Hello,
I am designing a temperature controller for growing human cells for AIDS research. We want to be able to measure the temperature at a certain location in our device (very close to the cells). The temperature will be around 37C, and we want to control it to +/- 0.5C. We have looked into different temp sensors and pretty much have settled on using NTC thermistors, such as the mf51e from Cantherm, or others from Honeywell. Now, to get the highest accuracy, we will calibrate the sensors AT 37C ourselves. My question is, to get high precision, and repeatability, what type of circuit should we use? I made a wheatstone bridge + differential amplifier circuit already in both LTSpice, and on the breadboard with 2% tol resistors. It seems to work okay, but I don't have a reference thermometer yet so I'm not sure how precise my results are, or repeatable. Also, although not in the attached picture, I designed a wheatstone bridge such that the range on the thermistor that correlates to 37+/-5C results in approximately 0-500mv, which is then amplified by an op amp.
Since the 0-500mv corresponds to 0-5V, that's 100mv/V. I'm not sure what my CMRR should be based on that information. In order to increase my CMRR I would need to use higher precision resistors... But I'm wondering if it's just cheaper and more efficient to buy an instrumental amplifier? I would still need a wheatstone bridge though. Does that need to be made with high precision resistors?
Also, the rest of the system consists of heater coils, powered by MOSFETs, and water pump(s), and a PIC18F4550. If I calibrate my temperature sensor to 37C and some other points around that temperature, I can store a calibration curve in the PIC. However, how do I design my electronics so that that calibration curve is accurate when the ambient temperature changes from 20C to 30C? That's about the fluctuation in temperature I'm looking at for room temperature. I know my wheatstone bridge resistors will change values and such due to temperature fluctuations, and the opamp will change a little, etc.
In the end, we'd like our PID to have a feedback from a temp sensor in the range of 37+/-5C, and with a resolution of 0.01C or greater, and an accuracy of 0.1C or greater. I know these are tight tolerances, but is this unreasonable?
Thanks,
Matt
I am designing a temperature controller for growing human cells for AIDS research. We want to be able to measure the temperature at a certain location in our device (very close to the cells). The temperature will be around 37C, and we want to control it to +/- 0.5C. We have looked into different temp sensors and pretty much have settled on using NTC thermistors, such as the mf51e from Cantherm, or others from Honeywell. Now, to get the highest accuracy, we will calibrate the sensors AT 37C ourselves. My question is, to get high precision, and repeatability, what type of circuit should we use? I made a wheatstone bridge + differential amplifier circuit already in both LTSpice, and on the breadboard with 2% tol resistors. It seems to work okay, but I don't have a reference thermometer yet so I'm not sure how precise my results are, or repeatable. Also, although not in the attached picture, I designed a wheatstone bridge such that the range on the thermistor that correlates to 37+/-5C results in approximately 0-500mv, which is then amplified by an op amp.
Since the 0-500mv corresponds to 0-5V, that's 100mv/V. I'm not sure what my CMRR should be based on that information. In order to increase my CMRR I would need to use higher precision resistors... But I'm wondering if it's just cheaper and more efficient to buy an instrumental amplifier? I would still need a wheatstone bridge though. Does that need to be made with high precision resistors?
Also, the rest of the system consists of heater coils, powered by MOSFETs, and water pump(s), and a PIC18F4550. If I calibrate my temperature sensor to 37C and some other points around that temperature, I can store a calibration curve in the PIC. However, how do I design my electronics so that that calibration curve is accurate when the ambient temperature changes from 20C to 30C? That's about the fluctuation in temperature I'm looking at for room temperature. I know my wheatstone bridge resistors will change values and such due to temperature fluctuations, and the opamp will change a little, etc.
In the end, we'd like our PID to have a feedback from a temp sensor in the range of 37+/-5C, and with a resolution of 0.01C or greater, and an accuracy of 0.1C or greater. I know these are tight tolerances, but is this unreasonable?
Thanks,
Matt
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