The ACQ-7700 acquisition interface can be used to collect a number of different types of signals. These can range from bio-potential, to pressure, to flow, or to signals from other laboratory hardware that have an analog out option. Collecting blood pressure using an ACQ-7700 and Universal XE is a very common procedure. In order to properly collect the signal, calibration of the equipment is recommended to ensure all pieces of the system are functioning as expected prior to collection.
How to Calibrate a Fluid Filled BP Transducer
The first step is to assign the proper pressure analysis to the channels that will be collected. Below shows the BP (Blood Pressure) analysis module selected in the Channel Input Setup menu. It is recommended to also configure the appropriate derived parameters, graph configurations and other settings desired at this point.
Once the configuration settings have been determined for the protocol, enter the 7700 Amplifier Setup dialog from the Hardware pull-down menu (see below).
This menu will provide access to all signal conditioners, such as the Universal XE, that are installed in the ACQ-7700 chassis. Each signal conditioner will have its own tab at the top of the 7700 dialog. Clicking on a specific tab will present that signal conditioner’s settings to the user to allow modification and calibration based on the specific signal collected.
The Universal XE is used to collect a host of different signal types and it commonly used for pressure collections using pressure transducers from various vendors. In most cases, the procedure is the same, regardless of the supplier of the pressure transducer. However, it is recommended to review the documentation for the pressure transducer to ensure the proper settings. In this example, a Becton Dickinson pressure transducer will be used.
General Tab - Label
Use the Label column to enter in any specific information about the signal being collected. Below shows AoP entered for channel A1. A1 refers to the signal conditioner in the first slot in the ACQ-7700. The Universal XE has four available channels, A1-A4. If a second signal conditioner is installed, this will show up as B.
The information entered under the Label column will automatically be transferred to the Channel Input Setup menu where the analysis was configured for each channel. This information helps to confirm that the input on the Universal XE is associated with the correct analysis in the Channel Input menu. Below shows the AoP label being automatically transferred to Channel 1 in the protocol.
General Tab - Mode, Filters, and Span
Next, select the proper Mode for each pressure channel. Clicking into a cell for a single input will allow you to change that specific input. Clicking on the column header, in the case Mode, will change the setting for all inputs at once. For blood pressure measurements, select Transducer mode. This will update the default settings for each channel for proper pressure collection.
In addition to changing to Transducer mode, you will notice that High and Low Pass filters have been updated. The High Pass filter has been automatically disabled as this type of filter is not desired when collecting pressures. The Low Pass filter has been updated to 100Hz to remove any high frequency interference outside of normal pressure signal content and the Span has been updated to -50 to 350mmHg. Span refers to the range that the signal conditioner will collect and display the signal. Signals that go above or below the range identified will be clipped (flat line) until they come back into the range specified here. Even though you may not collect pressures up to 350 mmHg, you may not need to adjust this range unless very small pressure signals are being collected. The software will automatically take care of scaling once a calibration is performed.
Configuration Tab – Value
Select the Configuration tab to expose the settings used for calibrating the pressure signal. Selecting Transducer from the Mode column on the General tab also populates the Configuration tab with the appropriate settings needed to configure a fluid filled transducer.
The Value column will display the current value that is seen on that input and will be used to verify calibration of the signal.
Configuration Tab – Excitation
Excitation is provided by the Universal XE signal conditioner and is used to complete the circuit for the transducer. While not every transducer requires excitation from the hardware, it is the most common configuration for fluid filled transducers and 5V is the default voltage used to power and set output voltages used during calibration.
Configuration Tab – Low and High Calibration
To perform a calibration, the Low/High Cal and Low/High Unit fields will be used. The Low and High Cal fields display the voltage that is read from the transducer. The Low and High Units are the actual values that will be used during the calibration process that are entered by the user and are associated with a specific voltage. In the example above, a Low Cal value of 0.000mV correlates to 0mmHg. A High Cal value of 2.500mV correlates to 100mmHg. The calibration is linearized across the Span of -50 to 350mmHg. Therefore, if the Universal XE received a value of 5.00mV from the transducer, a value of 200mmHg would be displayed.
The default settings for Excitation and Low and High Cal values are typical of most fluid filled transducers and are therefore automatically entered when selecting Transducer as the Mode. The association between the voltage (Low and High Cal) and pressure (Low and High Units) can be stated as 5uV/V exc/mmHg. The output from the transducer provides 5uV output for every volt of excitation per mmHg. If we have 5V excitation and calibrate at 100mmHg, the voltage from the transducer will be 2.500mV which is our default High Cal value.
Physical Calibration – Low Cal
Most transducers are not exactly 2.500mV at 100mmHg so it is important to calibrate each transducer. To physically calibrate the transducer a zero baseline should be established. Open the transducer to air so that no pressure is present on the transducer membrane that may cause an offset. Dependent on the position of the transducer at rest, there is usually some offset due to gravity or any other forces acting on the transducer causing a small amount of zero shift or offset.
Double click on the Low Cal cell for channel one. This opens a dialog box that enables the user to either enter the offset value in mV (if known), or “Measure” the value. By clicking on the Measure button, the software will accept an external voltage that will set the zero calibration of the channel to correspond to the value read.
With the transducer at rest and open to air, click on the Measure button. The offset value will be read and automatically inserted in the Low Cal cell. The example below indicates that there is 0.098mV offset with the transducer at rest. This is fine as we have removed that offset by physically calibrating the signal.
Next, double click on the High Cal cell for channel one. This opens the same dialog box seen above that enables the user to either enter the gain value in mV (if known) manually, or “Measure” the value. By clicking on the Measure button, the software will accept an external voltage that will set the gain calibration of the channel to correspond to the value read. Apply 100mmHg pressure to the transducer by using a manometer or other source and click on the Measure button. The gain value will be read and automatically inserted in the High Cal menu. The example below indicates that the gain calibration value is 2.467mV. This voltage is now associated with 100mmHg. Calibration of that channel has now been completed.
If desired, you can check linearity by changing the pressure that is applied by the manometer. This value will be shown under the Value column. Note that the value column is updated every one second. If desired, and based on application, adjusting the Span to a tighter range can be performed under the General tab by simply clicking in the Low and High Span cells and entering the desired limits.