How to Use P1 and P2 on the UNI-T UT366A Digital Manometer Malaysia

The UNI-T UT366A Digital Manometer has two pressure ports, identified as P1 and P2. These ports allow technicians to measure pressure at two separate points or calculate the pressure difference between them.

Correct hose connection is important. Reversing the pressure points may change the sign of the displayed value even when the actual pressure difference remains the same.

What Are P1 and P2?

P1 and P2 are the UT366A’s two pressure inputs.

They can be used for:

Independent P1 pressure measurement
Independent P2 pressure measurement
Differential-pressure measurement
Positive-pressure checking
Negative-pressure checking
AHU filter pressure-drop measurement
Fan inlet and outlet pressure comparison
Pitot tube airflow measurement

In differential mode, the instrument compares the pressure connected to P1 with the pressure connected to P2.

Understanding Positive Pressure

Positive pressure means the measured point has a higher pressure than the reference point.

Examples include:

A supply duct compared with the surrounding room
A pressurised room compared with an adjacent corridor
The upstream side of a filter compared with the downstream side
The discharge side of a fan compared with its inlet

The UT366A uses a green LED indicator for positive pressure.

The displayed sign still depends on how the hoses are connected and which port is being used as the reference.

Understanding Negative Pressure

Negative pressure means the measured point has a lower pressure than the reference point.

Examples include:

A return duct compared with the surrounding room
An exhaust system pulling air from an area
A fan-inlet section operating below ambient pressure
A negatively pressurised room compared with an adjacent area

The UT366A uses a red LED indicator for negative pressure.

A negative value is not automatically an error. It may correctly represent the pressure condition being measured.

Understanding Differential Pressure

Differential pressure is the difference between two pressure points.

It is commonly used for:

Checking pressure drop across an AHU filter
Comparing pressure before and after a coil
Measuring pressure across a fan
Comparing two rooms
Checking duct-pressure changes
Measuring velocity pressure with a Pitot tube

For example, if the upstream pressure is higher than the downstream pressure, the difference represents the pressure lost as air passes through the component.

Connecting the UT366A Across an AHU Filter

To measure filter pressure drop:

Identify the approved pressure-test point before the filter.
Identify the pressure-test point after the filter.
Connect one hose to the upstream point.
Connect the second hose to the downstream point.
Connect the hoses to P1 and P2 according to the required reading direction.
Select differential-pressure measurement.
Confirm the pressure unit.
Allow the reading to stabilise.
Record the value and compare it with the filter or AHU specification.

If the display shows a negative value when a positive pressure-drop value is expected, confirm the connection arrangement. Reversing the two hoses will normally reverse the displayed sign.

Using Only One Pressure Port

For gauge-pressure measurement, one port can be connected to the test point while the other side remains referenced to the surrounding pressure according to the instrument’s operating procedure.

This method can be used for:

Supply-duct pressure
Return-duct pressure
Fan discharge pressure
Fan inlet pressure
Room-pressure checking

Before starting, make sure the open reference is not affected by direct airflow, wind or pressure trapped inside a hose.

Connecting a Pitot Tube

A Pitot tube uses separate pressure paths to obtain:

Total pressure
Static pressure

The UT366A measures the difference between these pressures to determine velocity pressure and calculate air velocity.

When connecting the Pitot tube:

Follow the markings on the Pitot tube and pressure ports
Keep the two silicone hoses separate
Avoid crossing or reversing the connections
Make sure the hoses are not leaking
Point the Pitot tube directly into the airflow
Check the displayed sign before recording results

If the connections are reversed, the displayed pressure direction may be opposite to what is expected.

Why Does the Reading Change Sign?

The pressure sign may change because:

P1 and P2 have been reversed
The upstream and downstream hoses were swapped
The equipment’s airflow direction changed
The fan stopped or reversed
One pressure point became blocked
The reference point was affected by airflow
The system changed from positive to negative pressure

Always inspect the complete connection arrangement before assuming the UT366A has a fault.

Colour-Code the Hoses

Using two different hose colours helps prevent mistakes.

For repeated HVAC inspections:

Assign one colour to the upstream or high-pressure point
Assign the other colour to the downstream or low-pressure point
Use the same arrangement for every inspection
Label permanent pressure-test points
Record the connection method in the maintenance report

The compatible UNI-T Pitot tube is supplied with one black and one white silicone tube, helping technicians distinguish the two pressure paths.

Confirm the Pressure Unit

The UT366A supports multiple pressure units, including:

kPa
mbar
bar
psi
mmHg
inH₂O
inHg
mmH₂O
ftH₂O
hPa

Make sure the displayed unit matches the AHU, filter or equipment specification before comparing results.

Record More Than the Number

A useful pressure-measurement record should include:

Date and time
Equipment identification
P1 connection point
P2 connection point
Selected pressure unit
Equipment operating condition
Measured value
Positive or negative sign
Action taken

Without the connection details, another technician may misinterpret the recorded value.

Need Help with the UT366A Connection?

For assistance using P1 and P2 for filter pressure drop, duct pressure, room pressure or Pitot tube measurement, WhatsApp MTM Precision with photos of the instrument, hoses and test points.

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