Workshop 1 Measurement techniques and sensors

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The University of British Columbia GEOG 309 / Andreas Christen January 31, 2008 Workshop 1 Measurement techniques and sensors Goals 1 Use components and a multi-meter to understand measurement

You will be provided with with a multi-meter, a battery, a screwdriver, and different components (resistors, thermistors, cables, etc.) in a small plastic bag.

Using this equipment and following the hand-out does not expose you to any risk of electrical shock. Obviously, never connect anything to 110 V wall plugs!

It can measure different electrical variables (i.e. voltage, current, resistance) and we can use it to do a continuity test. Carefully read the multi-meter s sheet before you start any measurement.

1 The University of British Columbia GEOG 309 / Andreas Christen January 31, 2008 Workshop 1 Measurement techniques and sensors Goals 1 Use components and a multi-meter to understand measurement principles usually hidden in most field sensors. 2 Understand the operation modes of a data-logger in next week s workshop. 3 Prepare you for the use of sensors in your field projects. You will be provided with with a multi-meter, a battery, a screwdriver, and different components (resistors, thermistors, cables, etc.) in a small plastic bag. Please return all parts at the end of the course in the same packing. We will not work with high voltages. Using this equipment and following the hand-out does not expose you to any risk of electrical shock. Obviously, never connect anything to 110 V wall plugs! And do not short-cut battery connector terminals - this could destroy the batteries and cause fire. The multi-meter is as its name implies a laboratory multipurpose instrument. It can measure different electrical variables (i.e. voltage, current, resistance) and we can use it to do a continuity test. Carefully read the multi-meter s sheet before you start any measurement. You all know how a battery works: Basically, it stores energy in a chemical way and we can convert this energy in a controlled way to electrical energy. Many electrical sensors in environmental field research are installed at remote sites and we often do not have access to the power grid. Therefore, we prefer the use batteries to power sensors and data-loggers. For longer-term measurements a set-up with solar panels or wind turbines to re-charge batteries is appropriate (this is not needed in the field course).

2 2 1 Voltage Remember, voltage is a measure of the difference in electrical potential between two points in an electrical circuit. It is expressed in volts (V). As a first example, let us determine the voltage of your battery, i.e. the difference of electrical potential between the positive and the negative terminal. Task 1: Read your multi-meter s instruction sheet and measure your battery s voltage. Measured voltage Is your battery AC or DC? You see - voltages are relatively simple to measure. Many field sensors output voltages that are proportional to physical parameters. We can translate these voltages to a chart recorder or digitize them on a data-logger to record time-series. We will use a data-logger in next week s workshop. Today, let us create a sensor a primary transducer that outputs an electrical signal that is directly controlled by environmental parameters. There are numerous principles and techniques, and we cannot discuss all. As an illustration, we will build different thermometers. First, let us review a basic component the resistor. 2 Resistance and resistors A resistor is an electric component that resists a current. It is producing a voltage drop across its terminals. The resistance (in!) is a measure how strong the current is resisted. For a given current, the stronger the resistance, the higher the voltage drop (this is Ohm s Law). Task 2: Read the instruction sheet and find a way to measure resistances. Identify the resistance of the three brownish resistors with the four color bands located in your set. Resistor measured resistance labeled resistance

3 Use the color-coding table below to identify the resistances. Do your values and the values labeled by the manufacturer agree? If not, what could explain the disagreement?

3 3 Use the color-coding table below to identify the resistances. Do your values and the values labeled by the manufacturer agree? If not, what could explain the disagreement? 3 Thermistors A thermistor is a resistor that changes its resistance with changing temperature as most resistors do but in a well-defined way. In a first approximation, we can assume that the change of the resistance "R (in!) due to a change in temperature "T (in K) is linear: "R = k "T k is the linear temperature coefficient of resistance (in! K -1 ). There are Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC) thermistors. PTCs have a positive k, i.e. the resistance is increasing with increasing temperature, NTCs have a negative k, i.e. their resistance is decreasing with increasing temperature. The relationship between T and R is linear in a narrow range and the exact value of k depends on the material properties of the thermistor. In environmental sciences, thermistors are a widely used sensor to measure air, water, or soil temperatures.

