Reading the DELIXI air monitor

Unofficial. I wrote this from the printed manual and my own unit. DELIXI did not make, check or endorse it.

• Worry numbers: formaldehyde at 0.1 or more, TVOC at 0.6 or more, PM2.5 above 75.
• The digit colours mean nothing. PM2.5 shows in red even at a clean reading of 4. Read the number, or the row of faces.
• One high reading proves little. Perfume, alcohol wipes, smoke and cooking can all send it up falsely. Remove the cause, air the room and read again.
• Daily readings and the proper test are different things. The manual's test needs a room shut for 12 hours with the air-con off.
• Give it 180 seconds after switching on, and 5 to 10 minutes after moving to a new room.

• The screen
• Ranges
• Buttons
• A fair test
• What to do
• Every term
• Specs
• Manual gaps

The drawing shows the reading from my photo. Numbers match the list beside it.

Two things you may notice on your own unit. The dim 8-shaped marks behind the numbers are unlit parts of the display, and are normal. My unit also has faint Wi-Fi and Bluetooth symbols at the bottom right. The manual never mentions wireless features.

The first three tables are the manual's own bands. Green rows hold the reading from my photo. Treat it as an everyday reading, not the 12-hour test described further down.

Formaldehyde 甲醛 · mg/m³

Reading	Manual says	In plain words
below 0.1my photo: 0.008	Safe range	Nothing to do.
0.1 to 0.5	Odd smell, discomfort. Towards 0.5, can sting eyes and cause tears	Air the room, then retest properly.
over 0.5 to 0.6	Throat discomfort or pain	Leave the room and air it.
above 0.6	Nausea, vomiting, coughing, chest tightness	Get out. See a doctor if you have symptoms.

The manual's bands overlap (≥0.1, ≤0.5, ≤0.6). I have turned them into separate steps.

TVOC mg/m³

Reading	Manual says	In plain words
below 0.6my photo: 0.328	Safe range	Fine. Mine is past the halfway mark, so it is the one to watch.
0.6 or more	Over the limit, may affect breathing	Look for a cause first. This sensor reacts to many everyday smells.

PM2.5 µg/m³

Reading	Manual says	In plain words
0 to 35my photo: 4	Excellent	Good, but read the note below. WHO's daily guide is 15.
36 to 75	Good	Generous label. I would treat this as worth fixing if it lasts all day.
76 to 115	Light pollution	Close windows if it is outside air. Run a purifier.
116 to 150	Moderate pollution	Purifier on high. Find the source.
151 to 250	Heavy pollution	Stay out of that room if you can.
251 to 500	Severe pollution	Stay out. Check for smoke or fire.

The readings the manual gives no bands for

Item	My photo	How to judge it
PM1.0	1	No limit in the manual. It moves with PM2.5, so watch that instead.
PM10	3	WHO's daily guide is 45 µg/m³. China's indoor standard is 100.
Temperature	29 °C	Comfort is personal. Warmth can raise what furniture gives off, so compare readings taken in similar conditions.
Humidity	72 %	The one reading on my screen worth acting on. The US EPA advises staying below 60%, ideally 30 to 50%, to keep mould away.
Faces	first lit	Further right looks worse. The manual does not say which readings drive it.

How far to trust a number

The makers' stated error, applied to my photo. Small particle readings are mostly noise.

Item	Shown	True value could be
Formaldehyde (±15%)	0.008	0.007 to 0.009
TVOC (±15%)	0.328	0.279 to 0.377
PM2.5 (±10 µg/m³ below 100)	4	0 to 14
Temperature (±1 °C)	29	28 to 30
Humidity (±5%)	72	67 to 77

My photo shows PM2.5 at 4 and PM10 at 3. PM10 includes everything in PM2.5, so it cannot truly be lower. A gap that small sits inside the stated error, and is a good reminder not to read single digits too closely.

How the manual compares with outside limits

Item	This manual	China GB/T 18883-2022	WHO	Singapore SS 554
Formaldehyde, mg/m³	under 0.1	0.08 (1 hr)	0.1 (30 min)	0.1 (30 min)
TVOC, mg/m³	under 0.6	0.6 (8 hr)	none listed	2.3 (8 hr)
PM2.5, µg/m³	0–35 "excellent"	50 (24 hr)	15 (24 hr)	37.5 (24 hr)
PM10, µg/m³	none	100 (24 hr)	45 (24 hr)	not found

These limits are averages over the times shown, so a single glance at the screen is not a direct comparison. All figures come from RESET's published comparison tables, not the standards themselves. SS 554 covers air-conditioned non-residential buildings. RESET's numbers appear to be from the 2021 edition, and SS 554:2026 has since replaced it; I could not see the new limits. A TVOC figure also depends on the reference gas used, so numbers from different systems may not line up.

