How do we calibrate a thermistor in a resistance bridge?
Calibrate: to determine, check or rectify the graduation of (any instrument giving quantitative measurements) (The Random House College Dictionary, 1988).
Now consider that the chosen thermistor also has a tolerance. In this case it is ± 1 °C at 25 °C. Not only that, we have chosen our fixed resistor values by using the less accurate thermistor equation, , the only one for which we had published parameters from the manufacturer. If we wish to measure accurately using the circuit in Figure 4 with available parts, and avoid adding significantly to the datalogger measurement error, then we must calibrate it with a temperature bath and temperature measuring standard. For the purposes of this experiment, we will pretend that both are much more expensive than we actually own, and are stable and accurate to a few millidegrees. We must develop a correction equation that converts from the voltage output, Vt, of the bridge, to the temperature measured by the better standard in the calibration bath.
You have designed your bridge circuit by now, and chosen the values of Vs, Vspan, R1 and R2. We know that 0 °C is at one end of the span of measuring voltage. For the purposes of argument, let us suppose that Vs = 2V, Vspan = ± 250 mV and we have set 0 °C up at the +250 mV end of the span. If you have a different design, change the terms accordingly. This allows us to use the ice point of water at 0 °C as a fixed calibration point. We will put the thermistors in an ice bath and set R1 and R2 so that they remain close to the design values while setting the measured thermistor voltage, Vt, at the right end of the span. Then we will calibrate the thermistor/bridge circuits against a standard platinum resistance thermometer (SPRT) in a variable temperature bath. In all cases, we will used a datalogger to take the measurements. In this case, it is a Campbell Scientific, Inc., model CR10X.
We will follow this general procedure:
1. Make the ice bath and set it aside to temper.
Use distilled or dionized water for both the ice and the water in the bath. Impurities will usually cause the ice point to be depressed below 0 °C. The ice should be 1/2-inch cubes or smaller, down to slush. Put the thermistor assemblies in an insulated container and carefully pack them in ice so that they are about equidistant from each other and the boundaries of the container. Fill the container with the ice and add water about 2/3 of the way to the top, to keep the ice from floating. Ice from a freezer will begin at much below 0 °C. Let it set for as long as it takes for the ice above the water to start to melt, so that the entire mass equilibrates to 0 °C. You will be putting three or four dual thermistor assemblies in the same ice bath, so you will need about a quart or so in volume. When the ice starts to melt, it may be a good idea to cover the top with a block of insulation to keep the bath at 0 °C longer.
2. Construct the first trial bridge circuits and connect them to the datalogger.
You will be using a plastic prototyping block similar to the one in Figure 8. The holes in each row of five and twenty are connected with a metal spring clip. A wire inserted into any connected row will be connected electrically with any other wire inserted into the same row. The holes are spaced on a 0.100" grid, so that an 8- or 14-pin integrated circuit chip can be placed across the center slot, with 4 holes per row of five remaining to use in making connections. Figure 9 shows a typical arrangement of resistors for the circuits you will construct. The resistor leads are bent by simply holding the body of the resistor and gently pulling two fingers down on the leads.
Figure 10 shows the thermistors you will be calibrating, installed on the end of a three-wire instrument cable. The cable has three wire colors, red, black and clear. A green wire has been connected to the cable shield, a wrap of aluminum foil around the three inner wires. Short lengths of colored jacket have been placed over the thermistor leads to show to what color wires they connect in the cable. In this case both thermistors are connected to the black wire, and one each to the red and clear wires. These wires are also shown in the top of Figure 9, connecting to the bridge circuit on the prototype block.
Figure 11 shows the typical circuits for three pairs of thermistors connected to the CR10X datalogger. Where a line crosses another line but is broken, there is no connection. Electrical connections are where lines meet. For example all of the three bridge circuits connect via red wires to the same excitation terminal on the datalogger, EX1.
There are three thermistor assemblies, designated (Rta,Rtb), (Rtc,Rtd) and (Rte,Rtf). Notice the red, black and clear wires connecting the thermistor assemblies to the bridge circuits. The ellipses indicate cables. The bridge circuits are shown connected to the CR10X datalogger with four-wire modular telephone cable, coded black-red-green-yellow, which will be made available to you. You will place the tinned ends of the wires in the terminal holes as shown in Figure 12. Turn the terminal screw clockwise to clamp down on the wires. Then pull firmly but gently on all wires to assure that they have been caught. Figure 12 shows a (Rta,Rtb) bridge circuit connected with a different kind of four-wire cable at the bottom. The upper terminal strip shows connections for the SPRT. You should have a 22-page handout from the CR10X manual, entitled "CR10X Measurement and Control Module Overview", that goes into more explanation. If you don't get one and read it before the lab.
3. Connect the datalogger to a computer with the PC208E support software.
This is simple enough. Take the little blue ribbon cable and plug one 9-pin male connector into the female socket labeled "Serial I/O" on the datalogger. Connect the other end to the SC32A interface (Figure 13). Then connect a 25-pin male/female printer extension cable between the SC32A and a printer port on your computer.
4. Download the ice point measurement program from the computer to the datalogger.
Turn on the datalogger. There is a small toggle switch next to the green terminal block on the battery. Turn it away from the outside edge to turn on the datalogger. The computer you are going to use should already be set up with the PC208E software, and configured to work with the datalogger. You will be working either in MS-DOS or an MS-DOS window. Go to the proper directory and type "pc208e".
In the pc208e main screen, select file-open-station-OK. In the "Select Station File to OPEN" screen, use the tab key to get to the station *.stn names and select cr10lab.stn. Press Enter twice. You should now be back in the main screen. Select RealTime-Call. If you are successful, you will get an "*" in the lower dark blue window. Select Tools-Send Datalogger Program-OK. In the "Send Program to Datalogger" screen, select thrmice.dld and OK. The final "Information" screen should say "Download to Datalogger Successful". Click on OK.
Now select RealTime-Monitor. You should get a dark blue screen with the legend, "Enter:", in the lower righthand corner. On the keyboard type: L, then type 1,2,3,4,5,6,7,8,9,10,11,12 and enter. The middle screen should show outputs labeled Rta, Rtb, ..., Rtf, and Rabar, Rbbar, ..., Rfbar, with numbers to the right of each label. The Rt* labels are actually the consecutive voltage ratio measurements of Vt*/Vs, * = a,b,...,f, at the rate of about three per second. The R*bar labels are the averages of every five values. You will use these readouts to adjust your bridge circuits to the ice point.
The program thrmice.csi, from which thrmice.dld is compiled in the pc208e software, is shown in Listing 1. Instruction P5 does the three-wire bridge measurement. You should be aware of the form of it in case you have to change it to fit your particular bridge design. The first parameter is "2 reps". This, for example, makes one measurement each on consecutive single-ended channels 1 and 2, SE 1 and 2 on the datalogger (Figure 12), and puts the result, Vt/Vs (Figure 4), putting each measurement into consecutive locations Rta and Rtb (parameter 6).
You should have copies of the "CR10X Prompt Sheet". If you look under "Option Codes" you will find that parameter 2 is set to the ± 250 mV input range with an AC excitation that effectively nullifies 60 Hz noise. Parameters 5 and 4 set an excitation level of 2000 mV on excitation channel 1, E1 in Figure 12. The multiplier and offset are set to 1.0 and 0.0 in parameters 7 and 8, allowing a linear transformation of the measurement, if needed, but not used in this case.
If you wish to make changes to this program to accommodate your design, you can do so with either the pc208e software or with the CR10KD keyboard/display. If you have not learned how to do so by this lab, you must arrange with the instructor to do it. Please make as few changes as possible.