Category Archives: flow

Basics of Pneumotach Flow Measurement

Pulmonary function testing is a medical diagnostic procedure that measures how much air you are breathing and how fast. These parameters are measured with a device known as a pneumotach. A pneumotach is essentially a light screen that is inserted in the airflow. The pneumotach creates a known pressure drop that is directly proportional to the air velocity. A pneumotach is connected to an air tube that allows the patient to breath freely. As the air moves in and out of the patient’s lungs the flow of the air creates a small drop in pressure across the pneumotach screen. A sensitive pressure transducer, like the Validyne DP45, is connected to the pressure taps of the pneumotach and produces an analog signal proportional to the flow rate. This signal is integrated to volume so that the amount of air and its flow rate into the lungs is known at each instant in the inspiration/expiration cycle. The pneumotach typically is equipped with a heater to keep condensation from forming on the pneumotach screen. A typical system is shown below.

Pneumotach Flow Measurement

Pneumotach Flow Measurement

Typical System:
The key to quantifying these measurements is knowing the relationship between flow rate and pressure drop across the pneumotach screen. Pneumotachs come in several sizes, depending on the patient size. A pneumotach used for measuring lung capacity of an adult athelete will be different than a pneumotach designed to measure breathing in premature infants. It is important, of course, to select the correct pneumotach size for the intended measurement. In any case, every pneumotach style will have a chart showing the relationship between air flow and pressure drop. An example is shown below.

Pressure Differential vs Back Pressure






The Validyne transducer measures differential pressure so the conversion of the pressure signal to flow is a matter of some simple algebra. For example, the pneumotach chart above states that the pressure drop across the pneumotach will be 17 mm H2O at a flow rate of 800 L/Min. The Validyne DP45 in its most sensitive range, is calibrated such that 20 mm H2O = 10 Vdc signal output. Since the pneumotach flow/pressure curve and the DP45 transducer are nominally linear, we can easily convert the output signal of the DP45 demodulator from volts to L/Min, as follows:

800 L/Min = 17 mm H2O and the signal from the DP45 demodulator is 10 * (17/20) = 8.5 Vdc at this flow rate. Therefore flow in L/Min = 800/8.5 = 94.12 * Vdc So the data acquisition system simply needs to multiply the DP45 output signal in volts by 94.12 to have the flow rate in L/Min.

Selecting Accessories for the Recalibrating the P55 Pressure Transducer

The Validyne P55 pressure transducer has as its sensor a variable reluctance pressure sensor that can be re-ranged for different full scale pressure measurements. The sensor can be disassembled, a new sensing diaphragm installed and the unit re-calibrated to the new full scale pressure. Some 23 different full scale pressure diaphragms are available and this application note will describe how to select and order the parts needed to re-range the sensor and interface the signal to a PC.

Sensor Parts:
A typical P55 is shown below, with the external parts identified:

P55 Parts pressure transducer





First, remove the two Philips head screws holding the sensor to the P55 electronics housing. These are located on the underside of the housing. The wires from the sensor to the electronics are very short, so take care they do not break.

To disassemble a P55 sensor a torque wrench, T27 Torx socket and a vise are needed. The tools needed to disassemble the sensor are available from Validyne and are shown below:

torquewrench pressure transducer






The sensor can be disassembled by removing the four 10-32 Torx T27 body bolts. When disassembled, the sensor body pieces separate and the sensing diaphragm and o-rings are removed. These parts are shown below:

boltsorings pressure transducer






It is good practice to replace the body bolts and o-rings when changing the range of the P55. Various o-ring compounds are available (see ordering chart).

The sensing diaphragm may now be replaced with one of a different range. A typical sensing diaphragm is shown below:

diaphragm pressure transducer






To re-range a P55 sensor the full scale pressure must be known and the correct diaphragm part number ordered. The part number for a P55 diaphragm starts with 3- and is followed by a two-digit range code. The diaphragm in the photo above is p/n 3-22 and has a full scale range of 5.5 In H2O. The other available range codes for the P55 sensing diaphragm are shown in the chart below with their full scale pressures expressed in various engineering units.

P55Ranges pressure transducer








Re-assembly is simply the reverse of dis-assembly, taking care that the torque on the body bolts is 125 In-Lb. The vise is used to stabilize the sensor body during assembly and to allow the torque to be correctly transmitted to the body bolts.

Also be sure that the bleed screws are tightly seated – these use a 5/64” hex wrench, Validyne p/n K950-0781. The sensor is reattached to the housing using the two Phillips head screws.

Calibration Accessories:

The next step is to calibrate the P555 against a pressure standard. Validyne can supply model T140K calibrator kit that includes a pressure pump and reference standard – an example is shown below.

T140K pressure transducer








The T140K calibrator kit is available in six different versions covering the available DP15 full scale pressure ranges. To calibrate theP55 connect it the SI58 digital interface and have a voltmeter to observe the analog output signal of the P55 as it appears on the binding posts of the SI58. 

SI58 pressure transducer






The SI58 connects to any USB port on a PC and is supplied with software that allows changing the internal registers of the P55 to achieve an accurate calibration. Connect the re-ranged P55 to the SI58 and the SI58 to a PC. Connect a multimeter to the SI58 binding posts to observe the P55 output signal.  

P55Cal pressure transducer





Load the calibration software and follow the instructions for applying zero and full scale pressures using the T140K calibrator. The software will adjust the P55 microprocessor correction factors to produce an accurate calibration with the new sensing diaphragm.

The SI58 software also allows the user to compensate the P55 through temperatures. The temperature range can be selected by the user as applied by an environmental chamber. 

SI58 Software pressure transducer