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A manometer is a differential pressure instrument. To use one, connect the high-pressure side to the point you are measuring and the low-pressure side to the reference, zero the instrument with both ports at the same pressure, then read the difference. If you want air velocity you need a pitot tube and the density of the air, not just the pressure number. The main traps are reversed ports, forgetting to zero, and reading a single-port gauge as if it were differential.
What the two ports are for, and what happens when you swap them
A manometer does not measure pressure. It measures the difference between the pressure at one port and the pressure at the other. That is why every serious HVAC manometer has two ports, usually marked plus and minus, or high and low. The number on the screen is the arithmetic result of subtracting one from the other.
If you connect the high side to the duct and leave the low side open to the room, the instrument reads duct pressure relative to room pressure. That is what you want for static pressure. If you connect one port upstream of a filter and the other downstream, the instrument reads the pressure drop across the filter. That is what you want for filter loading. If you swap the two hoses, the reading simply changes sign. It does not break the instrument, but it does mislead you if you are writing down a number without thinking about which side is which.
The practical rule is to decide, before you connect anything, what the reference is. For duct static pressure the reference is the space the duct serves. For a gas valve manifold the reference is atmosphere. For a clean room the reference is the corridor outside. Once you know the reference, the port assignment follows.
The Leaton dual-port meter Leaton professional air pressure meter and the EHDIS unit EHDIS dual-port manometer both publish a range of plus or minus 2.999 psi, which is about plus or minus 20.68 kPa. That is enough for most duct static pressure and gas pressure work, but it is not enough for high-pressure gas lines or for compressed air systems above roughly 20 kPa. The listing does not publish a burst pressure or a maximum safe overload, so do not assume the sensor will survive being connected to a compressed air line at several bar.
Zeroing: the step that decides whether the number means anything
Every differential pressure sensor has an offset. Even a good one drifts a little with temperature and with the position it is held in. Zeroing tells the instrument that the current pressure difference, with both ports open to the same air, is zero. After that, any reading is relative to that zero.
The procedure is the same on almost every handheld manometer. Leave both ports unconnected, or connect them together with a short piece of hose, so both sides see the same pressure. Press the zero or tare button. Wait for the display to settle. Then connect the hoses and take the reading.
Two mistakes are common. The first is zeroing with one port still connected to the duct. That tells the instrument that the duct pressure is zero, and every subsequent reading is wrong by that amount. The second is zeroing once at the start of the day and then taking readings in a hot roof space an hour later. The sensor offset moves with temperature. Zero again when the instrument has reached the temperature of the place you are working.
The listing for the Leaton meter publishes a repeatability of plus or minus 0.2 percent, with a maximum of plus or minus 0.5 percent of full scale, and a linearity and hysteresis figure of plus or minus 0.29 percent of full scale. Those numbers describe how consistently the instrument repeats a reading, not how close it is to the true pressure. The accuracy figure is separate: plus or minus 0.3 percent of full scale at 25 degrees Celsius. On a 20.68 kPa range, 0.3 percent of full scale is about 62 Pa. That is the number that matters when you are checking a filter drop of 100 Pa. It is also why a manometer is not the right tool for measuring very small pressures, such as the few pascals that decide whether a bathroom fan is pulling air through a trickle vent.
Units, ranges, and why the same pressure appears three times on the screen
Manometers display in several units because different trades use different conventions. Pascals are common in ductwork and clean rooms. Inches of water column are common in gas appliance work in North America. Millibar and hectopascal appear in meteorological and laboratory contexts. The instrument does not change what it measures; it changes only the arithmetic used to display it.
The conversion you need to know is that 1 inch of water column is about 249 pascals. One psi is about 6.895 kilopascals. One millibar is 100 pascals. If you are checking a gas valve that should be set to 3.5 inches of water column, that is about 872 pascals, or about 0.87 kilopascals. If the manometer is reading in psi, the same pressure is about 0.127 psi. Reading the wrong unit is a common cause of a number that looks plausible but is wrong by a factor of 249.
The Testo 510 testo 510 differential pressure meter publishes ten measurement units and a temperature and air density compensated sensor. That compensation matters because a pressure sensor responds to the actual pressure difference, but the density of the air in a duct changes with temperature. If you are converting a pressure reading to a velocity, the density correction is part of the calculation, not an optional extra.
| Unit | Equivalent in pascals | Where it is used |
|---|---|---|
| Pascal (Pa) | 1 | Duct static pressure, clean room differentials, filter drop |
| Inch of water column (in wc) | about 249 | Gas appliance manifold pressure, draft |
| Millibar (mbar) | 100 | Laboratory and meteorological work |
| Psi | about 6895 | Gas lines, compressed air, tyre and tank work |
| Kilopascal (kPa) | 1000 | General metric pressure work |
The table is arithmetic, not product data. The instruments themselves publish their ranges in the listing bullets, and where a range is not published you should not assume it.
From pressure to velocity: the pitot tube and the square root
A manometer measures pressure. If you want air velocity, you need a pitot tube and a calculation. The pitot tube has two ports: one facing the airflow, which sees the total pressure, and one perpendicular to the flow, which sees the static pressure. The difference between them is the velocity pressure. The manometer measures that difference.
