Category Archives: pressure transmitters

Differential Pressure Transmitters for Kiln and Furnace Applications

Many industries rely on large furnaces. For example, the gas and oil industry uses a great deal of heat to refine and process crude oil. The steel industry must heat metals as part of heat treatment processes that occur in special furnaces. With new EPA regulations and efficiency considerations, it’s important to keep pressure low and this has led to special draft range differential pressure transmitters like the DR800. This instrument provides several important benefits in furnace operations.

The Importance of Furnace Draft

The flow of air within a furnace is a vital to energy efficiency and heat control. But what is draft? Draft is the difference between atmospheric or room pressure and pressure within the furnace combustion chamber. This affects the flow of airneeded for the combustion process.

Drafts can be natural, forced, induced or balanced. It is important to monitor this airflow at all times, as it can greatly affect temperature and heat transfer. Draft range differential pressure transmitters are able to detect very low amounts of pressure and send their readings to a furnace control system. In fact, the DR800 can measure pressures as low as 0.1 inches H2O full scale, delivering accuracy to within 0.5 percent. The DR800 is extremely stable, even though large ambient temperature ranges.

Thanks to the DR800, pressure calibration is uncomplicated. It is made possible by including a Hi/Lo gain jumper and continuous the span adjust. You also can choose an LCD display for local indication.

The DR800 is very durable, but should you need repairs they can be done right in the field. You can easily remove its entire electronics housing without disassembling the unit. Plus, your maintenance personnel won’t have to carry specialized tools for the job. The NEMA 4 enclosure and standard process industry form factor make the DR800 easy to use.

To check out the differential pressure transmitters we have to offer, visit our home on the Web today at or call 818-886-8488.

Pressure Measurement in Engine Test Cells

Does synthetic oil reduce bearing wear? Do gasoline additives really improve combustion? These questions are investigated in a special laboratory known as an engine test cells. There are many pressure measurements needed on an engine: oil pressure, exhaust pressure, coolant pressures and any number of emission pressure measurements. Pressure transducers must be protected from the ambient environment of the test cell and still provide useful signals to a data acquisition system.

An engine test cell is a noisy, dirty and cramped room containing a running engine and everything needed to conduct the test and make the required measurements. The engine must have a sturdy mounting and a dynamometer to simulate loads. There must be piping to remove the exhaust gases and to bring in clean air. There must be adequate ventilation so that heat exchanged by the radiator can be removed from the test cell.

Pressure transducers used inside the engine test cell are mounted in a large protective enclosure that usually hangs from the wall or the ceiling. Sometimes a portable enclosure is used that has wheels so the sensors and transducers can be moved for different engine configurations.There are often several dozen transducers inside the enclosure and all of the power and signal wiring must be brought out to the data acquisition system outside the test cell. The plumbing for the transducers is a series of hoses that are connected to the outside of the enclosure and run to various points on the engine.

Engine Test Cell

Because the enclosure holding the transducers is tightly packed, the ideal transducer must be compact and easy to install. The cramped conditions in the typical test cell also require that the transducer be highly reliable; changing out a pressure transducer during a long-term engine test will inevitably create gaps in the test data. A high-level output from the transducer reduces cost for external signal conditioning for the data acquisition system.

The Validyne P55 has been successfully used in test cell environments for many years and provides the rugged stability needed for this challenging environment.  New versions of the P55 with a CAN Bus interface are now able to connect directly into the data stream from the engine’s processors.

Stand-Alone Pressure Transducer or Sensor + Electronics?


Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

pressure transducer






Type 2 – A variable reluctance sensor and supporting carrier demodulator electronics.

pressure transducer







Type 1 category products include models P55, P61, P66, the P895 family and the DR800 and P532 process transmitters.

Type 2 category products include models DP15, DP360/363, DP103 with carrier demodulator models CD15, CD23/223, CD280 and CD17.

The transducers in both categories measure the same pressure ranges – so why would you choose one type over another?

Cost Effective DC Power and DC Signal:

Type 1 category transducers are generally more cost effective per point than are the sensor + electronics (Type 2) category. The Type 1 products come ready for DC power and produce a high-level DC signal, +-/5 Vdc or 4-20 mA. The Type 1 transducers include temperature compensation and are also available with higher accuracy because we can program corrections to sensor errors into the microprocessors in these products.

Type 1 products are generally ‘plug and play’ devices and are ideal for permanent installations.

Type 1 products, however, do not lend themselves to the changing of pressure ranges easily. It is possible to disassemble the sensor on a P55, for example, and replace a damaged diaphragm or install a diaphragm with a new range – but the correction factors and temperature compensation in the microprocessor will not be matched to the new assembly. Validyne can do this – and include new temperature compensation and error correction factors – but this takes time and has a cost.

Easy Range Changing:

The biggest reason to use Type 2 products is for convenient range changing. A DP15, for example, will be easier to disassemble and easier to replace a diaphragm than the Type 1 units. The sensor will be easier to calibrate with the zero and span adjustment ranges built into the external carrier demodulators. If fast frequency is important, the smaller variable reluctance sensors can be more conveniently close-coupled to piping than the larger Type 1 units and the electronics supporting Type 2 sensors have a higher low pass filter frequency available – up to 1 Khz.

Type 2 products are best suited to laboratory settings where pressure ranges are frequently changing, where a digital display is needed and where installation flexibility is important.

