Optical Flow Meter based on Time of Travel Method

 
This article is about an DIY optical flow meter for Measuring Urine Output. But you can measure also water flow.

Abstract
Critical care units have sophisticated commercial monitoring devices for measuring vital signs automatically but urine output is one of the most relevant physiological parameter that is still measured and supervised manually. Several methodologies have been investigated and proposed to achieve accurate urine output monitoring. An alternative method, time of travel, provides with relatively low cost and simple sensing circuitry continuous accurate measurements of patient’s urine output. We demonstrate this method with a device that records the time duration a liquid column needs to travel, in a transparent tube, between two optical sensors. The average flow rate is then calculated from the time duration, the distance of the two sensors, and the diameter of the tube. This device is able to measure urine flows of 1.5 ml/hr in 30 sec. The accuracy of the system increases as flow rates decrease. The measuring intervals get shorter than 3 sec for flow rates higher than 15 ml/hr. We also discuss which parameters can affect the accuracy of measurements.

 Introduction

Critical ill patients are exposed to threading life conditions. Early recognition of symptoms can help medical doctors to have an early diagnosis and effective treatment. Critical care units are equipped with sophisticated commercial monitoring devices that are capable of sensing most of the patient’s critical parameters 24 hours a day. Heart rate, blood pressure, blood levels of oxygen saturation, respiratory rate are just a few examples. All these parameters are being recorded automatically and can be inspected at any time. If a parameter is out of a specific range of values, this is being considered as a corresponding symptom and the devices generate audible warnings to alert the health care staff. The overall result is early diagnosis and a considerable reduction in the workload of the healthcare staff and reduction of a repetitive and monotonous task which is prone to errors.

Urine output is one of the critical parameters and often measured manually with ordinary urine meters. Such a urine meter usually consists of a flexible plastic tube connected to one side with patient’s Foley catheter and the other end is connected to a graduated container where the urine is collected. Every hour the nursing staff manually records the reading of the container of each patient and operates a valve which releases the urine into a larger container. Manual measurements require handling of the urinary collection system, visual assessment and manual data recording, actions that are easily affected by human errors.

Urine output usually is measured in ml per hour (ml/hr). Oliguria is defined as a urine output that is less than 1 ml/kg/h in infants and less than 0.5 ml/kg/h in adults. Anuria is defined as less than 2ml/hr (les than 50ml urine output per day). Polyuria is a condition usually defined as excessive or abnormally large production or passage of urine (at least 2.5 or 3L over 24 hours in adults).

Although conventional urinary output measurement is prone to errors this constitutes a simple and accurate method for measuring urine volume in the ICU (deviations of 2.6%) (1). A device capable of measuring the patients’ urine flow rate and supervising the attainment of the established therapeutic goals, would release the healthcare staff from a considerable amount of work, and would permit measurements to be carried out more frequently.

 Related Work

Flowmeter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. There are many kinds of flow meters for industrial use. Flow meters are classified by the method they use for measuring (ultrasonic, electromagnetic, differential pressure flowmeters, thermal mass flow sensors, etc). Industrial flow meters haven’t been used for urine output measurement for many reasons. High cost, low urine flow rates, chemical compatibility of urine with flowmeter’s wetted parts and the restriction of reusing components of the device that is in contact with the urine, are just a few reasons.

The first commercial device which automatically measures the urine output of critical care patients dates from 2009. Medynamix (has been bought by Flowsense) developed a device, Urinfo 2000, that measures urine output every minute by counting the number of drops of urine produced by the patient. Its accuracy is reported 8% (+/- 25 mL) (2).

Other methods have been also investigated in the last few years. A siphon based method has been studied and authors reported measuring an error of +/-2% (3).  A volumetric method with capacitance sensor also reported to have a measurement error of +/-1.5%. (4).

An ideal urine output flowmeter should be relatively accurate, low cost, safe, should carry out flow rate measurement and totalization, should measure flow rate of 0.5 ml/hr up to 500ml/hr (urine flow rates higher than 500ml/hr are very rare) with acceptable accuracy in all range of flow rates and with continuous recording or at least should have the minimum time intervals between measurements. The range of urine flows between 5ml/hr to 200ml/hr is the most common in everyday practice and measurements in this range should be the most accurate. The measurements should not be affected by the temperature, pressure and urine characteristics as density, pH. The ideal flowmeter should have a local display on the flow meter and an electronic signal output, should need minimum calibration and maintenance. The components of the device in contact with the urine should be possible to be replaced easily by the healthcare staff, should be low cost and also should last as long as possible. The flow meter should be small size, with easy installation and measurements ideally should not be affected from position (horizontal/vertical) and even better from any movement.

Materials and Methods

‘Time of travel’ describes a variety of methods that measure the time that it takes for an object, particle or acoustic, electromagnetic or other wave to travel a distance through a medium. The travelling object may be detected directly or indirectly.

The bubble time of flight method is an ‘old fashion’ method that uses a bubble as the travelling object. In medicine was proposed in 1960 as an accurate method for determination of the blood flow during haemodialysis (5). Nowadays, this method is been used in Microfluidics as a simple, low cost, precision, fluid-flow sensor. (6) For our project this method had several problems which we will discuss later.

