What is the 250T Two-wire RTD Transmitter Failsafe Hook-Up Approach? – 8401068
Two-wire Transmitters and Their “Hook-up” to Achieve a Failsafe Configuration
The attached analysis references Acromag’s 250T series RTD (100 ohm platinum)
This type of hook-up is typically not used because the effects of lead-wire resistance inaccuracies are additive to the RTD measurement. However, when addressing the problem of FAILSAFE, this configuration offers the most predictable output behavior. Also, H the RTD hook-up is properly done, the inaccuracies due to lead wire resistance can be minimized. The following is a list of most probable failures that could happen to the input sensor. An analysis will be performed to address each failure and predict the output of the transmitter.
- Lead wires +/-could break open
- The RTD sensor could fail open up
- The input could become grounded
- The input +/-could become shorted
Conditions/Assumptions for Failsafe Hook-up Approach:
- RTD type: 100 ohm platinum
- Lead-wire length: 50ft (or shorter)
- “-” & “L” connection: internal
- Max. temp. change for lead wires: 50°C
- Lead wire gauge: 17 AWG (0.005 ohms/ft)
- RTD break detection: UP
Based on these conditions, we can derive accuracies for this type of approach.
- Total lead wire resistance:
- 2 * 50 ft * (.005 ohms/ft) = 0.5 ohms (static value)
- NOTE: This lead wire resistance is a static value which means that at 25°C, R’s effect can be calibrated out of the transmitter. The value of the most concern, however, is the dynamic resistance change of the lead wires over temperature
- The following equation is a good approximation for copper wire resistance with temperature changes of up to 50°C.
- Rt= R(25°C) * [1 + (6 T)(0.004)]
- Rt= 0.5 * (1 + (50)(0.004)]
- Rt= 0.6 ohms
- Thus, the dynamic resistance change of the lead wires is:
Dynamic change= 0.1 ohms (maximum temperature change= 50°C)
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