A process plant’s technician was mystified about how to get a typical gauge pressure transmitter to read in the vacuum range. “All our gauges are 0 to 30 inches mercury, and that’s what we need to transmitter output to be. But the transmitter you sent us just stays around 4mA when we pull a vacuum.”
We walked out to the reactor vessel to look at the installation. The transmitter’s Low side port was open, its high side port was plumbed into a tee along with a conventional bourdon tube pressure gauge reading gauge pressure vacuum.
I could see why he was confused. The mechanical gauge goes from 0 to 30. I asked what range he used to configure the pressure transmitter. His answer, “0 to 30 inches mercury, same as the mechanical gauge.”
So, what was happening?
I’d run into this before, and realized that not everyone was as luck as me. I had Mrs. Mann for 7th grade arithmetic, and she drilled and drilled her class on the number line.
I’m grateful she did the drill, because that number line describes how I think of vacuum, where atmospheric pressure is at zero, negative numbers in red are vacuum, and the blue positive numbers are positive pressure.
Wrap the red numbers around a gauge dial and you get a vacuum gauge with negative number (left). Wrap both negative and positive numbers around a gauge dial and you get a compound range for both negative and positive pressures (right).
A differential pressure transmitter has high and low ports, so the port that is connected to the process dictates whether the pressure values are displayed as positive negative numbers. But a gauge pressure transmitter has the only the single ‘high’ port so that vacuum values will always be displayed as negative values.
I asked the operator how important reading as positive or negative numbers was. In this case, he said it wasn’t an issue: the critical part was getting the 4-20mA output to respond.
Rather than tackle the number line concept, I pulled out a vacuum gauge with negative numbers on the scale and explained that this is how the electronic pressure transmitter deals with vacuum — as negative pressure. When the transmitter is ranged 0 to 30 as positive numbers, the transmitter only measures positive pressure. It isn’t measuring vacuum.
When the vacuum pump isn’t running, the transmitter sees 0.0 inHg pressure and outputs 4.0mA. When vacuum the vacuum pump turns on, the transmitter can go ‘down to’ maybe 3.8 or 3.9mA and then bottoms out. (I’m guessing he had Mrs. Mann’s sister for his 7th grade math class, because he picked up on the concept right away.)
The solution was to range the transmitter for negative pressure so that it measures vacuum and its 4-20mA output moves proportional to the vacuum. We opened the top cover on the Siemens DSIII transmitter and using the push buttons cycled through the settings.
The engineering units were set correctly for inches mercury (inHG).
Mode 5, the Lower Range Value (LRV), was at 0.0 and Mode 6, the Upper Range Value (URV) was at 30.0. We made one change, we changed the URV from 30.0 to -30.0 (note the negative sign).
The display showed zero pressure or vacuum and the output was 4.0mA. When he turned on the vacuum pump on the 4-20mA signal rose higher and higher as the pump pulled a higher vacuum and the vacuum gauge needle showed higher vacuum.
Siemens’ spec sheet only mentions vacuum in terms of a lower measuring limit. Diaphragm sensing technology accurately measures down to -14.25 psi (0.44 psia [absolute]). This is an accuracy limit, not an operational limit. Applying a higher (more intense) vacuum will not damage a Siemens transmitter, but achieving the documented accuracy below the -14.25 psi is not guaranteed.
Either a Siemens DP transmitter (7MF4433) or a gauge pressure transmitter (7MF4033) can be used for vacuum or compound measurements. Whether the 4-20mA output rises or falls with higher vacuum and whether the indicated vacuum values are positive or negative depends on how the transmitter is piped to the process and its configuration.
The table below shows the sign of the displayed vacuum values and direction of the 4-20mA for different setups combinations.
The red rows are invalid setups because the 4-20 current signal bottoms out with any application of vacuum.
If you use preset range electronic transmitters, you need to be aware of where atmosphere or vacuum falls on the current signal.
For WIKA’s industrial line, there’s a vacuum model and a compound model.
On the vacuum model, 4.0mA is vacuum, 20.0mA is atmosphere.
On the compound range model, 4.0mA = 30” inHg vacuum, 9.26mA = atmospheric pressure , and 20.0 mA = 30.0 psig positive pressure.
But beware. As always, Caveat Emptor applies. These numbers will always differ slightly depending on the manufacturer.
Want to learn more? See all the pressure transmitters available from Lesman.