Fig. 1 The Optical Flow Meter

The device presented in this paper is based on the ‘time of travel’ method and uses liquid (urine) column in a tube as the traveling object which is being detected by optical sensors. Its operation is pretty simple. Urine enters the inlet and moves inside to the plastic tube, past the sensor block toward the outlet. As this happens, air is injected into the tube by a pump. This creates separate columns of liquid that move inside the tube, and toward the optical-sensor block. The meniscus that is formed by these columns of fluid against the plastic tube walls is measured by the optical sensors (the crescent shape of the fluid interface is called a meniscus). Since the meniscus travels at the same rate as the column of fluid, measuring the rate of meniscus-travel gives a direct correlation to the urine flow. Two infrared sensors located within the sensor block time the travel rate of the meniscus, and this volume-over-time measurement is then converted to a flow rate and displayed on a digital readout. The process then repeats itself as the pump creates another air gap. Urine flows from outlet into an ordinary graduated container that collects the urine.

The main parts of this device are two pairs of optical infrared sensors (emitter and receiver), a T-type round plastic tube, peristaltic air pump, microcontroller and a display screen.

Fig. 2 Optical Sensor

- As a sensor block we used a dual optical sensor (OMRON EE-SX1031 Photomicrosensor). The distance between the two pair of sensors is 7.1 +/- 0.13 mm with a 0.5-mm-wide aperture for each sensor (Fig.2). The distance between emitter-phototransistor for each pair is 3.4 mm and height of each element is 2-2.1 mm. An adaptor was used to keep the tube aligned with the sensors.

  

- A T-type connector was used to combine liquid input and air input.

Fig. 4 Stepper Motor

- The peristaltic pump is made of a 5 volt geared stepper motor with motor controller and a universal aluminium mounting hub for 5mm Shaft Pair, handmade rollers and handmade housing a 3D printed peristaltic pump (Fig.4)

Fig. 5 Microcontroller (Arduino Nano V3.0).

- We used an open source single-board microcontroller (Arduino-NanoV3.0, ATMEGA328P) which includes a 16 MHz crystal oscillator. (Fig.5)

-  As a digital readout we used a 1.8" TFT LCD Module Serial Display.

-  Other components we used: rotary encoder (0.2 $USD), 18mm speaker (3.5 $USD), wireless Bluetooth transceiver module, general plastic case 20x60x100 mm, some screws.

Detection principle

The detection of air or liquid in a transparent tube is based on their difference in refractive indexes. An air filled tube disperses light while a liquid-filled tube focuses light, Fig 6. The reason is that the refractive indexes of liquid and tube are quite different (Fig.6) (7).

 

Optical Sensors

Fig. 6 The principle of detecting liquid in a glass capillary

using the difference of refractive indexes.

(a) Air-filled capillary disperses light.

(b) Liquid-filled capillary focuses light.

(Adapted from [Z. Yang and R. Maeda, Proceedings of 1^st

 Annual international IEEE-EMBS Special Topic Conferenc

on Microtechnologies in Medicine & Biology, Lyon, FRANCE, p. 288 (2000)])

The optical sensor we used is a dual infra-red (IR) (IR is chosen over visible light for it is less susceptible to interfering light) and consists of two pairs of IR emitter and a phototransistor. Each element is about 2mm height and 0.5mm wide. These sizes were found to be very large for sensing a tube with OD 2.65mm. The phototransistor could not sense the difference between tube with air and tube with fluid. So we made an adaptor which positioned the tube to the upper level of the sensors and side by side with the phototransistor. With this configuration only 0.5mm of the emitter was active and the phototransistor could sense the difference properly.

The optical sensors powered as the sensor’s datasheet propose. The phototransistors’ analog signal measured from analog inputs of the microcontroller. The microcontroller contains a 10 bit analog to digital (ADC) converter. The tube aligned in precise position by measuring the same voltage in both sensors when tube was filled with air or water. The trigger point for the timer start/stop was set at midway of sensors maximum and minimum values.

We could measure sensors distance with 0.1mm accuracy. The systemic error because of wrong measurement between two optical sensors could be +/- 1.4%.

The tube

Surface tension, which is a measurement of surface energy, is the property (due to molecular forces) by which all liquids through contraction of the surface tend to bring the contained volume onto a shape having the least surface area. Surface tension has the dimension of force per unit length or of energy per unit area. The two are equivalent – but when referring to energy per unit of area most engineers use the term surface energy, which is a more general term in the sense that it applies also to solids and not just liquids. Polyethylene (PE) and polypropylene (PP), have critical surface tensions of 31 and 29 mN/m respectively. The tube we used is made of Polytetrafluoroethylene PTFE
(Teflon™) plastic which has lower surface tension 18 dynes/cm. The surface tension of distilled water is 72 dyn/cm at 25 °C (77 °F). The lower the surface tension of the solid substrate relative to the surface tension of a liquid, the less will be its “wettability”, and the higher the contact angle. As a general rule, acceptable bonding adhesion is achieved when the surface tension of a substrate is approximately 10 dynes/cm greater than the surface tension of the liquid. In this situation, the liquid is said to “wet out” or adhere to the surface (Surface Wetting & Pretreatment Methods).

