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

Give me a simple formula to calculate the bore of orifice plate with datas avilable instead of BS 1040.I want to calculate the approximate bore size only.

0 Answers  

---

What are the two states of a controller

3 Answers  

---

What is the formula to know the error of weighfeeder after having drop test.

3 Answers  

---

i saw in my company drawing seat cable size as mensioned like this 12p*15 sqmm,12t*1.5sqmm, 9c*1.5sqmm. now what my que is what is mean by p , t , c.

3 Answers  

---

Hello-I know this not an engineering question,I am working in gulf & i want to do correspondence diploma in instrumentation,can any one give me the address of any university in which this courses are available.

2 Answers  

---

---

what is the different between deflection type measurement and null type measurement in RTD?

0 Answers   SHR Solutions,

---

Can anyone say that formula of Converting the Diff. Pressure into Flow in Kg/Cm2 or any explaniation of Orifice Plate, Beta Ratio , How to calculate

1 Answers  

---

generally transmitter power supply having some range(12v to 35v) but the out put(4-20 mA) must same for given input.If we vary the power supply voltage then output mA also to be change according to V=IR. Is there any voltage regulator in the transmitter ?

1 Answers   Indian Steel,

---

1. What is the main differences between GE Mark IV and Mark V Turbine Control System? 2. what is the main differences between GE Mark V and Mark VI?

2 Answers   GE,

---

Where can i take training in intools software for Instrumentation design??

9 Answers  

---

Explain types of valves?

0 Answers  

---

what is droop mode n frequency mode in wood ward governor?

1 Answers   TATA,

---