The short answer
Differential Pressure
A differential pressure transmitter measures the difference between the pressures at its high and low connections, independent of the common pressure applied to both up to its static pressure rating. In water and wastewater it measures head loss across filters, screens, and strainers, transmembrane pressure across membranes, the differential across pumps for head and condition monitoring, flow through orifices and venturis, and level in closed vessels. It is connected through a three-valve or five-valve manifold whose block and equalizer valves must be operated in the right order so that full line pressure is never applied to one side alone, and it is zeroed with the equalizer open at line pressure. Its span is small compared with the line pressure, so it is ranged for the differential of interest and protected against overrange by the manifold procedure.
Key points
- It reads high side minus low side, and ignores the pressure common to both up to the static rating.
- Span is small, line pressure is large: the manifold sequence exists so that one side never sees full line pressure alone.
- To zero: equalizer open, both sides at line pressure, then trim. To run: close the equalizer, then open the low side block.
- Filter head loss, strainer differential, membrane TMP, pump differential, orifice flow, closed-tank level: one instrument, six jobs.
- Static pressure shifts the zero a little; a transmitter zeroed at line pressure is right at line pressure.
What it measures
The sensing element sees two pressures, one on each side of a diaphragm or a pair of diaphragms, and produces a signal proportional to their difference. A pressure applied equally to both sides deflects nothing and is not measured; that is the static or line pressure, and the transmitter is rated for the maximum it will tolerate. The differential it is ranged for is usually a small fraction of the line pressure: a filter with 100 inches of water of head loss on a system at 60 psi, which is about 1,660 inches of water, is a differential of 6 percent of the line pressure. Applying the full line pressure to one side alone, by opening the wrong valve, overranges the cell by a factor of sixteen; transmitters are built to survive that up to their overrange rating, and the manifold procedure exists so that it does not happen anyway.
Ratings
- Span and range
- The differential between 4 and 20 mA. Transmitters have a wide turndown, so one model covers head loss on a filter and flow through an orifice with different ranges.
- Static pressure rating
- The maximum common pressure both sides may carry. Water plant pressures are far below it on most transmitters.
- Overrange
- The pressure one side may see alone without damage. Usually the static rating; exceeding it can shift the calibration permanently.
- Static pressure effect
- The zero and span shift a small amount per unit of line pressure; negligible in water service, real in high-pressure service, and removed by zeroing at line pressure.
- Accuracy
- A fraction of a percent of span at the calibrated range; turning a transmitter down to a small span from a large upper range limit costs some of it.
The manifold
A three-valve manifold has a block valve on each side and an equalizer between them. A five-valve manifold adds vent or drain valves. The sequence protects the cell and makes zeroing possible.
- 1
Putting into service
Start with both blocks closed and the equalizer open. Open the high side block: line pressure reaches both sides through the equalizer and the transmitter reads zero. Close the equalizer. Open the low side block. The transmitter now reads the differential.
- 2
Zeroing
With the high side block open, close the low side block, then open the equalizer. Both sides are at line pressure and the reading should be zero; trim if it is not. Close the equalizer, then reopen the low side block.
- 3
Taking out of service
Close the low side block. Open the equalizer. Close the high side block. Vent or drain through the vent valves if fitted.
Applications
| Application | High side | Low side | Typical range | Purpose |
|---|---|---|---|---|
| Gravity filter head loss | Above the media, or the influent channel | Filter effluent | 0 to 120 inches of water | Backwash initiation, filter run tracking |
| Strainer or screen differential | Upstream | Downstream | 0 to 10 psi | Cleaning alarm and interlock |
| Membrane transmembrane pressure | Feed side | Permeate side | 0 to 15 or 0 to 30 psi depending on the membrane | Fouling trend, clean-in-place trigger, integrity |
| Pump differential | Discharge | Suction | 0 to the shutoff head | Head for the pump curve, wear and blockage detection |
| Orifice or venturi flow | Upstream tap | Throat or downstream tap | 0 to 100 inches of water, square root | Flow where a spool meter is not fitted |
| Closed tank level | Bottom tap | Vapor space | Liquid head between taps | Level independent of vessel pressure |
Pump differential
A differential transmitter across a pump, or two pressure transmitters whose readings are subtracted, gives the pump head, which together with speed and flow places the pump on its curve. Head that falls at a given speed and flow means wear or recirculation; head that rises with flow falling means a blockage downstream. The head in feet is the differential in psi times 2.31 divided by the specific gravity, corrected for the elevation difference between the two taps and the difference in velocity head where the suction and discharge pipe sizes differ.
