Absolute Pressure Transmitter

Absolute pressure transmitter with 4-20mA output; amplified voltage , strain gauge & digital output absolute pressure sensors; for measuring total pressures with reference to perfect vacuum.

Product Details

Absolute pressure transmitter

with 4-20mA output;

amplified voltage , strain gauge & digital output absolute pressure sensors;  

for measuring total pressures with reference to perfect vacuum.

What is Absolute pressure?

Absolute pressure is a particular type of pressure measurement,

which is always referred to a perfect vacuum.

Absolute pressure ranges are often labelled with an ‘abs’ or ‘a’ suffix,

to the pressure units proceeding a pressure value, e.g. 1013.25 mbar abs or 1013.25 mbara.

Absolute pressure is referred to the vacuum of free space (zero pressure).

In practice, absolute piezoresistive pressure sensors measure the pressure,

relative to a high vacuum reference sealed behind its sensing diaphragm.

The vacuum has to be negligible compared to the pressure to be measured.

Huaheng Instrument's absolute pressure sensors offer ranges from 1 bar,

or even 700 mbar as well as barometric pressure ranges.

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Absolute pressure (AP) transmitter Introduction

Absolute pressure (AP) transmitter

measure relative to a perfect (full) vacuum pressure (absolute zero pressure);

So, AP transmitters are not affected by fluctuations in the local atmospheric pressure.

All absolute pressure measurements are positive.

The letter ‘a’ or the abbreviation ‘abs’ in the unit of measure

(i.e., inH₂O(abs) or psia) indicates an absolute pressure measurement.

Huaheng Instrument absolute pressure transmitters,

use digital DPharp sensor technology to get accurate,

reliable readings to you.

Absolute pressure transmitter from Huaheng offers a broad variety of communication possibilities.

Another highlight of our sensors is the measurement accuracy of up to 0.04 %.

To provide high flexibility, remote seals can be fitted to any of the SMT series of transmitters. For further information, please see the relevant datasheet.

Absolute pressure transmitters are a type of pressure transmitter used in pressure measurement.

Absolute pressure sensor definition

The absolute pressure transmitter enables absolute pressure measurement in equipment,

such as degassing systems, distillation columns,

evaporators and crystallizers, allowing pressures at 10 MPa.

The absolute pressure transmitter,

has a measured absolute pressure signal on one side of the delta chamber,

and a high vacuum reference chamber on the other side, which is then converted to a 4-20 mA DC signal output.

absolute pressure transmitter

Absolute pressure transmitter working principle

Working principle:

During operation, the isolation diaphragm and filling liquid on the high,

and low-pressure sides transfer the process pressure to the filling liquid,

and then the filling liquid transfers the pressure,

to the sensing diaphragm in the center of the sensor.

The sensing diaphragm is a tensioned elastic element,

whose displacement varies with pressure

(for GP gauge transmitters,

atmospheric pressure is applied to the low-pressure side of the sensing diaphragm).

AP absolute pressure transmitter, the low pressure side always maintains a reference pressure.

The greatest displacement of the sensing diaphragm is 0.004 inches (0.1 mm),

and the amount of displacement is proportional to the pressure.

The capacitive plates on both sides detect the position of the sensing diaphragm.

The difference in capacitance between the sensing diaphragm and the capacitive plate,

is converted to a corresponding current, voltage or digital HART

(High Speed Addressable Remote Transmitter Data Highway) output signal.

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Absolute pressure transmitter calibration

How to calibrate the absolute pressure transmitter?

1. Install the transmitter,

open the terminal side,

connect and power up as shown in Figure 1 or Figure 2,

and warm up for 20 minutes.

2. Unscrew the trademark nameplate fixing screw on the top of the transmitter,

to expose the two change potentiometer screws shown in Figure 3,

as the transmitter full-point change potentiometer.

Adjust the zero and full scale of the transmitter as follows.

Absolute pressure transmitter calibration

3. Apply pressure to the inlet port.

At the lower pressure limit,

use a screwdriver to adjust the zero potentiometers so that the ammeter indicates a current of 4.00 mA.

4. When pressing to the upper rated pressure,

use a screwdriver to adjust the full potentiometer so that the ammeter indicates a current of 20.00 mA.

5. Repeat steps 3 and 4 above. Until the output of the transmitter meets the specified requirements.

6. After the change is completed,

install the sealing cover on both sides of the transmitter and tighten to keep the seal.


(1) There is a certain amount of mechanical space for the zero and span change screws.

The easiest way to overcome this space is to make the overshoot larger before the reverse rotation.

(2) When the transmitter is operating at a temperature extreme or vibration is high,

it may affect the zero and span change screws.

To improve the stability of the zero position,

and range setting in these environments,

after the final change is completed,

the change screw is slightly raised upward,

to make the contact between the potentiometer port,

and the surface of the change screw notch.

Applications of AP transmitters

An absolute pressure transmitter is used in applications,

where the pressure of a gas or liquid,

is isolated from atmospheric pressure changes,

for example when testing a sealed pressure vessel for leaks over a long period of time.

Example Industrial Applications:

Any critical storage and delivery application (i.e., toxic gases).

