Category Archives: Data Acquisition

A Closer Look at the USB2250 Data Acquisition

As a leading manufacturer of variable reluctance pressure transducers, we at Validyne Engineering make it our top priority to provide our customers with the latest technological advances. That is why we are pleased to introduce the USB2250 Data Acquisition.

What is a USB2250 Data Acquisition?

usb2250bThe USB2250 is a sensor interface that provides real world data acquisition for your PC through the USB port. Up to 16 different sensor inputs that are accepted by the USB2250 in any mix or combination with no external signal conditioning required.

All in One Configuration via the USB2250 Data Acquisition

When we say that up to 16 sensors are accepted no matter the mix, we mean no additional equipment or configuration is required. Thermocouples, RTDs, strain gages, LVDTs, potentiometers, VR sensors and low-level DC voltages are all wired directly to the terminal block..
In addition, the USB2250 Data Acquisition features 10 full scale input ranges from ±20 mV to ±10.24V full scale all with 16 bits of resolution. There is zero offset correction for low-level measurements. The USB2250 Data Acquisition also provides polynomial linearization for thermocouples and RTD’s. It produces a floating-point value directly in engineering units.

Included Software

Data Acquisition

Software includes a GUI configuration utility that gives the user the opportunity to set sensor type, gain range, channel, and other parameters. Easy Sense 2250 data acquisition software is included with Graphic Capabilities and Trigger Functionality included in the Premium Easy Sense version.

A LabVIEW VI is available for seamless integration to your existing software setup.

Exciting Uses and Possibilities

Data acquisition is the process of measuring electrical or physical phenomenon like voltage, current, pressure, temperature, or position with a computer. The USB2250 is a vital tool for those working in the oil, automotive, or medical industries and many more!

At Validyne, we work hard to provide our customers with the best equipment experience no matter where they operate. Providing equipment like the USB2250 not only makes the job easier, but is a more cost effective solution for sensor inputs to a PC.

Car Door Seal Testing

Car Door SealWhen automobile doors are closed it is expected that the sealing surfaces around the edges of the door will contact the frame properly so that the passenger cabin is weatherproof and the inside protected from rain and water. With all the different styles of doors, frames and gasket materials each new model must be tested to verify that the sealing system is effective.

One way to test the seal is to measure the pressure rise inside the passenger cabin when the door is slammed shut. If the seal is effective there will be a brief rise in pressure. We have probably all experienced this – Volkswagen Beetles were notorious for the ear discomfort on door closings as they were deigned to be waterproof. So a balance between sealing effectiveness and comfort is desired and testing the pressure rise is one way to verify that the right combination of sealing materials is being used.

The measurement of a pressure rise in the passenger cabin requires a pressure transducer with sensitivity to low pressures and fast dynamic response. One automobile manufacturer uses the Validyne DP45 to measure pressure spikes on the order of 400 Pa having a duration of 10 mSec. The DP45 is available in full scale ranges as low as 220 Pa and has a flat dynamic response on the order of 60 Hz and can thus capture a transient whose rise time is 4 mSec.

The system is comprised of the following Validyne Parts:

The system cal is convenient because of the low pressures involved – we calibrate the system here prior to shipment. The customer attaches DC power (9 to 55 Vdc) to the connector and also the 0 to +5 Vdc signal wires to a high speed data acquisition system. The transducer has 1/8” female NPT ports and these are fitted with adapters by the customer to plastic tubing that is run to the inside of the automobile passenger cabin. The door is slammed several times at various velocities and the resultant pressure rises recorded. On this basis the gasket seal and firmness can be evaluated.

Reducing Glove Box Filter Costs

Glove Box Filter

Handling radioactive materials must be done in glove box. A glove box is a clear plastic enclosure with rubber gloves attached to the sides so that an operator can handle material inside the glove box, but air and radioactive dust are not allowed to escape. The air pressure inside the glove box must be controlled so that it is always less than the ambient atmosphere. A series of risers, ducts, fans and filters connected to the glove box keeps the pressure inside the glove box lower than the outside atmosphere so that no dust can escape.

If the airflow velocity up the riser from the glove box is too high, radioactive dust is carried into the exhaust system, and these particles are trapped by a series of special filters. When a filter needs to be changed, it is very expensive to dispose of the dirty filter because it is charged with radioactive dust and dirt. The key to reducing filter disposal costs is to limit the exhaust airflow velocity so that dust is not carried out of the glove box and into the filters, while at the same time insuring that the pressure inside the glove box is always less than the ambient atmosphere.

Measurement of the air velocity in the glove box risers is accomplished with a pitot tube and a sensitive differential pressure transducer. As air velocity increases, the differential pressure across the pitot tube increases. The very low velocity needed to exhaust air from the glove box, without raising dust translates into a very low pressure drop across the pitot tube. This pressure drop is as little as 0.02 In H2O under normal operating conditions. A very sensitive differential pressure sensor, combined with a high level DC signal output, is needed to provide the air velocity signal to the system controller.

A special version of the Validyne P532 provides the low pressure measurement required for such a control system and these are used at a major North American nuclear fuel rod processing facility.

Connecting the USB2250 Data Acquisition System to a String Pot

A string pot is a displacement measuring sensor that can conveniently measure displacements of a few inches to a few feet. The string pot is something like an ordinary tape measure but instead of a ruler there is a cable attached to a spring-wound potentiometer so that the distance the cable moves changes the position of the pot wiper. Simply mount the string pot securely and connect the cable to the moving part to be measured. When the potentiometer is powered, the output of the string pot is a voltage proportional to the displacement of the cable.

String pot construction is shown below along with a typical enclosed unit.

String Pot





String Pot






Electrically, the string pot is a simple potentiometer and it can be powered by the USB2250 and the signal received as a single-ended DC voltage. The connection diagram is shown below:

Data Acquisition

The +5 V power is supplied by the USB2250 terminal block. The position of the wiper changes the voltage at the A-In terminal from 0 to +5 Vdc, depending on the position of the wiper.


A simple scale factor is all that is needed to convert the voltage into a reading in inches. For example if a string pot has a 7 inch displacement, the signal will be 0 to +5 V from 0 to 7 inches. The scale factor for Easy Sense to convert the voltage signal into a reading in inches is 7 inches/5 Vdc = 1.4

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.