Setting the Range of an RTD transmitter
1. RTD element range
>We use OMEGA's -200 to +850C RTD's
I can't find an RTD with a 850°C top end, the usual high end for wire wound platinum elements is 600°C. This looks more like a spec for a thermocouple.
I should have been clear. We do use -200C to 850C RTD's for a different application. The reason I brought up this RTD is it reports the correct temperature with this very same signal conditioner with temperature within 0.4 Deg. C of what is reported by the Mass Flow Controller.
But I do not get accurate results with the -30C - 260C rated RTD with the Signal conditioner set to -25 to 255C and the 4-20mA signal scaled to this range value in the PLC. 30C is well above and below the min max range of the RTD. So therefore one should expect it to report the temperature correctly.
The RTD transmitter's output is 4-20mA. The transmitter's output range is scaled over some temperature range according to what makes sense for the application.
I have already done this. However, what I have not tried yet is narrowing the range even further and see if it would yield accurate results.
5. What output range makes sense in this application?
The perceived problem is that the transmitter's output range DIP switches does not have a -30 to 260°C range. Some other range has to be selected, and the PLC's analog input range changed to match the selected transmitter output range.
For me, range of 15C 40C would be more than enough for purpose of testing.
The issue I am trying to understand is why the RTD with a min max range of -30 to 260 does not report temperature correctly when signal conditioner is set to a range of -25 to +255C. Which is within the min max range of the RTD. The 4-20mA signal is also scaled in the PLC to this signal conditioner range setting and not the actual min max range of the RTD. RTD should be reporting the correct temp with these settings.
In post #1, the process temperature range is described as
>the process loop which can never be above 32 C or below 28 C.
During testing, ambient gaseous Nitrogen is flown thru the system. We neither heat nor cool the gas. However, customer does and we are not privy to what gases and what temperatures their heated or chilled to. This happens after this subassembly get installed inside their process equipment.
Also, in post #1, it states
>RTD - OMEGA 4 Wire -30 to 260 C PT100 RTD
The -30 to 260°C range suggests some recommended range.
What is the source of the -30 to 260°C range?
Is -30° to 260°C the receiver's analog input range?
Is -30° to 260°C a customer specification?
-30C to 260C is a customer spec'd RTD. We cannot change that.
It seems to me -30° to 260°C to be an overly wide range for a process that has to maintain a temperature of 28 to 32°C.
If I had to pick, I'd use 0-50°C because the process temperature range of 28 to 30°C falls near the middle of a 0-50° range, but I don't know all the process details.
Agreed. I would try narrowing down the range.
The suggestion that the RTD's sensor range of -200 to +850C is not appropriate because of the stated 28 to 32°C process range; a wide range such as -200 to +850C would result in low resolution at the process operating range.
-200C to 850C RTD was used for testing. Signal condition range set to this value and 4-20mA signal scaled accordingly, this RTD was reporting the temperature correctly.