Because atmospheric conditions can fluctuate,

it is imperative that these systems be accurate and use a reference that is static.  

Applications in HVAC, water treatment,

food, paper, pharmaceutical,

chemical, petrochemical, offshore,

sub-sea and nuclear industries can be satisfied with this range of transmitters.

The difference between absolute, gage and differential pressure

To select the right pressure sensor for a specific application,

besides the pressure range,

first of all, the type of pressure measurement has to be considered. Pressure sensors measure a certain pressure,

in comparison to a reference pressure,

and can be divided into absolute, gage and differential devices. These terms will be explained,

on the basis of the piezoresistive pressure sensors of First Sensor.

Absolute pressure

Absolute pressure is referred to as the vacuum of free space (zero pressure).

In practice absolute piezoresistive pressure sensors,

measure the pressure relative to a high vacuum reference sealed,

behind its sensing diaphragm.

The vacuum has to be negligible compared to the pressure to be measured.

Huaheng's absolute pressure sensors,

the offer ranges from 1 bar or even 700 mbar as well as barometric pressure ranges.


Absolute pressure sensors are used to measure atmospheric pressure barometers or altimeters.

For these applications,

special barometric pressure ranges are offered, e.g. from 600...1100 mbar or 800...1100 mbar. (product examples: HCA-Baro, HDI)

Gage pressure

Gage pressure is measured relative to the ambient atmospheric pressure.

The average atmospheric pressure at sea level is 1013.25 mbar.

Changes in the atmospheric pressure due to weather conditions

or altitude directly influences the output of a gage pressure sensor.

A gage pressure higher than ambient pressure is referred to as positive pressure.

If the measured pressure is below atmospheric pressure,

it is called negative or vacuum gage pressure.

In general, a vacuum is a volume of space that is empty of matter. According to its quality vacuum is divided into different ranges such as e.g. low, high and ultra high vacuum.

Gage pressure sensors only offer one pressure port.

The ambient air pressure is directed,

through a vent hole or a vent tube to the back side of the sensing element and thus compensated.


A typical example of a gage pressure measurement is the control of tyre pressure. Here, the correct positive pressure above ambient pressure determines greatest tyre performance. (product examples: HCE, SSI)

Differential pressure

Differential pressure is the difference between any two process pressures p1 and p2.

So, differential pressure sensors must offer two separate pressure ports,

with the tube or threaded connections.

First Sensor's amplified pressure sensors are able to measure positive,

and negative pressure differences,

i.e. p1>p2 and p1<p2.

These sensors are called bidirectional differential pressure sensors with ranges of e.g. -1...+1 bar or -2.5...+2.5 mbar.

In contrast, unidirectional differential pressure sensors,

only operate in the positive range (p1>p2)

e.g. from 0...1 bar or 0...2.5 mbar and the higher pressure has to be applied to the pressure port defined as "high pressure".


Differential pressure sensors are used in medical devices,

to determine respiratory flow or in HVAC applications to control air flow.

An internal restriction to the flow channel,

such as a laminar flow element,

or an orifice plate generates a minimal pressure drop to the gas flow,

which is a measure of the volumetric flow rate.

Differential pressure sensors measure this pressure drop across the element (product examples: HCLA).

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How to calibrate Rosemount absolute pressure transmitter

Our absolute pressure transmitters can basically replace Rosemount transmitters,

in actual use conditions, so check the Rosemount transmitter for the above steps.

About Huaheng Instrument

Huaheng Instrument is one of China's leading suppliers in the field of sensor systems.

In the growth market of sensor systems,

Huaheng Instrument develops and produces standard products,

and customer-specific solutions for the ever-increasing number of applications,

in the industrial, medical, and mobility target markets.

Our goal here is to identify, meet and solve the challenges of the future,

with our innovative sensor solutions early on.

Huaheng supplies Absolute pressure transmitters with 4-20mA output,

for measuring pressure with an absolute type reference.

By selecting from a large range of materials,

we are able to customize the transmitter and supply remote seals ( Remote pressure transmitter ). We temperature compensate for the whole assembly to maintain the highest level of performance.

Selecting from 316SS, Hastelloy, Monel, Tantalum, Gold, and Ceramic,

we can custom build a transmitter to our own high standards,

or to your individual specifications.

Huaheng Instrument is recognized as a China leader,

for low draft range measurements,

or for dp measurements at a very high static pressure.

Absolute and relative sensors

Huaheng Instrument supplies a variety of absolute and relative sensors:

SMT3151GP/AP Pressure/Absolute Pressure Transmitter

SMT3151TGP/TAP Direct Mount Pressure / Absolute Pressure Transmitter (T Type)

3151 High Pressure Transmitter

Remote Seal Pressure Transmitter

Capacitive Pressure Transducer

4-20mA Remote Pressure Sensor Working

Diffused Silicon Pressure Transmitter

Explosion-Proof Pressure Transmitter

Low Cost Industrial Sensor Pressure Sensor

Silicone Oil Differential Pressure Transmitter

BP200 Diffused Sillicon Pressure Transmitter

Oem Smart Pressure Transducer / Pressure Sensor