This manual covers the 733882 and 733999 DLP 2.5 Low Pressure Transducer. The primary application for this transducer is the measurement of respiratory flow in combination with a pneumotachometer. The pneumotachometer is based on the principle of pressure drop along a pipe. Any pipe offers some resistance to gas flow owing to the internal resistance of the medium, i.e. the gas viscosity. The pressure drop along the pipe can be used as a measure of the flow. If the flow is sufficiently low so that the flow is laminar, then the pressure difference measured between two points along the pipe is proportional to the flow.

The basic sensor is a monolithic silicon piezoresistor. The resistive element is embedded on a thin silicone diaphragm. The special manufacturing technique as well as the configuration of the membrane result in a high sensitivity and a low temperature drift. The sensor is mounted in a user friendly plastic case. The transducer can be fixed on a stand using the removable mounting rod. Electrically the transducer is equivalent to a Weatstone bridge and can be connected to any DC bridge amplifier. 

Technical Description

Pressure connection

The transducer is a differential pressure transducer and therefore equipped with two pressure ports. These are labeled (+) for the positive pressure side of the membrane and (-) for the negative pressure side.

Pressure media

The pressure transducer can be used without restrictions with dry air. If water vapor containing air is used as media the user must take care that condensing water drops will not enter the ports. To prevent this, the transducer must be mounted in an inclined position with the ports looking downwards.

Electrical Connection:

This transducer can be connected to any type of full bridge amplifier also sometimes named strain gauge amplifier.

Application

The DLP2.5 is equipped with a stainless steel rod for mounting on a laboratory stand or on a mounting rod on a unit such as a system for isolated perfused lung. The rod can be mounted in two different positions depending on the requirements. Position 1 in line with the pressure inputs, position 2 in 90 degrees to the pressure inputs. The rod shows a small hole where a little screwdriver can be introduced to tighten the rod on the transducer. 

Connections

The DLP2.5 is a differential pressure transducer Therefore it has two pressure input ports. The ports are named (+) and (-). The output signal of the transducer is proportional to the pressure difference between Input port (+) and input port (-). If the input port (-) is left open to air the transducer can be used to measure a pressure referenced to the atmospheric  pressure, of course in the range of ±2.5cmH₂O. 

The input ports have a diameter of 3mm and are connected to the differential pressure source (Pneumotach) via tubing. As we are dealing with very low pressures it is recommended to keep the dead volume of the connecting tubing as small as possible, that means as short and as narrow as possible to reduce the compressible air volume and the mass of air to displace. 

Example of connection to a pneumotachometer

A pneumotach is a device generating a small pressure difference proportional to the airflow passing. The DLP2.5 is used to measure that pressure difference and therefore generate an electrical signal proportional to the airflow. The pneumotach consists of a tubing with an airflow resistance built in. The pressure difference across this resistance is measured with the DLP2.5.

The pressure difference is available on two ports of the pneumotach which must be connected to the two pressure inputs of the DLP2.5. The (+) input port of the DLP2.5 is always connected to the port of the pneumotach on the side where the airflow centers the pneumotachometer during inspiration.

That is different if the pneumotach is used in combination with an intubated / tracheotomized animal, or with a plethysmograph box. With an intubated / tracheotomized animal the side of the pneumotach not connected to the animal is the side where the air enters the pneumotach during inspiration (+ input port). With a pneumotach in combination with a plethysmograph box, we measure the changes of the body volume. During inspiration ,the body volume increases and air escapes from the plethysmograph box, the side of the pneumotach being connected to the box is connected to the (+) input port of the DLP2.5.

Standard HSE Pneumotach for Mice PTM0.9 or Rats PTM1.2

Output Signal of the HSE Pneumotachs for mice or rats

Calibration

Note: In the following procedure we expect that an amplifier TAM-A is used as well as the software "PULMODYN" 

Zero Adjustment

Zero adjustment (on the bridge amplifier) is used to correct inaccuracies in the sensor system at pressure = zero.  Zero adjustment has to be performed before each measurement, and should if necessary be repeated during longer series of measurements. Ensure that bridge amplifier and pressure transducer are at their operating temperature during zero adjustment. With both input ports open to air adjust he zero of the PLUGSYS TAM-A module. Follow the procedure described in the relevant instruction manual of the amplifier.

For the TAM-A

• Put the switch "CAL-OFF-MEAURE" in position "MEASURE"
• Press the "AUTO" button for 3 seconds
• The LED above the button should go on for a few seconds and automatically go off. The fixed and the mobile LEDs on the bar graph should now be overlapping.
• If the LED above the "AUTO" button is flashing, the range for the auto-ZERO adjustment is too small. Use the delivered small screwdriver and turn the "COURSE" and "FINE" trimmers to have the fixed and mobile LEDs on the bar graph overlapping. Restart the operation by pressing the "AUTO" button.

Calibration Value

Calibration before or during use allows the complete measurement chain to be properly matched. The measurement chain can take different forms. It consists essentially of pressure transducer, bridge amplifier, and pressure indication, recorder and/or data acquisition system by PC and appropriate software. Calibration is absolutely essential in order to obtain accurate measurements which can be evaluated. Regular calibration is also important in order to recognize any instrumental errors which may arise.
A known pressure 10mmH₂O is applied to the pressure transducer (+) input port the pressure indication is then
adjusted by altering the gain of the bridge amplifier. Calibration therefore requires a very precise pressure standard which provides the necessary calibration pressure (e.g. HSE KAL 84L, part number 73-0017 for the
230V version, 73-0016 for the 115V version).

PLEASE NOTE: the most accurate results are obtained when selecting the calibration pressure as close as possible to the measured pressure. As pneumotachometers mainly show pressure differences in the range of 0-10mmH₂O using a calibration pressure of 10mmH₂O would be a good choice.

