I am using an Omega HSTH-44000 series thermistor to measure temperatures inside residential refrigerators, freezers, and dishwashers. Here is the link: http://www.omega.com/pptst/HSTH-44000.html
I will be using a CR1000 and the total cable length (sensor cable + shielded extension cable) will be 15-25 meters.
My questions are:
- Which of the available resistance values would you recommend for the thermistor? The available values are 2252, 5000, or 10,000 ohms.
- What resistance value would you recommend for the fixed resistor?
- It is a little more convenient to install the fixed resistor at the CR1000 wiring panel instead of at the sensor end. Are any sensor accuracy consequences to locating the fixed resistor at the wiring panel?
- Would you recommend installing the fixed resistor between EX and SE or between SE and ground? Obviously, the thermistor would be installed on the opposite side of the voltage divider/half bridge relative to the fixed resistor.
- Would you recommend using analog ground or power ground of the CR1000 wiring panel for the grounded end of the voltage divider/half bridge?
- Would you recommend connecting the extension cable's shield to analog ground or to power ground on the CR1000 wiring panel?
Thanks in advance for any recommendations and explanations of the recommendations! These are questions that have puzzled me (and possibly others?) for a long time.
you will find many useful tips in 107L manual of CSI and maybe they use the same thermistor (10K at 25°C)
Smile
@Smile - I read through the 107 manual before writing my post. It discusses considerations related to the 107L, but does not discuss the selection and circuit design of a third-party thermistor element with a CSI datalogger.
IF you use the 10k variant, which will the smallest errors for the extra cable length, you would best follow the design given in the 109-L manual which uses a single resistor at the logger. This can be used with all the loggers. I would advise using a separate formula, rather than the Therm109 instruction so you can use the published Steinhart-Hart parameters that Omega give in their spec sheets for the bare thermistors. You need to use the BrHalf instruction and solve for the resistance of the thermistor before applying the Steinhart-Hart equation.
Regarding the 109-L and 10 kOhm thermistor, I'm trying to understand the pros and cons of selecting various thermistor and fixed resistor values in a custom configuration. I know the 10kOhm thermistor reduces the effect of cable length, but I suspect there are cons to go along with that pro.
Regarding the fixed resistor location, do you know why it is better to locate the resistor at the logger instead of near the sensor?
Finally, in the past, I have successfully used these Omega thermistors using the BrHalf instruction and a custom equation with Omega's Steinhart-Hart parameters, so that's not my concern. My concerns are those stated in my original post. I feel like I am randomly picking thermistor and fixed resistor values and wiring/grounding configurations. Sooner or later, that's going to come back to bite me when I have a problem or I realize that there was a much better way to do things. Additional insight into my original questions would be greatly appreciated.
Dear mwassmer,
with reference to the manual of 109, in my opinion, you should:
"My questions are:
- Which of the available resistance values would you recommend for the thermistor? The available values are 2252, 5000, or 10,000 ohms.
->Smile 10K
- What resistance value would you recommend for the fixed resistor?
->Smile 24.9K Ω, 0.1% 10..15 ppm (I think it is a good choice if very stable temperature, also depends on what environment works the CR1000, if the temperature is stable is perhaps less important)
- It is a little more convenient to install the fixed resistor at the CR1000 wiring panel instead of at the sensor end. Are any sensor accuracy consequences to locating the fixed resistor at the wiring panel?
->Smile Near the WP of CR1000, at the end of cable.
- Would you recommend installing the fixed resistor between EX and SE or between SE and ground? Obviously, the thermistor would be installed on the opposite side of the voltage divider/half bridge relative to the fixed resistor.
->Smile See FIGURE 6-1. 109 Thermistor Probe Schematic in the 109 manual
- Would you recommend using analog ground or power ground of the CR1000 wiring panel for the grounded end of the voltage divider/half bridge?
->Smile See FIGURE 6-1. 109 Thermistor Probe Schematic
- Would you recommend connecting the extension cable's shield to analog ground or to power ground on the CR1000 wiring panel?
->Smile See TABLE 5-1. Wiring in the 109 manual and however if the original cable is shielded always use the scheme described in the manual of the 109, but if the original cable is not shielded cut it as close as possible to the thermistor, easy to be processed and leave with a good shielded cable type belden 8761 (a pair with screen)
Besides You will also find interesting tips in Technical Note 15-95AS at http://s.campbellsci.com/documents/us/technical-papers/csl_thermistors.pdf
Smile
* Last updated by: smile on 1/31/2014 @ 10:26 AM *