Ever changed out a "faulty" transmitter only to find out that the problem was somewhere else in the loop?
Have any loops that require adjustment after it rains?
Ever spent hours swapping out all the instruments in a loop only to find that the problem was due to a corroded junction?
The solution is a PIE Calibrator with built-in Loop Diagnostics
(US Patent# 7,248,058).
When a 4-20mA control loop is first installed the instruments, conduit and wiring are new, clean and dry. The 2-Wire transmitter gets its operating voltage from a loop power supply or control system input board, regulates current based on the input signal and the corresponding current is measured by the control system.
After time, moisture or chemical fumes come in contact with the connections at the instruments or in junction boxes. This can cause bridges between the control signal lines and earth ground. Some unknown amount of current starts to “leak” from the control signal and is undetected by the control system. When this happens in the leg where a transmitter is located the control system will see a higher current than the transmitter is controlling. With this type of fault the control system is measuring the leakage current in addition to the current controlled by the transmitter. The following scenario is an example of troubleshooting a loop with moisture present in a junction box.
Scenario - The control room pages you and tells you that the boiler controlled by loop Temp 42 is running hot
Using a “Brand X” calibrator
- Verify the current signal at the control room – it is higher than it should be – 15 minutes
- Get a transmitter from stores and go to the transmitter site– 30 minutes
- Configure the transmitter – 15 minutes
- Remove the existing transmitter and replace it – 15 minutes
- Check in the control room – current still high
- Open up panels and locate a flooded junction box with corroded terminals – 25 minutes
- Remove moisture, clean terminals and reseal junction box – 5 minutes
Elapsed time: 1.75 hours
Bonus Question – what do you do with the transmitter you replaced?
- Break open the loop again to put it back into service (assuming it was working correctly) – 15 minutes
- Take it back to the shop to verify its operation before putting it into stores as a spare – 30 minutes
- Throw it away – 0 minutes and hundreds of dollars!
Scenario - The control room pages you and tells you that the boiler controlled by loop Temp 42 is running hot
Using a PIECAL 434 Automated Diagnostic Calibrator
- Disconnect the loop from the power supply and drive the loop using the built-in power supply of the PIECAL 434 – 15 minutes
- Observe that the PIECAL 434 indicates that there is 0.51mA of current leaking in the loop (see photo)
- Open up panels and locate a flooded junction box with corroded terminals – 25 minutes
- Remove moisture, clean terminals and reseal junction box – 5 minutes
Elapsed time: .75 hours
Bonus Savings–no transmitter replacement!
One instrument technician, after seeing a demonstration of the PIECAL mA Diagnostic Calibrator said “It is worth the extra few dollars to buy a PIECAL Calibrator with the loop diagnostics and never use the feature than to buy a calibrator without it and need to use the diagnostics!”
Try a PIECAL 434 Automated Diagnostic Calibrator and see how much time and money you can save.
