10 Questions - Pressure Transmitters

Hi my name is Lee Hamlett, I'm with Yokogawa, I'm a product manager I've been here 12 years.

What is pressure?

Well pressure is very simple, its just the amount force applied over a certain area. This can apply to gas, liquid, anything that applies a certain force over a certain amount of area. well pressure is one of

Hi my name is Lee Hamlett, I'm with Yokogawa, I'm a product manager I've been here 12 years.

What is pressure?

Well pressure is very simple, its just the amount force applied over a certain area. This can apply to gas, liquid, anything that applies a certain force over a certain amount of area. well pressure is one of the four unique measurements inside of a plant. The others being flow, temperature and level. What unique about pressure is pressure can actually be used to infer the other three. Flow by using a pressure transmitter across a Venturi, a Pitot tube or a orifice plate you can infer what the flow of the liquid or gas that you are measuring. With level in a tank, you can use the hydrostatic head pressure principle to measure the amount of liquid in a tank. Lastly and not used very much is, using pressure to measure temperature. This is where you have a certain amount of gas in a closed area and as the temperature changes, the pressure inside the chamber changes and by the change of pressure you know the change in temperature is. 

 

What are some basic traits of pressure that are important in industrial applications?

Well theres three attributes that are important. First , in a chamber the pressure is the same no matter where your located inside the chamber, so whether you're at the top, the bottom, the left the right, the pressure is the same no matter where you're at in the chamber. The second attribute is if you have the same amount of gas in a chamber, you change the size of the chamber the pressure changes in the chamber to meet the size you've changed to, so if you make a smaller chamber, the pressure increases. The third attribute is the way that temperature affects pressure, if we have a given size chamber with a gas in it and you increase the temperature, the pressure will increase in it or if you decrease the temperature the pressure will decrease. Those three attributes are very important to know. 

 

What is pressure reference?

Well when you measure pressure what you're really doing is comparing two pressures to each other and theres a couple ways to qualify that . Theres a gauge pressure measurement that compares the pressure that you are measuring to the atmospheric pressure. Now thats very similar to what you do with your car tire, when you measure the pressure in your car tire you actually comparing the pressure in the tire to the outside of the tire. The problem with that is if theres any change in the local pressure it can look like your accuracy is off, and pressure changes when a thunderstorm comes through, a cold front or anything like that. The local atmospheric pressure can change. Now the next one is absolute pressure. Thats where you compare your pressure you are wanting to measure to absolutely zero pressure. Zero pressure doesn't exist anywhere, so its very hard to build a transmitter that references, you have to suck the vacuum out of the of a chamber to compare to. Thats very good because it doesn't have the variation that you get with gauge pressure. Problem is they are very expensive. Some customers will put up with using a gauge reference  because its less expensive or instead of the absolute pressure thats more accurate. And then theres also a differential pressure reference and thats where you're just comparing two separate pressures.

 

How do you measure pressure in an industrial application?

Well you use a pressure transmitter. Pressure transmitters have sensors in them that are designed to measure the pressure. How they do that, is they have a physical attribute in the sensor that changes when pressure changes. So you're actually measuring the change of that physical attribute to infer what the pressure change is. theres several different ways of doing that, theres sensors on the market that use frequencies, capacitance and inductance. They are all kinds of sensors out there and you can find transmitters that use all the different technologies.

 

Is that all a pressure transmitter does?

No. The transmitter takes information from the sensor and transmits it or communicates with something else that  needs the information. That can be a controller , a DCS or even a cloud based data collection program. Whats important is that the transmitter do it quickly, accurately and reliably. Thats whats important.

 

How does the transmitter communicate?

Well most transmitters are hardwired to whatever needs the information but there also wireless solutions out there that broadcast the information to where it needs to go.

 

What language does the transmitter communicate in?

Wells theres two, theres an analog communication and theres also a digital or a combination of the two. Analog you have a 1-5 volt DC or 4-20 milliamp signal that corresponds to the variation in the pressure. With digital you have a protocol and thats a language that the controller and transmitter speak with each other to get the information. Some popular versions of this are HART, FieldBus, MODBUS. Some companies even have a proprietary communication language. There is a lot to chose from but make sure all of your equipment speaks the same language.

 

What characteristics should you look for in a pressure transmitter?

As we discussed before, you want your transmitter to quickly, accurately and reliably get the signal to where it needs to go. Unfortunately, with a lot of pressure transmitters out there the specifications that convert to these qualities are hard to understand. When it comes to quickly, you're looking for response time. You want a fast response time. With accuracy, thats easy, thats usually referred to as reference accuracy. The one that is a little tough to understand is reliability which has to do with the stability of the product. So what you want to look for is the response time, the accuracy and the stability of the transmitter you are looking at. 

 

Anything else you should look for?

All applications and all pressure transmitters are different. all sensors all manufactures, everybody has their strengths and weaknesses. You just need to evaluate what your application needs and pick the unit that best suits your application.

 

Where can I find more information?

A good source of information would be yokogawa.com/us there you'll find these questions and answers as well as other things that you can use to help you evaluate transmitters.