- Range
- 0-1 kPa to 0-10 MPa, with 0-3000 Pa, 0-30 kPa and 0-100 kPa as the low spans
- Accuracy
- 0.075, 0.1, 0.2 or 0.5 class depending on the form
- Output
- 4-20 mA with HART, or RS-485; 0-10 V on the micro-differential form
- Supply
- 24 V DC as standard, 9-32 V DC on the transmitter shown
- Process connection
- Flanged DN50 or DN80, with NPT 1/4 on the micro-differential form
- Diaphragm
- 316L stainless, Hastelloy or tantalum
- Display
- LCD with a 0-100 % scale
- Ingress protection
- IP67, with IP65 versions
Most pressure instruments measure one pressure. A differential transmitter measures two and reports the difference, which is why it appears wherever the difference carries the meaning: flow across an orifice, level in a tank, the pressure drop across a filter, the draught in a furnace.
That also makes it the instrument that is most often quoted wrongly. The range and the static pressure are separate figures, and a cell chosen for a small difference can still be destroyed by the line pressure it sits on.
Product overview
The instrument has two pressure sides separated by a diaphragm. The material covers spans from 0-1 kPa to 0-10 MPa, with 0-3000 Pa, 0-30 kPa, 0-100 kPa and 0-10 MPa named as common ones, and offers 0.075, 0.1, 0.2 and 0.5 class accuracy.
The output is 4-20 mA with HART as the common form, with RS-485 on the communication form and 0-10 V on the micro-differential form. The enclosure is IP67 on the main form, and the transmitter shown carries a nameplate marked 9-32 V DC supply, 4-20 mA/HART output and an ambient range of -20 to 85 °C.
How a differential pressure transmitter works
A diaphragm separates the high and low sides. The pressure difference deflects it, and the deflection is converted into an electrical signal proportional to that difference. Because the diaphragm sees both pressures, the static pressure on the line acts on both sides at once and largely cancels; what remains is the difference.
Three consequences follow. The instrument needs two process connections, not one, and the piping that brings them has to be arranged so that both legs see the same static head — which is what a manifold and a pair of impulse lines are for. A blocked or partly filled leg becomes a measurement error rather than a fault, because the transmitter keeps reading a plausible difference. And the diaphragm is the only part in contact with the medium, so its material decides what the instrument can survive.
How to select the right transmitter
What the difference is used for. Flow across an orifice, level in a tank and pressure drop across a filter all use the same instrument but are specified differently. The first needs a span matched to the calculated differential; the second needs the static head of the vessel accounted for; the third needs a low span and a connection that will not block.
Range and static pressure, separately. The differential range is what the instrument measures; the static pressure is what it sits on. A 0-1 kPa cell on a 4 MPa line is a different order from the same cell on a duct.
Single flange, double flange or remote seal. A single flange suits a pipe. A double flange with two capillaries suits level in a closed tank, because the second seal puts the vessel head on the low side so that it cancels. A remote seal is used where the medium is hot, viscous or corrosive enough that it should not reach the transmitter body.
Diaphragm material. 316L stainless covers most duties; Hastelloy and tantalum are for media that would attack it. This is the choice that decides whether the instrument lasts, and it is best settled with the medium list rather than after the order.
Installation and configuration
The transmitter is bolted to the process by its flange, or by an NPT 1/4 port on the micro-differential form. The impulse lines, the manifold and the orientation matter more than the instrument: air trapped in a liquid leg and condensate in a gas leg both produce a steady offset that looks like a real reading.
The zero is set with the keys beside the display, with both sides vented or equalised. The material gives no installation torque, no straight-run requirement and no impulse-line diameter, and none is invented here.
Measurement limitations and selection considerations
The first limitation is that both legs must be sound. A differential transmitter does not fail when a leg blocks; it keeps reading, and the error appears as a slow disagreement with a total taken elsewhere.
The second is the low end. A cell built for megapascals cannot resolve a few hundred pascals, which is why the material offers a separate micro-differential form with a 0-3000 Pa span and a 0-10 V output rather than extending the main range downwards.
Several figures are unsettled and are listed as information required rather than filled in: the static pressure rating, the temperature effect, the fill fluid, the seal material and the flange standard. The flange standard is the one that most often delays an order, because the drilling has to match the line that is already there.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
