Untuk mengontrol suatu proses, dibutuhkan informasi mengenai kuantitas dan kualitas ciri-ciri fisik proses itu. Instrumen-instrumen ukur dipakai untuk mendapatkan informasi ini. Kontrol yang lebih ketat membutuhkan pengukuran yang lebih akurat. Beberapa istilah yang lazim dipakai dalam system pengukuran adalah proves variable, range, zero, span, error, linearitas, akurasi. Sekarang akan kita bahas masing masing dari istilah diatas.
1. Proses Variabel
Proses variabel adalah besaran phisik atau besaran kimia karena berbagai pengaruh proses. Tekanan, temperature, flow dan level adalah variabel phisik; sedangkan kandungan oksigen dan nilai pH adalah variabel-variabel kimia.
Range adalah mengambarkan batasan sinyal yang berhubungan dengan instrumen input ataupun instrumen output. Batasan sinyal terendah dari suatu sinyal input adalah kuantitas instrumen terendah yang diukur, sedang batasan maksimumnya adalah nilai tertinggi. Sebagai contoh, suatu proses mempunyai batas atau range tekanan dari 100 kPa sampai 500 kPa. Maka alat instrumenasi proses ini tidak dapat digunakan untuk mengukur nilai dibawah 100 kPa atau diatas 500 kPa.
Nilai terendah suatu sinyal input atau sinyal output disebut zero, meskipun nilainya tidak nol. Sebagi contoh, range input transmiter tekanan mungkin 0 – 1000 kPa sedang range outputnya 20 sampai 100 kPa. Dari sini, nilai zero sinyal output digambarkan dengan 20 kPa. Transmiter temperatur dapat mengukur temperatur anatara 50oC dan 120 oC, sedang nilai outputnya bervariasi dari 20 sampai 100 kPa. Dalam hal ini, nilai zero pada range input dan output masing-masing adalah 50 oC dan 20 kPa.
Span input dan output dari suatu instrumen berhubungan langsung dengan range input ataupun range outputnya. Span adalah selisih aljabar antara nilai range teratas dengan nilai range terendah.
Error adalah selisih antara nilai yang diukur dengan nilai yang sebenarnya. Sebagai contoh, jika pressure gage menunjukkan 216 kPa ketika tekananya nyatanya 220 kPa, maka errornya adalah – 4kPa.
Linieritas menggambarkan kedekatan hubungan antara input dengan output dari suatu instrument yang digambarkan seperti sebuah garis lurus ; hal tersebut adalah, sebuah gris lusrus dari 0% input dan 0% output sampai 100% input dan 100% output. Jika hubungan ini menyimpang maka timbul ketidak linieran. Ketidak linieran output biasanya dinyatakan dalam persentase skala penuh atau full scale output.
Akurasi dari sebuah instrumen dapat didefinisikan sebagai kedekatan antara pengukuran atau output yang menggambarkan nilai nyata. Akurasi biasanya dinyatakan dengan persentase span.
DASAR DASAR KALIBRASI INSTRUMENT
From sea to shining sea, or at least from one side of the Hudson to the other, politicians you have barely heard of are being accused of wrongdoing. There were so many court proceedings involving public officials on Monday that it was hard to keep up.
In Newark, two underlings of Gov. Chris Christie were arraigned on charges that they were in on the truly deranged plot to block traffic leading onto the George Washington Bridge.
Ten miles away, in Lower Manhattan, Dean G. Skelos, the leader of the New York State Senate, and his son, Adam B. Skelos, were arrested by the Federal Bureau of Investigation on accusations of far more conventional political larceny, involving a job with a sewer company for the son and commissions on title insurance and bond work.
The younger man managed to receive a 150 percent pay increase from the sewer company even though, as he said on tape, he “literally knew nothing about water or, you know, any of that stuff,” according to a criminal complaint the United States attorney’s office filed.
The success of Adam Skelos, 32, was attributed by prosecutors to his father’s influence as the leader of the Senate and as a potentate among state Republicans. The indictment can also be read as one of those unfailingly sad tales of a father who cannot stop indulging a grown son. The senator himself is not alleged to have profited from the schemes, except by being relieved of the burden of underwriting Adam.
The bridge traffic caper is its own species of crazy; what distinguishes the charges against the two Skeloses is the apparent absence of a survival instinct. It is one thing not to know anything about water or that stuff. More remarkable, if true, is the fact that the sewer machinations continued even after the former New York Assembly speaker, Sheldon Silver, was charged in January with taking bribes disguised as fees.
It was by then common gossip in political and news media circles that Senator Skelos, a Republican, the counterpart in the Senate to Mr. Silver, a Democrat, in the Assembly, could be next in line for the criminal dock. “Stay tuned,” the United States attorney, Preet Bharara said, leaving not much to the imagination.
Even though the cat had been unmistakably belled, Skelos father and son continued to talk about how to advance the interests of the sewer company, though the son did begin to use a burner cellphone, the kind people pay for in cash, with no traceable contracts.
That was indeed prudent, as prosecutors had been wiretapping the cellphones of both men. But it would seem that the burner was of limited value, because by then the prosecutors had managed to secure the help of a business executive who agreed to record calls with the Skeloses. It would further seem that the business executive was more attentive to the perils of pending investigations than the politician.
Through the end of the New York State budget negotiations in March, the hopes of the younger Skelos rested on his father’s ability to devise legislation that would benefit the sewer company. That did not pan out. But Senator Skelos did boast that he had haggled with Gov. Andrew M. Cuomo, a Democrat, in a successful effort to raise a $150 million allocation for Long Island to $550 million, for what the budget called “transformative economic development projects.” It included money for the kind of work done by the sewer company.
The lawyer for Adam Skelos said he was not guilty and would win in court. Senator Skelos issued a ringing declaration that he was unequivocally innocent.
THIS was also the approach taken in New Jersey by Bill Baroni, a man of great presence and eloquence who stopped outside the federal courthouse to note that he had taken risks as a Republican by bucking his party to support paid family leave, medical marijuana and marriage equality. “I would never risk my career, my job, my reputation for something like this,” Mr. Baroni said. “I am an innocent man.”
The lawyer for his co-defendant, Bridget Anne Kelly, the former deputy chief of staff to Mr. Christie, a Republican, said that she would strongly rebut the charges.
Perhaps they had nothing to do with the lane closings. But neither Mr. Baroni nor Ms. Kelly addressed the question of why they did not return repeated calls from the mayor of Fort Lee, N.J., begging them to stop the traffic tie-ups, over three days.
That silence was a low moment. But perhaps New York hit bottom faster. Senator Skelos, the prosecutors charged, arranged to meet Long Island politicians at the wake of Wenjian Liu, a New York City police officer shot dead in December, to press for payments to the company employing his son.
Sometimes it seems as though for some people, the only thing to be ashamed of is shame itself. Finding Scandal in New York and New Jersey, but No Shame