Type 2 products, however, do not have built-in temperature compensation, must be calibrated by the user with an appropriate pressure standard and are generally more expensive per measurement point.

Interfacing 4-20 mA Current Loops to the USB2250

Many pressure transducers and other field instruments use the two-wire 4-20 mA current loop for both power and signal. The 4-20 mA current loop is economical to install, using the same two wires for power and signal. It is also ideal for sending a signal over long distances – up to a mile or more – with high resistance to noise. Most data acquisition devices, however, are configured to accept voltage signals. This application note will describe how to interface a standard two-wire 4-20 mA current loop to the USB2250 sensor interface.

A typical 4-20 mA transmitter receives power from an external power supply. The power supply must be able to provide enough voltage and current to power the transmitter under all operating conditions. The transmitter will require some voltage just to produce a signal and the power supply must provide this plus any power needed to overcome any resistances placed in the current loop. The maximum amount of current required by the transmitter will be at least 20 mA, but it is best to select a power supply that will provide for 25 or 30 mA through the loop to allow for over-range indication by the transmitter.

To interface to a data acquisition device such as the USB2250 a resistor is placed in the loop and the voltage drop across the resistor will be connected to the USB2250 as a single-ended voltage input. To see how this works, assume the following conditions:

Minimum voltage required by the transmitter = 12 Vdc
Maximum loop current = 25 mA
Interface Resistor = 250 Ohms
Wire or other miscellaneous resistances in the loop = 20 Ohms

To calculate the voltage required for the power supply, we add up the voltage drops in the loop:

12 Vdc for the transmitter
Voltage drop through the wire = 0.025 * 20 = 0.5 Vdc
Voltage drop through the interface resistor = 0.025 * 250 = 6.25 Vdc

Adding these voltage drops together, the minimum voltage provided by the power supply must be 12 + 0.5 + 6.25 = 18.75 Vdc to push 25 mA through all the resistances in the loop. For a single loop the power supply will need to be rated for at least 18.75 * 0.25 = 0.47 W, but typically a single power supply will power many loops before wattage ratings become an issue.  We can use a 24 Vdc power supply – just as long as it is greater than 18.75 Vdc.

The diagram below shows how the current loop is interfaced to the USB2250 terminal block. Note that the power return is common to the USB2250 signal ground. The voltage drop across the resistor is 1 Vdc when the signal is 4 mA and 5 Vdc when the signal is 20 mA, being proportional in between.

4-20 mA USB2250

USB2250 scale and offset factors in Easy Sense software are used to convert the 1 to 5 Vdc input into engineering units. If, for example, the 4-20 mA signal is for a 100 psig pressure transducer, then at an input of 1 Vdc the pressure is 0 psig and at 5 Vdc the pressure is 100 psig.

The algebra is simplified by first determining the scale factor = change in pressure/change in voltage = 100/4 = 25.

Multiply the scale factor by -1 to obtain the offset factor: 25 * -1 = -25.

Check by determining the readings at each end point:

At 1 Vdc the reading will be R = (25 * 1) -25 = 0 psig

At 5 Vdc the reading R = (25 * 5) -25 = 100 psig

The USB2250 will read the input from the current loop and provide readings in psig or any other engineering units.

Soil Consolidation Testing

Soil consolidation testing is used to predict the ability of a certain soil to bear a load safely. Perhaps the most spectacular example of this is the leaning tower of Pisa. Construction was started in the 1173 and the tower took 200 years to complete. During the first 5 years of construction the soft soil on one side of the foundation began to consolidate under the weight of the stone structure and it began to lean. Construction was halted, for a variety of reasons, until 1272 and by this time the soil had stabilized and the rest of the tower was curved to compensate. Eventually, in the late 20th the tower structure was reinforced and remains a famous tourist attraction.


Large buildings today are built on foundation soil that has been tested for compaction before construction begins, and this is why modern buildings that are heavier and taller than the Tower of Pisa can be constructed.

Soil Compaction Testing:
To test soil for unsuitable compaction, a test of several samples is performed in a soils laboratory. The idea is to simulate the conditions of load and water pore pressure in the soil and then measure the amount it compacts, or is compressed under the load. Here is a sketch of a soil compaction test cell.

This seems complicated but the idea is that a static load is applied to piston that transmits the force to the soil sample contained within the test cell. Water is introduced and kept to the pressure expected for the construction site and this pressure is sensed and controlled by a pressure transducer. The force of the load is sensed by a series of strain gage load cells and the movement of the sample as it compacts is measured by an LVDT.
A compaction test may take a few hours or several days, depending on the soil conditions and applied load. The idea is to verify that the soil will be strong enough to bear the weight of the building to be built on that particular site.

Commercial Soil Compaction Testing System:


In this example a servo-controller is used to apply a continuous load to the sample instead of heavy weights which would make for a bigger and less stable fixture. The load cell just beneath the top platten measures the actual force applied and a spring-loaded LVDT measures the change in thickness of the soil sample as it consolidates.

Validyne pressure transducers are used to measure pore pressures and our USB2250 signal conditioning can be used as the data acquisition interface to record pressure, the force sensed by the load cell and the sample displacement from the LVDT signals. The USB2250 can accept all three sensor types directly, including other sensors such as themocouples and DC voltages. The variety of sensor types used in soil compaction testing – load cells, LVDTs and pressure transducers – make the USB2250 an ideal sensor interface. Validyne will work with customers to integrate the USB2250 into existing soil test software.