The hydrophobic properties of the main body of the tube prevent the formation of typical bubbles when air is injected in the tube. Instead, a stagnant bubble is created. In fact these kinds of bubbles, with large tensions in small round tubes they stick in the tube. As air is injected, they get larger and when their radius become larger than the tube radius, then they plug the tube, they prevent any draining and actually separate the liquid column with an air gap (Fig.7, 8, 9). (8).The air gap now will ‘travel’ with the same speed as fluid due to pressure difference, independently of the tube position.

Fig. 7 Air gap plugged in PCV tube. Water was used in this situation.

Fig. 8 Bubble in PVC tube. Ethyl alcohol instead of water was used

(surface tension for ethyl alcohol is 22.3 dyn/cm).

Fig. 9 The liquid at the wall drains under

gravity withno pressure gradient.

(Adapted from [Ellipsoidal model of
the rise of a Taylor bubble in a round tube])

Typical bubbles draining water between bubble surface and tube wall (Fig. 9). Because of drainage at the wall, they don’t move at the exact same speed as the fluid. They can move upwards if the tube is in vertical position even if the fluid is stationary (Ellipsoidal model of the rise of a Taylor bubble in a round tube, Fig.9)). Due to these properties, typical bubbles would make our device more inaccurate, unstable and position-dependent.

The air injector (T-junction) of the tube was placed 2mm before the first sensor (sensor A). Sensor A acted as negative feedback for air pump by stopping it when air in the tube was sensed. Air-injector was placed at this distance to be slightly larger than the tube diameter. At this position the time intervals of measurements would be the minimum and the needed air could be injected.

The PTFE tube we used was a common tube and its diameter was referred to be 2mm. By measuring diameter with 0.1mm accuracy the systemic error could be 6.6%. 

The pump

The T-tube has two legs. One is used as a fluid inlet and contains fluid. The other is used as an air inlet and is been connected to rubber latex tube at the end, which is part of the peristaltic pump. This one contains filtered air. The main body is used as an outlet and contains both liquid and air. In vertical position of the T-tube, fluid inlet leg has positive hydrostatic pressure and the outlet negative hydrostatic pressure. The pressure at T-junction level will be equal to the sum of inlet hydrostatic pressure and outlet hydrostatic pressure of the tube. Depending on the position of input and output T-junction can have positive or negative hydrostatic pressure. If it has positive pressure then liquid tends to flow toward the pump side (air inlet). If it has negative pressure air will pass in the main body. If one of these conditions happens as we measuring then the flow at the outlet tube will change and the measurement will have serious unpredictable systemic error.

Fig. 9 The peristaltic pump.

So, the rubber latex tube should remain closed all the time and air should be injected under control. A peristaltic air pump was selected for this reason. A geared stepper motor has been used for the pump as more accurate. A motor controller for the stepper motor connected to microcontroller.

Flow rate of pump was set at about 20ml/hr. The relatively low air flow didn’t influence air gap formation even at high fluid flow rates. Low air flow selected because trapped air and rubber latex tube were acting as a spring. Sadden movements of pump were creating small oscillations of the fluid in outlet tube which affected measurements especially at higher flow rates. Due to this, air pump should slightly ‘push’ rather ‘press’ air. Also, the length of latex tube and the length of outlet were minimized.

The microcontroller

The microcontroller is been programmed as follow:

1)                If no measurement is in progress and liquid is sensed in both sensors then start peristaltic pump (inject air) until air is sensed at sensor A. This creates separate columns of liquid that move inside the tube.

2)                If air is sensed at sensor A, stop the pump and wait water-air interface (meniscus) to pass. This is sensed as air to liquid change.

3)                If water-air interface passes from sensor A, then start the timer. Now measurement is in progress.

4)                If  a measurement is in progress and sensor A senses air then cancel measurement.

5)                If water-air interface passes sensor B and measurement is not canceled then stop timer, measurement ended successfully.

6)                Calculate time duration and flow rate. Display all data on the screen.

7)                Start over.

8.      References

1. Accuracy of conventional urinary output monitoring in the ICU
2. Accuracy and Ease of Use of a Novel Electronic Urine Output Monitoring Device Compared with Standard Manual Urinometer in the Intensive Care Unit
3. A Low-Cost Device for Monitoring the Urine Output of Critical Care Patients
4. A Device for Automatically Measuring and Supervising the Critical Care Patient’s Urine Output
5. The Accuracy of the Bubble flow Method for determination of the blood flow during haemodialysis
6. Simple, high-precision, microliter per minute, fluid-flow sensor
7. Z. Yang and R. Maeda, Proceedings of 1^st Annual international IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine & Biology, Lyon, FRANCE, p. 288 (2000)
8. Ellipsoidal model of the rise of a Taylor bubble in a round tube


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