A REPEATABLE METHOD
How to use it
- Check tubing, measurement range and units. Follow the model manual to zero with both ports at the same pressure, disconnected from the test pressure; do not zero away the pressure difference you intend to measure.
- Label the + and − connections and the two test locations. For filter drop, use the specified static-pressure test points upstream and downstream, with sound tubing and no kinks or leaks; an open hose facing the airflow is not equivalent to a static-pressure tap.
- Run the system at a documented fan speed and allow the reading to stabilize.
- Record the signed pressure and unit, test locations, filter model, fan setting, system mode and date. Compare with the manufacturer limit or a clean-filter baseline at comparable airflow; matching a switch setting alone does not prove equal airflow.
- Disconnect carefully, cap permanent test ports and repeat using the same hose arrangement next time.
INTERPRET THE MEASUREMENT
How to read the result
- The displayed difference is p(+) − p(−). The + port need not be above atmospheric pressure; it only needs to be higher than the other port for a positive result. Reversing hoses reverses the sign under unchanged conditions.
- At a fixed airflow, filter pressure drop usually increases with loading. In a real system the airflow may fall, so a lower drop does not by itself prove a clean filter or adequate ventilation.
- Pascals, inches of water column and millibars differ greatly; always record the unit.
- Illustrative sign check: upstream pressure −50 Pa and downstream pressure −120 Pa, both relative to the same ambient reference, give a filter drop of (−50) − (−120) = +70 Pa. Swapping the connections gives −70 Pa; this is not a filter-replacement threshold or an airflow measurement.
TROUBLESHOOT THE METHOD
Common problems and what to check
Problem
The zero drifts before connection.
What to do
Let the meter acclimatize, shield it from drafts and inspect tubing. Repeat the manual zero procedure with both ports at the same pressure, disconnected from the test pressure. A zero setting is not a calibration or a correction for leaking hoses.
Problem
The filter drop looks lower when it is dirtier.
What to do
Check fan speed, bypass around the filter, blocked return paths and hose placement. Reduced airflow can reduce measured drop.
Problem
The reading pulses rapidly.
What to do
Use time averaging or a damping setting and check that the probe is not in strongly turbulent flow.
STOP BEFORE THE HAZARD
Safety boundary
- Use proper test ports and keep hoses, clothing and tools away from fans, belts and electrical compartments.
- Do not drill into equipment until hidden coils, wiring, refrigerant lines and manufacturer instructions are checked.
- Combustion appliances and pressure-critical rooms require trained assessment under local rules.
- Check the exact model measurement and maximum applied-pressure limits separately: an overload or burst limit is not a usable measuring range. A small differential does not establish that both applied pressures are safe. Do not use a duct-air micromanometer as a water, fuel-gas or refrigerant gauge.
SMART BUYING GUIDE
What to look for when choosing
- Choose a low-pressure range and resolution appropriate to residential filters and duct systems.
- Zeroing, averaging and a bidirectional display matter more than a very high maximum range.
- For airflow work, pay for compatible probes and a documented conversion method rather than a generic calculation.
Types of tool
Best for: Filter drop and room-pressure spot checks.
Best for: Tracking cycling systems and pressure changes over time.
Best for: Trained duct testing with compatible pitot or flow devices.
Common functions
Bidirectional pressureZero and relative modesTime averagingMultiple pressure unitsData logging
Worth paying more for
- Good low-range resolution and zero stability
- Leak-resistant fittings and useful averaging
- Logging for variable systems
Usually not worth the extra cost
- Industrial maximum pressure for household ducts
- Automatic airflow values without a defined probe
- Many units when the basic sensor is unstable
Sources & instructions
- TSI application note TSI-107 — differential sign, static pressure and Pitot velocity pressure (PDF)
- TSI DP-Calc 5815 manual — model-specific example: zero control, measurement range and overpressure warning (PDF)
AI-generated topic illustration; do not use the depicted scale for measurements.