Answer / carl_ellis

A 2 wire, loop powered transmitter can not output a negative current, like -13mA. The 4-20mA output is always positive.

However, a remote seal DP transmitter used for level will 'see' a negative pressure with nothing in the vessel and the LRV (lower range value) for the transmitter will be a relatively negative value, close to the (negative) span of level range.

A DP transmitter with remote seals used for level measurement will ALWAYS have a zero level reading that is a relatively large negative number because of the head pressure of the fill fluid in the capillaries.

The reason for the negative value is that the fill fluid in the high side capillary (which connects to the lower flange) pulls away from the transmitters high side diaphragm as it presses down on the lower seal by the force of gravity.

Oppositely, the fill fluid in the low side capillary (which connects to the upper flange) exerts a positive static pressure on the transmitter's low side diaphragm because the seal is up high and the fill fluid presses down on the low side transmitter diaphragm, again by the force of the acceleration of gravity.

A differential transmitter works by subtracting the low side pressure from the high side pressure:

High side minus low side = differential

As an example, in a vessel 7m tall, the transmitter has two remote seals, each 4 meters in length. The upper seal has to be above the liquid level in the head space above the liquid. The medium is water, SG = 1.00.

Assume a fill fluid with a Specific Gravity of 1.00, 4m of high side capillary and 4m of low side capillary. With nothing in the vessel (zero level), the applied pressures would be:

high side pressure = approx - 3.75m H2O (negative 3.75 meters, water column)
low side pressure = approx 3.75m H2O (positive 3.75 meters, water column)

high side minus low side = differential
-3.75m minus 3.75m = - 7.5m (negative 3.75m minus 3.75m = negative 7.5m)

The LRV (lower range value) would be approximately -7.5m H2O.

The transmitter needs to be ranged properly, with the appropriate lower range value for zero level.

Carl Ellis
Measure First

i am having information technology branh ....... can i prepare for psu exam

0 Answers  

---

How we select the range of level transmitter in closed vessel ?

4 Answers   Rosemount,

---

why 4ma show higher zero ? i have transmiter flow renge 0-1000 m3/h but 4 ma show in monitoring 200 m3/h and 20 ma show 1000 m3/h i want when 4ma show 0 m3/h and 20ma 1000m3/h

3 Answers  

---

What is a PID controller and how can it to applied to our everyday life?

4 Answers  

---

Why we not use 2-20Ma or 1-20Ma

5 Answers  

---

---

Dear All can any one told me that VFD Make ABB Model ACS800-01 used for dual Source like 1.DC Source applied at terminal UDC+,UDC- 2. 3 phase 400AC,50Hz

1 Answers  

---

Why Differential pressure transmitter is located below the tappings points for steam & condensible vapor flow measurement application?

2 Answers  

---

How a conductivity sensor & analyzer work at boiler, whats the principle & science is beyond this,How conductivity sensor sence & convert signal & move toward analyzer & analyzer show reading in percent? also what is temperature compensate option in conductivity analyzer

0 Answers  

---

I WANT TO KNOW SOME BASIC CONCEPTS RELATED TO INSTRUMENTATION

4 Answers  

---

what do you mean by Loop checking ? What are the cold loop check and hot loop check? what are the difference between these two loops?

1 Answers  

---

I am manjunath having exp in maintenance of chemical plants,know i want to do a design course INTOOLS,after doing design course can we get jobs in design field.sinve i dont have design exp..

0 Answers  

---

how magentic flow tx. vortex flow tx & ultrasonic flow tx works and its prinicle?its callibtration prosedure?

1 Answers  

---