
- An air purifier
- An air-conditioning outlet
- A kitchen
- A person walking past
- A dusty surface
Even a short distance can affect the reading when air is moving unevenly around the room.
### Different Response Times
One instrument may react quickly when particle levels change, while another may update more gradually.
During rapidly changing haze conditions, one display may rise or fall before the other catches up.
### Different Sensor and Calculation Methods
Many portable PM2.5 monitors use optical particle-sensing methods, but manufacturers may use different:
- Sensor designs
- Airflow arrangements
- Sampling intervals
- Internal algorithms
- Display averaging periods
- Particle conversion methods
Two models can therefore show different numbers while following a similar upward or downward trend.
### Humidity and Environmental Conditions
High humidity can influence the way some optical particle sensors respond.
Avoid comparing monitors immediately after moving them between different temperature or humidity conditions. Allow both instruments to adjust to the same room environment first.
### Dirty Air Inlets or Sensors
Dust accumulation around the air inlet can affect airflow and sensor response.
Inspect the instrument according to the manufacturer’s instructions. Do not insert tools, compressed air or cleaning liquids into the sensor unless the procedure is specifically approved for that model.
### Different Instrument Condition
Readings may differ if:
- One monitor is new and another has been used for a long period
- One instrument was stored in a dusty location
- The air inlet is partially blocked
- Battery or power conditions are unstable
- One monitor has been exposed to smoke or high particle levels repeatedly
- The instruments use different factory settings
## How to Perform a Side-by-Side Comparison
### Step 1: Use the Same Room
Place both monitors in one indoor room away from strong particle sources.
### Step 2: Position Them Close Together
Place the instruments at the same height with enough space for their air inlets to operate freely.
Do not stack one monitor on top of the other or block any ventilation openings.
### Step 3: Avoid Unstable Locations
Keep both monitors away from:
- Open windows
- Air purifiers
- Air-conditioning vents
- Kitchens
- Doors with frequent movement
- Direct sunlight
- Fans blowing directly at the instruments
### Step 4: Allow the Readings to Stabilise
Do not compare only the first number displayed after switching on the instruments or moving them into the room.
Allow both units to operate under the same conditions and observe the readings over a reasonable period.
### Step 5: Compare the Trend
Ask:
- Do both readings rise when a window opens?
- Do both decrease when the air purifier operates?
- Do they follow the same general direction?
- Is the difference stable or constantly increasing?
- Does one monitor remain unchanged when conditions clearly change?
The trend is often more useful than expecting both displays to match exactly at every moment.
## When Is the Difference More Concerning?
Further checking may be appropriate when:
- One monitor shows almost no response to changing conditions
- One reading remains extremely high in clean-room conditions
- One instrument consistently displays zero
- The difference becomes much larger over time
- The fan or air inlet appears blocked
- The monitor was dropped or exposed to liquid
- An error message appears
- The instrument behaves differently after storage
Repeat the comparison in another room before concluding that the monitor has a fault.
## PM2.5 Monitor vs Official Outdoor API
An indoor PM2.5 monitor should not be expected to display the same number as Malaysia’s outdoor API.
The API is a regional outdoor index based on official monitoring data. A portable monitor measures particle concentration at its immediate indoor location.
Differences may be caused by:
- Distance from the official station
- Indoor filtration
- Closed windows and doors
- Local traffic or construction
- Different units and reporting methods
- Indoor activities
- Timing of data updates
Use the outdoor API for regional awareness and the indoor monitor for checking the actual occupied space.
## Avoid Adjusting Readings Without a Reference
Users should not apply an arbitrary correction simply because two monitors show different numbers.
Before making any adjustment:
- Confirm correct placement
- Repeat the side-by-side test
- Check the manufacturer’s instructions
- Inspect the air inlets
- Compare trends over time
- Use an appropriate reference method when accuracy verification is required
A second consumer monitor is useful for comparison, but it is not automatically a calibration reference.
## Choosing a Monitor for Consistent Indoor Checks
For repeatable comparisons:
- Use the same monitor when tracking one room over time
- Keep the measurement position consistent
- Record room and operating conditions
- Avoid comparing different parameters or units
- Follow the manufacturer’s maintenance guidance
- Keep the monitor away from contamination when stored
MTM Precision supplies **Temtop air-quality monitors** for homes, offices, schools and workplaces. Customers experiencing unusual readings may WhatsApp us a clear photo of the monitor, model label, display and measurement location for an initial assessment.
**MTM Precision Sdn Bhd**
**Showroom & Service Centre:** No. 29-1 & 29-2, Jalan Bandar 18, Pusat Bandar Puchong, 47160 Puchong, Selangor, Malaysia
**Tel:** 03-8080 7172
**WhatsApp:** +6016-660 7346
**Email:** mtmpre@yahoo.com
**Website:** www.mtmpre.com.my
29 Sep 2026
