Yokogawa Draft Pressure Transmitter

A common application for a Draft Pressure transmitters are in air handling systems.

Yokogawa uses their digital DPharp sensor to design a transmitter for this specific application.


Traditional-mount Differential Pressure Transmitter designed for Draft Range applications based on the EJA-E Series.


Traditional-mount Differential Pressure Transmitter designed for Draft Range applications based on the EJX-A Series.

Which Series Draft Pressure Transmitter is Right for You?

± 0.09%
± 0.1%
± 0.2%
Minimum Span
0.1 inH2O
0.4 inH2O
Response Time
150 msec
450 msec
FMEDA Report
IEC 61508 Certified (SIL 2)
Specification Conformance

Draft Pressure Transmitter Introduction

Level transmitter configuration can be very time consuming. Calculations required to determine proper range values for traditional transmitters can become complex due to the physical layout of an application.

With maintenance shops getting smaller, finding equipment that allows us to do more with fens becomes a priority. DPharp transmitters with advanced software functionality can eliminate these complex calculations.

Draft Pressure Transmitter Application

Using typical smart or conventional products, all the following must be considered:

The specific gravity of the processSGP
Precise location of 0% and 100%

Specific Gravity of the capillary fill fluid
(or sealing fluid used in impulse piping)

Exact orientation oi the transmitter to the vesselH2
Vertical distance between the process conn.H1

Depending on the application, the vessel may be open (referencing atmosphere) or closed (under a blanket pressure).

Elevation is typically used when the vessel is closed. To reference the blanket pressure, a low side remote seal may be used (or a wet leg). The capillary on remote seal creates a negative force on the transmitter equal to the vertical height times the specific gravity of the fill fluid.

Elevation = (H1 + H2) x SGFF

Suppression is a positive pressure created on the high-pressure side of the transmitter typically due to the transmitter being positioned below the 0% process connection. Suppression is present in both open arid closed vessels. Suppression is equal to the vertical distance between the 0% process connection and the transmitter times the Specific Gravity of the fill livid.

Suppression = H2 x SGFF

Span is the vertical distance between the process connections times the process medium’s Specific Gravity.

Span = H1 x SGP

Figure 1: Closed Tank
Figure 1: Closed Tank

Now that you have the ElevationSuppression, and Span, the calibration values can be calculated for the 0% (Empty) arid the 100% (Full).

Cal Value (0%) = Suppression – Elevation

Cal Value (100%) = (Suppression + Span) – Elevation

Example: (using figure 1)

SGP0.9H210 inches
SGFF0.8H120 inches

Cal Value 0%) = Suppression – Elevation
Cal Value (0%) = (H2 x SGFF) – (H1 + H2 x SGFF)
Cal Value (0%) = (10 x 0.8) – (20 + 10) x 0.8
Cal Value (0%) = 8 – 24
Cal Value 0%) = -16 inH2O

Cal Value (100%) = (Suppression + Span) – Elevation
Cal Value (100%) = ((H2 x SGFF) + (H1 x SGP)) – (H1 + H2) x SGFF
Cal Value (100%) = ((10 x 0.8) + (20 x 0.9)) – (20 + 10) x 0.8
Cal Value (100%) = (8 + 18) – 24
Cal Value (100%) = +2 inH2O

Therefore, Calibrated Range would be:

-16 inH2O +2 inH2O

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