4 Task 3: Measure the resistance of the small, round thermistor (PNT105-ND, three color bars, see photo below) in different thermal environments. From the manufacturer we know that k = -4.2! / K, and the value at 25ºC is 100!. Using this information calculate the temperatures. 4 PNT105-ND Resistance Temperature Room air Your fingers Outdoor air Is this a PTC or an NTC thermistor? Task 4: Find a way to estimate the time constant " of the above thermistor. The time constant represents the time for a sensor to complete 63.2% of adjustment to a step change in its input.

Task 5: There is a second thermistor in your bag (KC001E-ND, metallic, larger but still round, see photo below). Use this thermistor and redo the measurements. Is this a PTC or an NTC thermistor?

5 Task 5: There is a second thermistor in your bag (KC001E-ND, metallic, larger but still round, see photo below). Use this thermistor and redo the measurements. Is this a PTC or an NTC thermistor? From your readings, and the results in Task 3 calculate its k-value and the resistance at 25ºC. 5 KC001E-ND Resistance Calculated k-value Room air Your fingers Outdoor air If we send a known current I through the resistor, we can use Ohm s law, measure the voltage drop V across the resistance, and calculate the resistance R. This is the way your multi-meter measures resistances. Further, we use resistors in environmental sensors to translate directional movements into resistances. A potentiometer is variable resistor that changes its value due to changing positions of a gliding contact (wiper) sliding around an annulus. This is used in wind vanes to measure wind direction (see Figure right). 4 Thermocouples Thermocouples are another widely applied sensor to translate temperature into an electrical signal (see last week s lecture). Thermocouples are made of two dissimilar metals / alloys. These form a simple circuit (see Figure on next page). If there is a thermal gradient between the cold junction and the exposed junction, a current will flow in the circuit, which will generate a very small voltage drop (Seebeckeffect). By using thermocouples we can only measure temperature differences. To record an absolute value, the temperature of one junction (usually the cold junction ) must be referenced against a known value. Most data-loggers have such an internal reference temperature.

The sensitivity of a Type T thermocouple is 41!V K -1. This is at the lower resolution end of your multi-meter. So let us measure a strong temperature gradient.

6 6 Left: Scheme of a thermocouple. Right: Photo of one end of a fine-wire thermocouple used to measure fluctuations of air temperature. Task 6: In your bag you will find a small piece of thermocouple cable (Type T, copper = blue shield / constantan = red shield). Use this cable to build a thermocouple. The sensitivity of a Type T thermocouple is 41!V K -1. This is at the lower resolution end of your multi-meter. So let us measure a strong temperature gradient. Use a cup of the provided hot water and measure the temperature difference between the hot water and the air temperature. Voltage created Temperature difference Difference hot water / air How fast does the thermocouple respond to a step change compared to the thermistors? Why? Thermopiles are thermocouples in series. Thermopiles amplify the small voltages measured in a single thermocouple (add them up) and therefore allow us to measure an increased voltage (n times the sensitivity, where n is the number of exposed junctions). Thermopiles are typically built into sensors that measure temperature gradients, such as soil heat flux plates, or radiometers.

7 7 Left: Scheme illustrating a thermopile. Right: A pyranometer has a built-in thermopile. The sensor translates short-wave irradiance (indirectly measured by the heat differences between an absorbing carbon plate and the instrument s body) into a voltage. Task 7: Cut your thermocouple into two pieces and create a thermopile with 2 cold and 2 exposed junctions. Again measure the temperature difference between the hot water and the air. Is the voltage changing as expected? Voltage created Difference hot water / air 5 Other sensing principles There are many other principles to translate environmental parameters into an electrical signal. Some examples are: Electric current - Wires and cables from sensors act like resistors and show temperature dependence - an effect that can falsify measurements. When dealing with long cables some devices output currents instead of voltages, because they can be conserved at the sensor output. Today, this is often replaced by digital protocols (see next week s workshop). Capacitance the electric charge stored in a material. This is used in some soil measurement sensors (soil moisture) or in air humidity sensors. Digital counters Counters are a way to record binary measurements. If a rain gauge tips it creates a short induction. By electronically counting the number of inductions (tips) on a data-logger, we know when and how much precipitation occurred. Coppers in rotating devices are another application of counters.

9/28/2010. Chapter , The McGraw-Hill Companies, Inc.

9/28/2010. Chapter , The McGraw-Hill Companies, Inc. Chapter 4 Sensors are are used to detect, and often to measure, the magnitude of something. They basically operate by converting mechanical, magnetic, thermal, optical, and chemical variations into electric