The relationship between velocity pressure and velocity is not linear. Velocity is proportional to the square root of the velocity pressure. The standard equation is velocity in metres per second equals the square root of two times the velocity pressure in pascals divided by the air density in kilograms per cubic metre. At standard conditions, with air at about 1.2 kilograms per cubic metre, a velocity pressure of 100 pascals corresponds to about 12.9 metres per second. A velocity pressure of 25 pascals corresponds to about 6.5 metres per second. Doubling the velocity quadruples the pressure, which is why a small error in a low pressure reading becomes a large error in velocity.
The Testo 0560 1510 testo 510i wireless differential pressure probe publishes a measurement menu that includes volume flow, with the pitot tube factor and dimensions entered in the app. The pitot tube itself is not included, and the listing says so. That is an honest limitation: the probe measures pressure, the app does the arithmetic, but the pitot tube is a separate instrument that determines whether the pressure you are measuring is the velocity pressure at all.
The mistake to avoid is treating a duct static pressure reading as a velocity reading. They are different quantities. Static pressure is the outward push of the air on the duct wall. Velocity pressure is the kinetic energy of the moving air. A duct can have high static pressure and low velocity, or the reverse. If you want to know whether a room is getting enough air, you need the volume flow, which is velocity multiplied by the duct cross-sectional area. That is a chain of measurements, and the manometer is only one link.
The maintenance nobody mentions, and the limits of the instrument
Manometers are low-maintenance instruments, but they are not maintenance-free. The most common failure is a blocked or leaking port. A speck of dust in the nipple, or a hose that has been kinked and has split, will make the instrument read a pressure difference that is not there. The symptom is a reading that does not change when you change the connection, or a reading that drifts slowly when it should be steady.
The second failure is a sensor that has been over-pressured. The listing for the Leaton and EHDIS meters publishes a range of plus or minus 2.999 psi, which is about 20.68 kPa. If you connect one of these to a compressed air line at 6 bar, which is about 600 kPa, you have applied roughly thirty times the full-scale pressure. The sensor may survive once, or it may not. The listing does not publish an overload rating, so the safe assumption is that any pressure above the stated range is a risk.
The third issue is temperature. The accuracy figures published for the Leaton and EHDIS meters are specified at 25 degrees Celsius. If you are working in a hot roof space or a cold plant room, the accuracy will be different. The listing does not publish a temperature coefficient, so you cannot calculate the error. The honest answer is that the number on the screen is less trustworthy outside the specified temperature, and you should not use a manometer for a critical measurement without allowing it to stabilise in the environment first.
Where the manometer stops being the right instrument is when you need to know about a gas that is not air, or when you need an alarm rather than a reading. A manometer measures pressure, not composition. It cannot tell you whether the pressure it is reading is caused by air, natural gas, or refrigerant. If you are checking for a gas leak, you need a combustible gas detector, not a manometer. If you are checking a CO level, you need a carbon monoxide monitor. A manometer is a measurement tool, not a safety device, and it does not replace a personal gas monitor in a confined space.
For related measurement techniques, the same principle of comparing two points applies to other instruments. A psychrometer compares wet-bulb and dry-bulb temperatures to find humidity, and the psychrometer guide explains that arithmetic. A CO2 monitor in a bedroom measures a different quantity, but the bedroom CO2 guide shows how the same logic of reference and trend applies. And if you are using pressure readings to judge ventilation, the guide to using CO2 data for ventilation explains why pressure alone is not enough.
The reading is only as good as the reference
A manometer is a simple instrument with a simple job: subtract one pressure from another and show you the result. The difficulty is not in the electronics. It is in deciding what the reference is, zeroing the instrument against that reference, and connecting the ports the right way round. Get those three things right and the number on the screen means something. Get them wrong and you will write down a figure that is precise to three decimal places and wrong by a factor of 249. The instruments named here publish their ranges and their accuracy figures; where a figure is missing, treat it as missing rather than guessing.
Frequently Asked Questions
Do I need a two-port manometer or will a single-port gauge do?
A single-port gauge measures pressure relative to atmosphere. That is fine for some gas pressure work, but it cannot measure a pressure drop across a filter or a velocity pressure from a pitot tube. If you are doing HVAC work, you need two ports. The Leaton and EHDIS meters are both dual-port.
How often should I zero the manometer?
Zero it before each measurement session, and again whenever the instrument has moved to a different temperature. The zero procedure takes a few seconds and it is the single most effective way to avoid a wrong reading.
Can I use a manometer to measure air velocity without a pitot tube?
No. A manometer measures pressure. Velocity requires a pitot tube to create a velocity pressure, and then a calculation that includes air density. The Testo 510i listing explicitly says the pitot tube is not included.
What does plus or minus 0.3 percent of full scale mean in real terms?
On a 20.68 kPa range, 0.3 percent of full scale is about 62 pascals. That is the accuracy figure published for the Leaton and EHDIS meters at 25 degrees Celsius. It means a reading of 100 pascals could be off by 62 pascals, which is why these instruments are not suitable for very low pressure measurements.
Is a manometer a safety device for gas work?
No. A manometer measures pressure. It does not detect gas, it does not alarm, and it does not replace a combustible gas detector or a carbon monoxide monitor. If you are checking for a leak, use the appropriate detector.
Why does my reading change sign when I swap the hoses?
Because the instrument is subtracting one port from the other. Swapping the hoses reverses the subtraction. The magnitude is the same; the sign tells you which port is at the higher pressure. It is not a fault.