H = 2.31 × ΔP / SG + (z_d − z_s) + (v_d² − v_s²) / 2g
- H = pump total head in feet
- ΔP = discharge pressure minus suction pressure in psi, at the taps
- SG = specific gravity of the liquid, 1.0 for water
- z_d − z_s = elevation of the discharge tap above the suction tap in feet
- v_d, v_s = velocities at the discharge and suction taps in feet per second; g = 32.2 feet per second squared
Installation notes
- Impulse lines of equal length and the same fill, both sloped so that air vents back to the process on liquid service.
- Taps at the same elevation on flow and pump service, or the elevation difference accounted for.
- The transmitter below the taps on liquid service so the lines stay full, with a vent at the transmitter.
- Diaphragm seals on sludge, membrane feed with solids, and chemical services, with matched capillaries.
- A gauge on each side for a sanity check, and access to the manifold from the floor.
Frequently asked questions
- Why not use two pressure transmitters and subtract in the controller?
- Because each has its own error, and subtracting two large numbers to get a small one multiplies the error. Two transmitters at 60 psi with a quarter percent error each can be wrong by a few tenths of a psi, which is a large fraction of a filter head loss of a few psi. A differential transmitter measures the small number directly. Two transmitters are fine when the differential is large relative to the errors, such as pump head.
- The transmitter reads a small negative value with the equalizer open. Is it broken?
- It needs a zero trim at line pressure, which is what the equalizer-open condition is for. A small shift with line pressure and temperature is normal; trim it and close the equalizer. A large negative reading with the equalizer open is a leg that is not full or a plugged equalizer.
- Can a differential transmitter be used as a plain pressure transmitter?
- Yes, with the low side open to atmosphere, it reads gauge pressure at the high side. The range limits and the static rating still apply, and a vented low side must be protected from water and insects.
- What causes the filter head loss reading to jump at the start of backwash?
- The flow reversal changes which tap is at the higher pressure, and a transmitter ranged for positive differential reads below zero or pins. Range it bidirectionally, or accept that the reading is meaningless during backwash and mask it in the controller.
Related topics
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- Impulse LinesThe tubing between the process tap and the pressure or DP transmitter, and everything that goes wrong in it: slope and routing for liquid and gas, elevation head and how to correct for it, air and condensate, blocking and manifold valves, freezing, plugging on dirty service, and the diaphragm seal alternative.
- Differential Pressure Flow MeasurementOrifice plates, venturis, and flow nozzles: the square-root relationship, why turndown is limited, where the square root is taken, impulse line rules, and when a DP element is still the right choice against a mag meter.
- Differential Pressure LevelMeasuring level in a closed or pressurized vessel with a DP transmitter: high and low side connections, dry legs and wet legs, zero suppression and elevation, the range calculation with a worked example, remote seals, and the density dependence.
- Pressure CalibrationWhat calibrating a pressure transmitter means: as-found and as-left, the five-point test and hysteresis, tolerance and the reference standard, sensor trim versus output trim versus reranging on a smart transmitter, gauge versus absolute, and intervals.
- Pressure Transmitter InstallationWhere and how to install a pressure transmitter so it reads the process and not its own plumbing: tap location, mounting above or below the tap, isolation and bleed valves, line slope, pulsation, elevation, freeze protection, seals, vibration, and sun.
Direct contact
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