To do this	Press	What happens
Switch on	Hold ⏻	Runs on 5V USB or its own battery. Wait 180 seconds for it to warm up.
Change brightness	Tap ▲	Screen brightness steps.
Alarm sound on or off	Hold ▼	Hold once for on, again for off. Not while setting the clock.
Reset TVOC to zero	Hold ▲ 5 seconds	One beep means done. Do this in clean air only (see below).
Set the clock	⏻ twice	Year flashes. ▲ or ▼ changes it. ⏻ moves on to month, hour, minute. Keep pressing ⏻ and it saves and exits.
Charge	Plug in USB	Battery bars run in sequence, then three bars when full. It can stay plugged in for long-term use without harming the battery, says the manual.

First time out of the box

Switch it on somewhere well ventilated for 30 to 60 minutes. This wakes the sensors up. Ignore every number during this time.

The manual's proper test

1. Keep the room shut for 12 hours at normal temperature and humidity.
2. Turn off anything that moves air: purifier, air-con, fan, humidifier.
3. Put the unit in the middle of the room, 80 to 150 cm off the floor.
4. Don't breathe on the vents or block them.
5. Keep perfume, paint, cigarettes and alcohol away.
6. Moving to another room? Wait 5 to 10 minutes for the numbers to settle.

What makes it read falsely high

The manual warns that these can send readings shooting up:

• Breweries and other places thick with alcohol vapour
• Make-up rooms and anywhere with strong alcohol-based scents
• Heavy perfume, toner or skincare on the person testing
• Alcohol-based cleaners being used nearby
• Smoking rooms, and kitchens during stir-frying
• Smoking or exhaling straight at the unit
• Holding it over a stirred tin of paint or anything strong-smelling

Temperature, humidity, alcohol, smoke and sharp smells all affect the formaldehyde reading. Small ups and downs between times of day or parts of a room are normal.

These are my suggestions, not the manual's.

If you see	Check first	Then
TVOC 0.6 or more	Perfume, wipes, cooking, new paint or furniture nearby	Open windows for 30 minutes and read again. If it stays high with the obvious causes gone, hunt for a hidden source.
Formaldehyde 0.1 or more	Any item on the false-high list above	Run the 12-hour test on two different days. If both are high, air the room hard and pay for a lab test before spending on any treatment.
Formaldehyde above 0.5, or sore eyes or throat	Don't stop to check	Leave the room and open it up. See a doctor if symptoms carry on.
PM2.5 above 35	Cooking, incense, candles, haze outside	Cooker hood during cooking. In haze, shut windows and run a HEPA purifier.
Humidity above 60%	Damp walls, wet laundry, windows open in the rain	Air-con dry mode or a dehumidifier, more so in rooms with cupboards and books.
A sudden spike that fades	Someone walked past in perfume, or sprayed something	Usually nothing. Only sustained readings matter.

Habits that make it more useful

• Watch the trend. Same spot, same time of day. A change matters more than one exact figure.
• Place it away from the stove, windows and a purifier's airflow, at about breathing height.
• Zero TVOC only in clean air, after airing the room well. Zeroing in stale air sets a false baseline, and later readings will come out too low.
• Use the 12-hour test sparingly, say after renovation or new furniture. In Singapore, a shut room with the air-con off soon stops being "normal" conditions, so test in the cooler hours.
• Match the filter to the problem. HEPA catches particles. It does nothing for gases, which need activated carbon. Notes on how I chose a purifier.
• Don't use it as the final word. Home sensors lose sensitivity with age, and TVOC sensors in particular react to alcohol more than to many other gases. Before paying for treatment or arguing with a contractor, get a proper lab test.
• Check the clock if the date looks wrong. It only matters if you log your readings.

On the screen

Formaldehyde甲醛 · HCHO

A colourless, sharp-smelling gas. Pressed-wood furniture, glues, some paints and cigarette smoke give it off.

Formaldehyde average甲醛均值

A running average of the formaldehyde reading. Steadier than the live figure. The averaging period isn't stated.

PM2.5 averagePM2.5 均值

Listed in the manual's features and drawn on its cover, but not visible on my unit's main screen.

TVOCTotal volatile organic compounds

One combined signal for many carbon-based gases that evaporate from paint, cleaners, perfume, cooking and more. It cannot tell you which gas it is seeing.

PMParticulate matter

Solid or liquid specks floating in air: dust, smoke, soot, pollen.

PM1.0

Particles up to 1 micrometre across. A human hair is roughly 70 micrometres.

PM2.5

Particles up to 2.5 micrometres. Small enough to reach deep into the lungs. The main haze measure.

PM10

Particles up to 10 micrometres. Includes all of PM2.5 and PM1.0.

Temperature温度

Air temperature at the unit, in degrees Celsius.

Relative humidity湿度

How much water vapour the air holds, as a percentage of the most it could hold at that temperature.

AQIAir quality index · 空气综合质量

Here, the unit's overall rating shown as four faces. It is not the same scale as Singapore's PSI or the US AQI number.

mg/m³Milligrams per cubic metre

Mass of gas in one cubic metre of air. 1 mg = 1,000 µg, so 0.328 mg/m³ equals 328 µg/m³.