Now apply the 10mmH₂O to the (+) input port.

For the TAM-A

• Have the switch "CAL-OFF-MEASURE" in position "MEASURE"
• The fix LED on the bar graph shows pressure zero and the mobile LED on the bar graph shows the pressure of 10mmH₂O.
• Now adjust the "GAIN" trimmer to bring the mobile LED to the required position on the bar graph. The fix LED shows the zero pressure, the top of the bar graph is 100% of the measuring range, if the mobile LED is at 50% (about 5V at the recording output of the amplifier), the maximum pressure which can be measured is then 20mmH₂O.
• This is a good adjustment for measurements with pneumotachometers.

Volume Calibration of a System

Before any measurement can be performed the entire measurement systems must be calibrated. it includes (the plethysmograph box), the pneumotachometer, the DLP2.5, the amplifier and the software. All these compounds are considered as an entity and calibrated in one step. 

Before starting calibration you must ensure that the pneumotachometer, the DLP2.5 and the amplifier are properly connected according to the relevant instruction manuals.

Note: In the following procedure we expect that an amplifier TAM-A is used as well as the software "PULMODYN" 

Calibration of a system

When the system is ready for calibration we will do the following:

• Adjusting the Transducer / Amplifier Baseline
• Entering a baseline value into the software
• Proceed with the volume calibration

Adjusting Baseline for Transducer and Amplifier

Now adjust the zero of the amplifier on the PLUGSYS TAM-A module. Follow the procedure described in the relevant instruction manual of the amplifier. 

For the TAM-A

• Put the switch "CAL-OFF-MEASURE" in position "MEASURE"
• Press the "AUTO" button for 3 seconds
• The LED above the button should go on for a few seconds and automatically go off. The fixed and the mobile LEDs on the bar graph should now be overlapping.
• If the LED above the "AUTO" button is flashing, the range for the auto-zero adjustment is too small. use the delivered small screwdriver and turn the "COURSE" and "FINE" trimmers to have the fixed and the mobile LED's on the bar graph overlapping. Restart the operation by pressure the "AUTO" button.

Entering the Baseline value to the software

Now select in the “Calibration & Scale” menu the “Start” button for “Lower Calibration Point”. This opens the small calibration window showed below, where the measured AD points and the corresponding voltage is displayed. For Baseline, the AD point value is around 2048 and the voltage should read near zero Volt. If the received value is stable quit this menu with “OK”. The zero value is now displayed. Take care that the assigned
physical value is set to 0 ml/min.

By starting the lower calibration the software reads the actual zero signal, after a stable reading 2048 (±3 to 5 points) click the OK button.

Proceed with the Volume Calibration

Select the “Start” button for “Upper Calibration Point” . This opens the small window below, where the calibration volume must be entered. Enter the volume of the calibration syringe used in this window e.g. 2ml for a rat pneumotachometer, 200 or 500µl for a mouse pneumotachometer . If you use a different value than 2ml for calibration enter your chosen value. The calibration volume should be in the range of the animal tidal volume (e.g. 2 to 4ml for rats or guinea pigs).

Enter the calibration volume:

Prepare a syringe, filled with the chosen volume of air (in our case 2 ml for a rat pneumotach or 0.5ml for a mouse pneumotach) connect the syringe to the pneumotach or the plethysmograph box. 

Continue by pressing "OK"

The next window is:

Click the "START" button to start the volume calibration. You hear a beep and in the upper scale of this window you see the measured zero line displayed in green color at 2048. Now inject the chosen syringe content of air
through the pneumotach or pull it out of the plethysmograph box. This results in a peak curve like displayed
below. This procedure only accepts positive going signals!

After 5 seconds the window is updates. Scale 2 (red curve) shows the peak with a magnified scaling. Begin and end of the peak are marked with green dotted lines. The peak flow is marked with a red dotted line. Please check
your curve. It depends a little on the speed you use to empty the syringe. Important is that the software detects a maximum (red line appears) and the beginning and the end of the peak (green lines). The area under the red curve is the same as the area under corresponding square wave. Scale 3 (blue) shows a simulated signal with
constant flow to reach the same volume over time. From this volume curve the software calculates by differentiation the corresponding airflow.

Check carefully this window for the localization of the lines. Important are the two lines showing the beginning and the end of the injection. If these lines are not properly placed repeat the procedure by using the "START" button. If they are well placed click on "OK." This will return you to the calibration window and the calibration for this restrainer is accomplished. 

Here the resolution and the maximal range for the signal is displayed. It is important to check these values. If the resolution is not as good as required (less than 0.1ml/sec) the gain of the amplifier must be increased, so that higher values for the upper calibration point are received. If the calculated range seems to small, the gain of the amplifier must be reduced. If gain has been changed also redo the lower calibration. 

AFTER HAVING DONE THE CALIBRATION PROCEDURE YOUR SYSTEM IS READY FOR MEASUREMENT. 

Faults, causes and remedies

No signal measured

• Amplifier TAM in in "CAL" or "OFF" position: set the amplifier in "Measure" mode.
• Tubing to pneumotachometer not connected or leaking
• Pneumotachometers side ports are blocked with mucus. Remedy clean the pneumotach. 

Signal out of range or signal clipping

• Too high of air flow or wrong "GAIN" calibration. Remedy recalibrate the measurement system. If the system is pressure calibrated use a lower "GAIN", e.g. 30% instead of 50%. If the system is volume calibrated, reduce the "GAIN" on the amplifier module TAM.

Signal is inverted, inspiration is displayed as a negative signal deflection. 

• Exchange tubing connected on (+) and (-) input ports, on the pneumotachometer or on the DLP2.5 side.

Technical Data

Electrical Connection: