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.

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.

Wind Testing of Metal Buildings

Metal buildings are relatively inexpensive, easy to construct and are used extensively for agricultural purposes in the western United States. But metal buildings are light and often present a large flat surface to winds. In areas like West Texas, where local wind conditions can be severe, testing metal structures under field conditions is an important part of specifying safe building codes.

Texas Tech has constructed a field test station in an open field near Lubbock to provide data on wind pressures that are generated under local conditions. A 30 ft X 45 ft X 13 ft prefabricated metal building was anchored to a rigid undercarriage that is mounted on a circular rail track. The entire building can be rotated to create different angles of attack to the prevailing winds. The surrounding area is flat farmland and is representative of the wind conditions in that part of the state. A meteorological tower was constructed nearby to record local wind speed, direction, temperature and barometric pressure. The diagram below shows the concept of the rotatable building.
A series of 100 pressure taps were drilled in the various flat surfaces to measure the pressures developed along the outside of the building. Inside, Validyne DP103 pressure transducers were used to measure the pressures.  The DP103 is sensitive to low pressures and has a relatively fast response.  The DP103 transducers were calibrated to +/-0.18 psi.  The recording rate of the transducer pressures is 40 times per second by a PC-based data acquisition system. Up to 96 simultaneous pressure measurements are possible and wind speeds of up to 90 mph can be accommodated.

The pressure data that is collected under various wind conditions and angles of attack are used to calculate the loading force under actual wind conditions so that the anchoring requirements and profiles of prefabricated metal buildings can be specified for maximum safety.

Connecting Strain Gages to the USB2250, Part II

The quarter-bridge strain gage configuration can be improved upon to increase the output signal into the USB2250 by adding another strain gage to the system. For the previous example, the second strain gage could be affixed to another part of the steel bar, as shown below.


The applied load and stress are the same as before, so the two strain gages will change in resistance by the same amount.

dR = Rsg * GF * E

dR = Change in resistance of the strain gage, Ohms
Rsg = nominal resistance of the strain gage, Ohms
GF = Gage Factor of the strain gage (stated by the manufacturer but usually about 2)
E = Strain, inches/inch

Assuming a gage factor of 2, the change in resistance in Ohms = 350 * 2 * 3.333 e-4 = 0.233 Ohms for each gage.

Because there are two strain gages we can construct a half-bridge circuit that will double the signal going into the USB2250:


The output of the bridges is now doubled.

In the example the strain gage resistance has changed from 350 Ohms at no load to 350.233 Ohms when 10,000 pounds is applied to the bar. The output signal of the Whetstone half-bridge is calculated as follows:

Vb = ((Ve * Rsg)/(R2 + Rsg)) – (Ve * R3)/(R3 + Rsg) = 0.0016637


Vb = voltage output of the bridge, Vdc
Ve = USB2250 excitation voltage, Vdc = 5
Rsg = Rsistance of the strain gages, Ohms = 350.233
R2 = Completion resistor, Ohms = 350
R3 = Completion resistor, Ohms = 350

So the circuit now produces 1.6637 millivolts at full load.

We can also express that as millivolts output per volt of excitation, or mV/V:

mV/V = 1000 * (Vb/Ve) = 0.33274

So the USB2250 will receive 0 to 0.33274 mV/V of signal from this circuit as the bar is loaded from 0 to 10,000 lbs.

To determine the scale factor for the USB2250 that will indicate applied load, we proceed as before:

Lb = mV/V * SF


Lb = Applied force in pounds
mV/V = signal from Whetstone bridge
SF = Scale Factor

Re-arranging this we get SF = Lb/mV.V = 10000/0.33274 = 30053

By entering 30053 into the Scale Factor box in the USB2250 software, we will now receive the readings as pounds of force applied to the bar.

Twice the signal coming from the strain gage circuit means that any noise present will be less of the total available signal. Because strain gages have such a small output, any chance to increase the signal should be implemented.