µg/m³Printed "ug/m³" on the screen

Micrograms per cubic metre. A thousand times smaller than mg/m³.

Air quality labels

优Excellent

PM2.5 from 0 to 35.

良Good

PM2.5 from 36 to 75.

轻度污染Light pollution

PM2.5 from 76 to 115.

中度污染Moderate pollution

PM2.5 from 116 to 150.

重度污染Heavy pollution

PM2.5 from 151 to 250.

严重污染Severe pollution

PM2.5 from 251 to 500.

安全范围Safe range

The manual's label for formaldehyde below 0.1 and TVOC below 0.6.

超标Over the limit

The manual's label for TVOC at 0.6 or more.

爆表Off the scale

The manual's word for a reading that suddenly shoots up falsely.

Buttons and actions

Calibration key校准键 · ▲

Its printed name. Used for TVOC zero and brightness.

Power key电源键 · ⏻

Switches on and steps through clock setting.

Mute key静音键 · ▼

Turns the alarm sound on and off.

USB portUSB 口

On the side of the unit. For charging and power.

Warm-up预热

The 180 seconds the sensors need after switch-on before numbers count.

Zeroing清零

Telling the sensor that the current air counts as zero TVOC.

Calibration校准

Adjusting a sensor against a known reference. This unit only offers TVOC zeroing to the user.

Alarm sound报警声

A warning beep you can switch on. The trigger level isn't stated.

Running-light charging跑马灯

The battery bars lighting up one after another, like a marquee.

Specification words

Measuring range测量范围

The lowest to highest value the unit can report.

Resolution分辨率

The smallest step on the display. It says nothing about how correct the value is.

Accuracy测量精度

How far the reading may be from the true value, written with ±.

Measuring principle测量原理

The physical method the sensor uses.

Electrochemical电化学 · formaldehyde sensor

The gas reacts at an electrode and produces a tiny current that rises with the amount of gas.

Semiconductor半导体 · TVOC sensor

Gases change the electrical resistance of a heated metal-oxide layer. It reacts to many gases, and alcohol strongly. Better for spotting changes than for exact figures.

Laser scattering激光散射 · PM sensors

A laser shines through the air. Particles scatter the light, and the sensor turns that into an estimated mass.

18650 lithium cell18650 锂电池

A standard cylindrical rechargeable cell, 18 mm wide and 65 mm long.

3.7V / 1200mAh

The cell's voltage and its capacity. Good for 6 to 8 hours here.

DC5V, 5V/1A

The power it needs: 5 volts direct current, up to 1 amp. Any ordinary USB charger.

LCD

Liquid crystal display, the screen type.

Q/LDL021-2021执行标准 · standard applied

The product standard printed on the manual. The "Q/" prefix marks a company's own standard in China, so it is not an air-quality limit.

Certificate / warranty card合格证 / 保修卡

The pass slip and the warranty card in the box.

Measure	Range	Step	Accuracy	Sensor type
Formaldehyde	0.000–1.999 mg/m³	0.001	±15%	Electrochemical
TVOC	0.000–9.999 mg/m³	0.001	±15%	Semiconductor
PM1.0, PM2.5, PM10	0–999 µg/m³	1	±10%*	Laser scattering
Temperature	−10 to 50 °C	1	±1 °C	Not stated
Humidity	20–90 %	1	±5%	Not stated

*From 0 to 100 µg/m³ the error is ±10 µg/m³. The display step for temperature and humidity is what my screen shows; the manual doesn't state it.

Other details. Model DLX-FTJ0506. LCD screen. Battery 3.7V, 1200mAh, 18650 lithium, 6 to 8 hours. USB power, 5V/1A. Time in hours and minutes, 24-hour format. Rated for homes, offices, cars, hotels, schools and public places.

In the box. The unit, a USB cable, the manual, and a certificate with warranty card.

Maker. Delixi Electric Ltd, Delixi High-Tech Industrial Park, Liushi Town, Yueqing, Zhejiang, China. delixi-electric.com. Customer hotline in China: 400-7566-777, extension 1718.

• How the four faces map to its five or six pollution levels
• Which readings feed the faces
• What reading sets off the alarm
• How long the formaldehyde average runs
• How to switch the unit off
• Whether the clock has a day step. It lists year, month, hour and minute only. If the day flashes, set it the same way.
• Limits for PM1.0, PM10, temperature or humidity
• How long the sensors last

• DELIXI DLX-FTJ0506 printed instruction manual (Chinese). Rewritten in my own words.
• US EPA: A Brief Guide to Mold, Moisture and Your Home
• RESET: comparing PM2.5 and PM10 targets around the world (July 2026)
• RESET: comparing TVOC and formaldehyde targets around the world (Aug 2026)
• Singapore Standards eShop: SS 554:2026
• Atmotube: TVOC sensors, what they can and can't do

Checked 8 Nov 2026. Sensor explanations are general knowledge about each type, not tests of this unit.